El Niño–Southern Oscillation

{{Short description|Global climate phenomenon}} {{redirect2|El Niño|ENSO|the Japanese motif|Ensō|other uses|El Niño (disambiguation)|and|Enso (disambiguation)}} {{redirect|La Niña|other uses|Niña (disambiguation){{!}}Niña}} {{multiple image | total_width = 500 | width1 = | image1 = NOAA_Nino.jpg | alt1 = Impacts of El Niño on climate | caption1 = | width2 = | image2 = NOAA_Nina.jpg | alt2 = Impacts of La Niña on climate | caption2 = | footer_background =  | footer_align = center | footer = Changes to temperature and precipitation are different during El Niño and La Niña, and vary from season to season.<ref name="Wald 2021">{{cite book |last1=Wald |first1=Lucien |title=Fundamentals of solar radiation |date=2021 |publisher=CRC Press |location=Boca Raton |isbn=978-0-367-72588-4 |chapter=Definitions of time: from year to second}}</ref> }}

'''El Niño–Southern Oscillation''' ('''ENSO''') is a global climate phenomenon that emerges from variation in winds and sea surface temperatures over the tropical Pacific Ocean. Those variations have an irregular pattern but do have some semblance of cycles. The occurrence of ENSO is not easily predictable. It affects the climate of much of the tropics and subtropics, and has links (teleconnections) to higher-latitude regions of the world. The warming phase of the sea surface temperature is known as "'''El Niño'''" and the cooling phase as "'''La Niña'''". The '''Southern Oscillation''' is the accompanying atmospheric oscillation, which is coupled with the sea temperature change.

El Niño is associated with higher than normal air sea level pressure over Indonesia, Australia and across the Indian Ocean to the Atlantic. La Niña has roughly the reverse pattern: high pressure over the central and eastern Pacific and lower pressure through much of the rest of the tropics and subtropics.<ref name="CPC ENSO">{{cite web |author=Climate Prediction Center |publisher=National Centers for Environmental Prediction |title=Frequently Asked Questions about El Niño and La Niña |url=http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#DIFFER |date=2005-12-19 |access-date=2009-07-17 |archive-url=https://web.archive.org/web/20090827143632/http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#DIFFER |archive-date=2009-08-27 |author-link=Climate Prediction Center}}</ref><ref name="Climate Change 2007">{{cite book |title=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change |editor=Solomon, S. |editor2=D. Qin |editor3=M. Manning |editor4=Z. Chen |editor5=M. Marquis |editor6=K.B. Averyt |editor7=M. Tignor |editor8=H.L. Miller |display-editors=3 |chapter=Observations: Surface and Atmospheric Climate Change |publisher=Cambridge University Press |location=Cambridge, UK |pages=235–336 |chapter-url=http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3.html |author=Trenberth, K.E. |author2=P.D. Jones |author3=P. Ambenje |author4=R. Bojariu |author5=D. Easterling |author6=A. Klein Tank |author7=D. Parker |author8=F. Rahimzadeh |author9=J.A. Renwick |author10=M. Rusticucci |author11=B. Soden |author12=P. Zhai |access-date=2014-06-30 |archive-date=2017-09-24 |archive-url=https://web.archive.org/web/20170924100014/http://ipcc.ch/publications_and_data/ar4/wg1/en/ch3.html |url-status=dead}}</ref> The two phenomena last a year or so each and typically occur every two to seven years with varying intensity, with neutral periods of lower intensity interspersed.<ref>{{cite web |url=http://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina/enso-description |title=El Niño, La Niña and the Southern Oscillation |publisher=MetOffice |access-date=2015-08-18 |archive-date=2023-10-27 |archive-url=https://web.archive.org/web/20231027063920/https://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina/enso-description |url-status=live}}</ref> El Niño events can be more intense but La Niña events may repeat and last longer.

A key mechanism of ENSO is the Bjerknes feedback (named after Jacob Bjerknes in 1969) in which the atmospheric changes alter the sea temperatures that in turn alter the atmospheric winds in a positive feedback. Weaker easterly trade winds result in a surge of warm surface waters to the east and reduced ocean upwelling on the equator. In turn, this leads to warmer sea surface temperatures (called El Niño), a weaker Walker circulation (an east–west overturning circulation in the atmosphere) and even weaker trade winds. Ultimately the warm waters in the western tropical Pacific are depleted enough so that conditions return to normal. The exact mechanisms that cause the oscillation are unclear and are being studied.

Each country that monitors the ENSO has a different threshold for what constitutes an El Niño or La Niña event, which is tailored to their specific interests.<ref name="December 2014 EU2" />  El Niño and La Niña affect the global climate and disrupt normal weather patterns, which as a result can lead to intense storms in some places and droughts in others.<ref name="NIWA El Niño/La Niña" /><ref name="How Much Do El Niño" /> El Niño events cause short-term (approximately 1 year in length) spikes in global average surface temperature while La Niña events cause short term surface cooling.<ref name=":1" /> Therefore, the relative frequency of El Niño compared to La Niña events can affect global temperature trends on timescales of around ten years.<ref name=":2" /> The countries most affected by ENSO are developing countries that are bordering the Pacific Ocean and are dependent on agriculture and fishing.

In climate change science, ENSO is known as one of the internal climate variability phenomena.<ref name="IPCC2021">IPCC, 2021: [https://report.ipcc.ch/ar6/wg1/IPCC_AR6_WGI_FullReport.pdf Climate Change 2021: The Physical Science Basis] {{Webarchive|url=https://web.archive.org/web/20231208092128/https://report.ipcc.ch/ar6/wg1/IPCC_AR6_WGI_FullReport.pdf |date=2023-12-08}}. [https://www.ipcc.ch/report/ar6/wg1/ Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20230526182346/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf |date=2023-05-26}} [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2391 pp. doi:10.1017/9781009157896.</ref>{{rp|23}} Future trends in ENSO due to climate change are uncertain,<ref name="Collins102">{{cite journal |author1=Collins, Mat |author2=An, Soon-Il |author3=Cai, Wenju |author4=Ganachaud, Alexandre |author5=Guilyardi, Eric |author6=Jin, Fei-Fei |author7=Jochum, Markus |author8=Lengaigne, Matthieu |author9=Power, Scott |author10=Timmermann, Axel |author-link10=Axel Timmermann |author11=Vecchi, Gabe |author12=Wittenberg, Andrew |date=23 May 2010 |title=The impact of global warming on the tropical Pacific Ocean and El Niño |url=https://hal.archives-ouvertes.fr/hal-00534052 |journal=Nature Geoscience |volume=3 |issue=6 |pages=391–397 |bibcode=2010NatGe...3..391C |doi=10.1038/ngeo868 |access-date=10 January 2019 |archive-date=14 September 2019 |archive-url=https://web.archive.org/web/20190914105415/https://hal.archives-ouvertes.fr/hal-00534052 |url-status=live|url-access=subscription }}</ref> although climate change exacerbates the effects of droughts and floods. The IPCC Sixth Assessment Report summarized the scientific knowledge in 2021 for the future of ENSO as follows: "In the long term, it is very likely that the precipitation variance related to El Niño–Southern Oscillation will increase".<ref name="IPCC2021" />{{rp|113}} The scientific consensus is also that "it is very likely that rainfall variability related to changes in the strength and spatial extent of ENSO teleconnections will lead to significant changes at regional scale".<ref name="IPCC2021" />{{rp|114}}

==Definition and terminology== [[File:SOI.svg|thumb|upright=1.4|The Southern Oscillation Index from 1876 to 2025. The Southern Oscillation is the atmospheric component of El Niño. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters.]] The El Niño–Southern Oscillation is a single climate phenomenon that recurs in three phases: Neutral, La Niña or El Niño.<ref name="ENSO:Nutshell">{{cite web |author1=L'Heureux, Michelle |date=5 May 2014 |title=What is the El Niño–Southern Oscillation (ENSO) in a nutshell? |website=ENSO Blog |publisher=NOAA |url=https://www.climate.gov/news-features/blogs/enso/what-el-ni%C3%B1o%E2%80%93southern-oscillation-enso-nutshell |url-status=dead |archive-url=https://web.archive.org/web/20160409220031/https://www.climate.gov/news-features/blogs/enso/what-el-ni%C3%B1o%E2%80%93southern-oscillation-enso-nutshell |archive-date=9 April 2016 |access-date=7 April 2016}}</ref> La Niña and El Niño are opposite phases in the oscillation which are deemed to occur when specific ocean and atmospheric conditions are reached or exceeded.<ref name="ENSO:Nutshell" />

An early recorded mention of the term "El Niño" ("The Boy" in Spanish) to refer to climate occurred in 1892, when Captain Camilo Carrillo told the geographical society congress in Lima that Peruvian sailors named the warm south-flowing current "El Niño", referring to the Christ Child, because it was most noticeable around Christmas.<ref>Carrillo, Camilo N. (1892) [https://www.biodiversitylibrary.org/item/183558#page/80/mode/1up "Disertación sobre las corrientes oceánicas y estudios de la correinte Peruana ó de Humboldt"] {{Webarchive|url=https://web.archive.org/web/20231030150743/https://www.biodiversitylibrary.org/item/183558#page/80/mode/1up |date=2023-10-30}} (Dissertation on the ocean currents and studies of the Peruvian, or Humboldt's, current), ''Boletín de la Sociedad Geográfica de Lima'', '''2''' : 72–110. [in Spanish] [https://www.biodiversitylibrary.org/item/183558#page/92/mode/1up From p. 84:] ''"Los marinos paiteños que navegan frecuentemente cerca de la costa y en embarcaciones pequeñas, ya al norte ó al sur de Paita, conocen esta corriente y la denomination Corriente del ''Niño'', sin duda porque ella se hace mas visible y palpable después de la Pascua de Navidad."'' (The sailors [from the city of] Paita who sail often near the coast and in small boats, to the north or the south of Paita, know this current and call it "the current of the ''Boy'' [el Niño]", undoubtedly because it becomes more visible and palpable after the Christmas season.)</ref><ref name="Definition">{{cite journal |author=Trenberth, Kevin E |date=December 1997 |title=The Definition of El Niño |journal=Bulletin of the American Meteorological Society |volume=78 |issue=12 |pages=2771–2777 |bibcode=1997BAMS...78.2771T |doi=10.1175/1520-0477(1997)078<2771:TDOENO>2.0.CO;2 |doi-access=free}}</ref><ref>{{Cite web |title=El Niño |url=https://education.nationalgeographic.org/resource/el-nino |access-date=2023-06-03 |website=education.nationalgeographic.org |language=en |archive-date=2023-06-05 |archive-url=https://web.archive.org/web/20230605082628/https://education.nationalgeographic.org/resource/el-nino/ |url-status=live}}</ref>

Over time the term has evolved and now refers to the warm and negative phase of the El Niño–Southern Oscillation (ENSO). The original phrase, ''El Niño de Navidad'', arose centuries ago, when Peruvian fishermen named the weather phenomenon after the newborn Christ.<ref>{{cite news |date=21 October 2015 |title=The Strongest El Nino in Decades Is Going to Mess With Everything |url=https://www.bloomberg.com/news/articles/2015-10-21/a-huge-el-nino-is-spreading-all-kinds-of-mayhem-around-the-world |access-date=18 February 2017 |work=Bloomberg.com |archive-date=11 February 2022 |archive-url=https://web.archive.org/web/20220211215114/https://www.bloomberg.com/news/articles/2015-10-21/a-huge-el-nino-is-spreading-all-kinds-of-mayhem-around-the-world |url-status=live}}</ref><ref>{{cite news |title=How the Pacific Ocean changes weather around the world |url=http://www.popsci.com/how-pacific-ocean-changes-weather-around-world#page-8 |access-date=19 February 2017 |work=Popular Science |language=en |archive-date=3 January 2022 |archive-url=https://web.archive.org/web/20220103035434/http://www.popsci.com/how-pacific-ocean-changes-weather-around-world#page-8 |url-status=live}}</ref>

La Niña ("The Girl" in Spanish) is the colder counterpart of El Niño, as part of the broader ENSO climate pattern. In the past, it was also called an anti-El Niño<ref name="USNOAA-NOS-NinoNina">{{cite web |date=February 10, 2020 |title=What are "El Niño" and "La Niña"? |url=https://oceanservice.noaa.gov/facts/ninonina.html |access-date=11 September 2020 |website=oceanservice.noaa.gov |publisher=National Oceanic and Atmospheric Administration |department=National Ocean Service |archive-date=11 January 2023 |archive-url=https://web.archive.org/web/20230111092825/https://oceanservice.noaa.gov/facts/ninonina.html |url-status=live}}</ref> and El Viejo, meaning "the old man".<ref>{{cite web |date=24 March 2008 |title=What is "La Niña"? |url=http://www.pmel.noaa.gov/tao/elnino/la-nina-story.html |access-date=17 July 2009 |publisher=National Oceanic and Atmospheric Administration |department=Tropical Atmosphere Ocean project / Pacific Marine Environmental Laboratory |archive-date=16 December 2008 |archive-url=https://web.archive.org/web/20081216020733/http://www.pmel.noaa.gov/tao/elnino/la-nina-story.html |url-status=live}}</ref>

A negative phase exists when atmospheric pressure over Indonesia and the west Pacific is abnormally high and pressure over the east Pacific is abnormally low, during El Niño episodes, and a positive phase is when the opposite occurs during La Niña episodes, and pressure over Indonesia is low and high over the east Pacific.<ref>{{cite web |url=https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/soilink.shtml |title=The Southern Oscillation and its Links to the ENSO Cycle |publisher=NOAA National Weather Service Climate Prediction Centre |website=www.cpc.ncep.noaa.gov |access-date=19 January 2024 |archive-date=19 January 2024 |archive-url=https://web.archive.org/web/20240119143151/https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/soilink.shtml |url-status=live}}</ref>

==Fundamentals== thumb|The West Pacific is typically warmer than the East Pacific. The warmer water in the West Pacific leads to: more clouds, more rainfall, and a lower air pressure. The buildup of warm waters towards the west also leads to a thicker layer of warm ocean water that lowers the depth of the thermocline.|alt=Diagram showing a cross-section of the Pacific and related phenomena

On average, the temperature of the ocean surface in the tropical East Pacific is roughly {{cvt|8|–|10|C-change}} cooler than in the tropical West Pacific. The sea surface temperature (SST) of the West Pacific northeast of Australia averages around {{cvt|28|–|30|C}}. SSTs in the East Pacific off the western coast of South America are closer to {{cvt|20|C}}.

Strong trade winds near the equator drive water away from the East Pacific and into the West Pacific.<ref name="BOM About ENSO" /> This water is slowly warmed by the Sun as it moves west along the equator,<ref name="ENSO and Australia" /> the wind stress acting on the ocean surface being balanced by a sea surface slope, one result of which is that the sea levels near Indonesia is typically around {{cvt|1.5|ft|round=0.5|order=flip}} higher than that near Peru.

The warm surface waters collect in the western Pacific, with the result that the thermocline, the transitional zone between the warmer waters near the ocean surface and the cooler waters of the deep ocean,<ref name="What ENSO IRI">{{cite web |title=What is ENSO? |url=https://iridl.ldeo.columbia.edu/maproom/ENSO/ENSO_Info.html |website=IRI/LDEO Climate Data Library |publisher=International Research Institute for Climate and Society |access-date=22 January 2024}}</ref> lies much deeper in the western Pacific,<ref name="Effects ENSO Pacific" /> where it has an average depth of around {{cvt|450|ft|round=10|order=flip}} compared to around {{cvt|90|ft|round=10|order=flip}} in the East Pacific.<ref name="Effects ENSO Pacific">{{cite web |title=Effects of ENSO in the Pacific |url=https://www.weather.gov/source/zhu/ZHU_Training_Page/tropical_stuff/enso/enso2.htm |publisher=National Weather Service |access-date=22 January 2024}}</ref> At depth, the sloping surface thermocline helps reduce the east–west pressure difference due to the sea level slope, but below the thermocline the pressure difference is still enough to drive the eastward flowing cold equatorial undercurrent.<ref name="Sarachik and Cane 2010">{{cite book |last1=Sarachik |first1=Edward S. |last2=Cane |first2=Mark A. |title=The El Niño-Southern Oscillation Phenomenon |date=2010 |publisher=Cambridge University Press |location=Cambridge |isbn=978-0-521-84786-5}}</ref>{{rp|12}}

The cooler deep ocean water replaces the outgoing surface waters in the East Pacific, rising to the ocean surface in a process called upwelling.<ref name="BOM About ENSO" /><ref name="ENSO and Australia" /> Along the western coast of South America, water near the ocean surface is pushed westward due to the combination of the trade winds and the Coriolis effect. This process is known as Ekman transport. Colder water from deeper in the ocean rises along the continental margin to replace the near-surface water.<ref name="OceanMotion">{{cite web |title=Wind Driven Surface Currents: Upwelling and Downwelling Background |url=https://oceanmotion.org/html/background/upwelling-and-downwelling.htm |website=Ocean Motion and surface currents |publisher=NASA |access-date=22 January 2024}}</ref>

This process cools the East Pacific because the thermocline is closer to the ocean surface, leaving relatively little separation between the deeper cold water and the ocean surface.<ref name="Effects ENSO Pacific" /> The northward-flowing Humboldt Current carries colder water from the Southern Ocean to the tropics in the East Pacific.<ref name="BOM About ENSO" /> The combination of the Humboldt Current and upwelling maintains an area of cooler ocean waters off the coast of Peru.<ref name="BOM About ENSO" /><ref name="ENSO and Australia" />

The West Pacific lacks a cold ocean current and has less upwelling as the trade winds are usually weaker than in the East Pacific, allowing the West Pacific to reach higher temperatures. These warmer waters provide energy for the upward movement of air. As a result, the warm West Pacific has, on average, more cloud and rain than the cool East Pacific.<ref name="BOM About ENSO">{{cite web |title=El Niño Southern Oscillation (ENSO) |url=http://www.bom.gov.au/climate/about/australian-climate-influences.shtml?bookmark=enso |website=About Australian climate |publisher=Bureau of Meteorology |access-date=22 January 2024 |archive-url=https://web.archive.org/web/20240122152621/http://www.bom.gov.au/climate/about/australian-climate-influences.shtml?bookmark=enso |archive-date=22 January 2024 |url-status=live}}</ref>

ENSO describes a quasi-periodic change of both oceanic and atmospheric conditions over the tropical Pacific Ocean.<ref name="BOM About ENSO" /> These changes affect weather patterns across much of the Earth.<ref name="ENSO and Australia">{{cite web |title=El Niño, La Niña and Australia's Climate |url=http://www.bom.gov.au/info/leaflets/nino-nina.pdf |publisher=Bureau of Meteorology |access-date=22 January 2024 |archive-url=https://web.archive.org/web/20240122181111/http://www.bom.gov.au/info/leaflets/nino-nina.pdf |archive-date=22 January 2024 |date=February 2005 |url-status=live}}</ref> The tropical Pacific is said to be in one of three states of ENSO (also called "phases") depending on the atmospheric and oceanic conditions.<ref name="ENSO Nutshell">{{cite web |last1=L'Heureux |first1=Michelle |title=What is the El Niño–Southern Oscillation (ENSO) in a nutshell? |url=https://www.climate.gov/news-features/blogs/enso/what-el-ni%C3%B1o%E2%80%93southern-oscillation-enso-nutshell |archive-url=https://web.archive.org/web/20140524153606/http://www.climate.gov/news-features/blogs/enso/what-el-ni%C3%B1o%E2%80%93southern-oscillation-enso-nutshell |url-status=dead |archive-date=May 24, 2014 |website=ENSO Blog |publisher=Climate.gov |access-date=22 January 2024 |date=5 May 2014}}</ref> When the tropical Pacific roughly reflects the average conditions, the state of ENSO is said to be in the ''neutral'' phase. However, the tropical Pacific experiences occasional shifts away from these average conditions.<ref name="BOM About ENSO" />

If the trade winds (blowing from east to west) are weaker than average; then both the upwelling in the East Pacific, and the flow of warmer ocean surface waters towards the West Pacific, lessen. This results in a cooler West Pacific and a warmer East Pacific, leading to a shift of cloud and rain towards the East Pacific. This situation is called El Niño. The opposite occurs if trade winds are stronger than average, leading to a warmer West Pacific and a cooler East Pacific. This situation is called La Niña and is associated with increased cloudiness and rainfall over the West Pacific.<ref name="BOM About ENSO" />

===Bjerknes feedback=== The close relationship between ocean temperatures and the strength of the trade winds was first identified by Jacob Bjerknes in 1969.<ref>{{cite journal |last1=Bjerknes |first1=J |date=1969 |title=Atmospheric teleconnections from the equatorial Pacific |journal=Monthly Weather Review |volume=97 |issue=3 |pages=163–172 |doi=10.1175/1520-0493(1969)097<0163:ATFTEP>2.3.CO;2 |bibcode=1969MWRv...97..163B }}</ref> Bjerknes also hypothesized that ENSO was a positive feedback system where the associated changes in one component of the climate system (the ocean or atmosphere) tend to reinforce changes in the other.<ref name="Wang et al. 2017">{{cite book |last1=Wang |first1=Chunzai |last2=Deser |first2=Clara |last3=Yu |first3=Jin-Yi |last4=DiNezio |first4=Pedro |last5=Clement |first5=Amy |chapter=El Niño and Southern Oscillation (ENSO): A Review |editor1-last=Glynn |editor1-first=Peter W. |editor2-last=Manzello |editor2-first=Derek P. |editor3-last=Enochs |editor3-first=Ian C. |title=Coral Reefs of the Eastern Tropical Pacific |series=Coral Reefs of the World |date=2017 |volume=8 |pages=85–106 |doi=10.1007/978-94-017-7499-4_4 |chapter-url=https://www.ess.uci.edu/~yu/PDF/Wang.et%20al.Ch4.2016.pdf |access-date=22 January 2024 |publisher=Springer |isbn=978-94-017-7498-7}}</ref>{{rp|86}} This process is known as ''Bjerknes feedback''.<ref name="Rise El Nino">{{cite web |last1=L'Heureux |first1=Michelle |title=The Rise of El Niño and La Niña |publisher=NOAA |url=https://www.climate.gov/news-features/blogs/enso/rise-el-ni%C3%B1o-and-la-ni%C3%B1a |archive-url=https://archive.today/20240924065235/https://www.climate.gov/news-features/blogs/enso/rise-el-ni%C3%B1o-and-la-ni%C3%B1a |url-status=dead |archive-date=September 24, 2024 |website=ENSO Blog |access-date=22 January 2024 |date=23 October 2020}}</ref> As an example, during the growth of El Niño, the reduced contrast in ocean temperatures across the Pacific results in weaker trade winds, further reinforcing the El Niño state.

Although these associated changes in the ocean and atmosphere often occur together, the state of the atmosphere may resemble a different ENSO phase than the state of the ocean or vice versa.<ref name="ENSO Nutshell" /> Because their states are closely linked, the variations of ENSO may arise from changes in both the ocean and atmosphere and not necessarily from an initial change of exclusively one or the other.<ref name="What El Nino Scripps">{{cite web |last1=Fox |first1=Alex |title=What is El Niño? |url=https://scripps.ucsd.edu/news/what-el-nino |website=Scripps Institution of Oceanography |publisher=University of California–San Diego |access-date=22 January 2024 |location=San Diego, California |date=5 October 2023}}</ref><ref name="Rise El Nino" /> Conceptual models explaining how ENSO operates generally accept the Bjerknes feedback hypothesis. However, ENSO would perpetually remain in one phase if Bjerknes feedback were the only process occurring.<ref name="Wang et al. 2017" />{{rp|88}}

Several theories have been proposed to explain how ENSO can change from one state to the next, despite the positive feedback.<ref name="Wang 2018">{{cite journal |last1=Wang |first1=Chunzai |title=A review of ENSO theories |journal=National Science Review |date=1 November 2018 |volume=5 |issue=6 |pages=813–825 |doi=10.1093/nsr/nwy104|doi-access=free}}</ref> These explanations broadly fall under two categories.<ref name="Yang et al. 2018">{{cite journal |last1=Yang |first1=Song |last2=Li |first2=Zhenning |last3=Yu |first3=Jin-Yi |last4=Hu |first4=Xiaoming |last5=Dong |first5=Wenjie |last6=He |first6=Shan |title=El Niño–Southern Oscillation and its impact in the changing climate |journal=National Science Review |date=1 November 2018 |volume=5 |issue=6 |pages=840–857 |doi=10.1093/nsr/nwy046|doi-access=free }}</ref> In one view, the Bjerknes feedback naturally triggers negative feedbacks{{clarify|What negative feedbacks?|date=January 2024}} that end and reverse the abnormal state of the tropical Pacific. This perspective implies that the processes that lead to El Niño and La Niña also eventually bring about their end, making ENSO a self-sustaining{{clarify|self-sustaining or self-limiting?|date=January 2024}} process.<ref name="Wang et al. 2017" />{{rp|88}} Other theories view the state of ENSO as being changed by irregular and external phenomena such as the Madden–Julian oscillation, tropical instability waves, and westerly wind bursts.<ref name="Wang et al. 2017" />{{rp|90}}

===Walker circulation=== {{main|Walker circulation}} Bjerknes proposed that ENSO was due to an east–west overturning atmospheric cell above the Pacific, which he named the Walker Circulation after Gilbert Walker who discovered the Southern Oscillation during the early twentieth century. Bjerknes proposed that the strength of the cell depended on the east–west temperature gradient along the Equator, the rising branch of the cell being associated with high sea temperatures, convection and rainfall in the western Pacific, while the downward branch occurs over cooler sea surface temperatures in the central and eastern Pacific.<ref name="Trenberth 2022">{{Cite book |last=Trenberth |first=Kevin |title=Chapter 12: El Niño. In: The changing flow of energy through the climate system |date=2022 |publisher=Cambridge University Press |isbn=978-1-108-97903-0 |location=Cambridge New York, NY Port Melbourne}}</ref>

During the growth of El Niños, the increased sea surface temperature in the east reduces the east west temperature gradient and reduces the strength of the overturning. This in turn reduces the easterly winds at the ocean surface, reduces the upwelling of cold water and eventually results in even warmer sea surface temperatures in the eastern Pacific.<ref name="Trenberth 2022"/>

=== Southern Oscillation === {{multiple image | total_width = 300 | direction = vertical | image1 = Fig4a ENSOindices Nino3.4only large.png | caption1 = The regions where the air pressure are measured and compared to generate the Southern Oscillation Index | image2 = Soi-map.png | caption2 = The Southern Oscillation Index correlated with mean sea level pressure. }}

The Southern Oscillation is the atmospheric component of ENSO. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti (in the Pacific) and Darwin, Australia (on the Indian Ocean).<ref name="Aus">{{cite web |date=2002-04-03 |title=Climate glossary — Southern Oscilliation Index (SOI) |url=http://www.bom.gov.au/climate/glossary/soi.shtml |access-date=2009-12-31 |publisher=Bureau of Meteorology (Australia) |archive-date=2017-12-26 |archive-url=https://web.archive.org/web/20171226231406/http://www.bom.gov.au/climate/glossary/soi.shtml |url-status=live}}</ref>

El Niño episodes have negative SOI, meaning there is lower pressure over Tahiti and higher pressure in Darwin. La Niña episodes on the other hand have positive SOI, meaning there is higher pressure in Tahiti and lower in Darwin.

Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water, in part because of deep convection over the warm water. El Niño episodes are defined as sustained warming of the central and eastern tropical Pacific Ocean, thus resulting in a decrease in the strength of the Pacific trade winds, and a reduction in rainfall over eastern and northern Australia. La Niña episodes are defined as sustained cooling of the central and eastern tropical Pacific Ocean, thus resulting in an increase in the strength of the Pacific trade winds, and the opposite effects in Australia when compared to El Niño.

Although the Southern Oscillation Index has a long station record going back to the 1800s, its reliability is limited due to the latitudes of both Darwin and Tahiti being well south of the Equator, so that the surface air pressure at both locations is less directly related to ENSO.<ref name="EQSOI">{{cite web |last1=Barnston |first1=Anthony |date=2015-01-29 |title=Why are there so many ENSO indexes, instead of just one? |url=https://www.climate.gov/news-features/blogs/enso/why-are-there-so-many-enso-indexes-instead-just-one |access-date=2015-08-14 |publisher=NOAA |archive-date=2015-09-05 |archive-url=https://web.archive.org/web/20150905133507/https://www.climate.gov/news-features/blogs/enso/why-are-there-so-many-enso-indexes-instead-just-one |url-status=dead}}</ref> To overcome this effect, a new index was created, named the Equatorial Southern Oscillation Index (EQSOI).<ref name="EQSOI" /><ref name="Indexes">{{cite web |author=International Research Institute for Climate and Society |title=Southern Oscillation Index (SOI) and Equatorial SOI |url=http://iridl.ldeo.columbia.edu/maproom/ENSO/Time_Series/Equatorial_SOI.html |access-date=2015-08-14 |publisher=Columbia University |archive-date=2015-11-17 |archive-url=https://web.archive.org/web/20151117025947/http://iridl.ldeo.columbia.edu/maproom/ENSO/Time_Series/Equatorial_SOI.html |url-status=live}}</ref> To generate this index, two new regions, centered on the Equator, were defined. The western region is located over Indonesia and the eastern one over the equatorial Pacific, close to the South American coast.<ref name="EQSOI" /> However, data on EQSOI goes back only to 1949.<ref name="EQSOI" />

Sea surface height (SSH) changes up or down by several centimeters in Pacific equatorial region with the ENSO: El Niño causes a positive SSH anomaly (raised sea level) because of thermal expansion while La Niña causes a negative SSH anomaly (lowered sea level) via contraction.<ref>{{Cite web |title=El Nino |url=https://eospso.nasa.gov/sites/default/files/publications/ElNino-LaNina_508.pdf |access-date=1 Nov 2025 |website=eospso.nasa.gov |date=27 June 2023 }}</ref>

== Three phases of sea surface temperature == The El Niño–Southern Oscillation is a single climate phenomenon that quasi-periodically fluctuates between three phases: Neutral, La Niña or El Niño.<ref name="ENSO:Nutshell" /> La Niña and El Niño are opposite phases, which require certain changes to take place in both the ocean and the atmosphere before an event is declared.<ref name="ENSO:Nutshell" />

The cool phase of ENSO is La Niña, with SST in the eastern Pacific below average, and air pressure high in the eastern Pacific and low in the western Pacific. The ENSO cycle, including both El Niño and La Niña, causes global changes in temperature and rainfall.<ref name="CPC ENSO2">{{cite web |author=Climate Prediction Center |author-link=Climate Prediction Center |date=19 December 2005 |title=Frequently Asked Questions about El Niño and La Niña |url=http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#DIFFER |url-status=live |archive-url=https://web.archive.org/web/20090827143632/http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#DIFFER |archive-date=27 August 2009 |access-date=17 July 2009 |publisher=National Centers for Environmental Prediction |language=en-US}}</ref><ref>{{cite book |author1=Sergey K. Gulev |title=Climate Change 2021: The Physical Science Basis. The contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |author2=Peter W. Thorne |author3=Jinho Ahn |author4=Frank J. Dentener |author5=Catia M. Domingues |author6=Sebastian Gerland |author7=Daoyi Gong |author8=Darrell S. Kaufman |author9=Hyacinth C. Nnamchi |publisher=Cambridge University Press |editor1=Valérie Masson-Delmotte |location=Cambridge, UK |chapter=Changing state of the climate system |editor2=Panmao Zhai |editor3=Anna Pirani |editor4=Sarah L. Connors |editor5=C. Péan |editor6=Sophie Berger |editor7=Nada Caud |editor8=Y. Chen |editor9=Leah Goldfarb |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_02.pdf |editor10=Melissa I. Gomis |editor11=Mengtian Huang |editor12=Katherine Leitzell |editor13=Elisabeth Lonnoy |editor14=J. B. Robin Matthews |editor15=Thomas K. Maycock |editor16=Tim Waterfield |editor17=Özge Yelekçi |editor18=R. Yu |editor19=Botao Zhou |author10=Johannes Quaas |author11=Juan Antonio Rivera |author12=Shubha Sathyendranath |author13=Sharon L. Smith |author13-link=Sharon L. Smith |author14=Blair Trewin |author15=Karina von Shuckmann |author16=Russell S. Vose |access-date=2024-01-18 |archive-date=2022-03-02 |archive-url=https://web.archive.org/web/20220302154841/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_02.pdf |url-status=live}}</ref>

{{multiple image | align = center | direction = horizontal | image1 = ENSO - normal.svg | caption1 = Neutral phase: Equatorial winds gather warm water pool toward the west. Warm pool in the west drives deep atmospheric convection. In the east local winds cause nutrient-rich cold water to upwell at the Equator and along the South American coast. | image2 = ENSO - El Niño.svg | caption2 = El Niño phase: Warm water pool approaches the South American coast. The absence of cold upwelling increases warming. Warm water and atmospheric convection move eastwards. In strong El Niños the deeper thermocline off South America means upwelled water is warm and nutrient poor. | image3 = ENSO - La Niña.svg | caption3 = La Niña phase: Warm water is farther west than usual. }}

=== Neutral phase === If the temperature variation from climatology is within 0.5 °C (0.9 °F), ENSO conditions are described as neutral. Neutral conditions are the transition between warm and cold phases of ENSO. Sea surface temperatures (by definition), tropical precipitation, and wind patterns are near average conditions during this phase.<ref>{{cite web|url=http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#NEUTRAL|title=Frequently Asked Questions about El Niño and La Niña|author=Climate Prediction Center Internet Team|publisher=National Oceanic and Atmospheric Administration|date=2012-04-26|access-date=2014-06-30|archive-date=2020-05-02|archive-url=https://web.archive.org/web/20200502083245/https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#NEUTRAL|url-status=live}}</ref> Close to half of all years are within neutral periods.<ref>{{cite web|url=http://iri.columbia.edu/our-expertise/climate/forecasts/enso/archive/200203/technical.html|title=More Technical ENSO Comment|author=International Research Institute for Climate and Society|publisher=Columbia University|date=February 2002|access-date=2014-06-30|archive-date=2014-07-14|archive-url=https://web.archive.org/web/20140714125528/http://iri.columbia.edu/our-expertise/climate/forecasts/enso/archive/200203/technical.html|url-status=live}}</ref> During the neutral ENSO phase, other climate anomalies/patterns such as the sign of the North Atlantic Oscillation or the Pacific–North American teleconnection pattern exert more influence.<ref>{{cite web|url=http://www.nc-climate.ncsu.edu/climate/patterns/ENSO.html|title=Global Patterns – El Niño-Southern Oscillation (ENSO)|author=State Climate Office of North Carolina|publisher=North Carolina State University|access-date=2014-06-30|archive-url=https://web.archive.org/web/20140627031846/http://www.nc-climate.ncsu.edu/climate/patterns/ENSO.html|archive-date=2014-06-27}}</ref>

=== El Niño phase === [[File:Sst_9798_animated.gif|thumb|250x250px|A loop of the 1997–98 El Niño event showing extreme sea surface temperature (SST) anomalies in the east tropical Pacific]] El Niño conditions are established when the Walker circulation weakens or reverses and the Hadley circulation strengthens,{{citation needed|date=January 2024}}{{clarify|this implies that the walker circulation causes El Nino. That is a strong claim and must be strongly supported by a good source, please specify which source makes this claim, preferably with a quote to that effect.|date=January 2024}} leading to the development of a band of warm ocean water in the central and east-central equatorial Pacific (approximately between the International Date Line and 120°W), including the area off the west coast of South America,<ref>{{cite web |title=Australian Climate Influences: El Niño |url=http://www.bom.gov.au/watl/about-weather-and-climate/australian-climate-influences.shtml |access-date=4 April 2016 |publisher=Australian Bureau of Meteorology |archive-date=24 March 2016 |archive-url=https://web.archive.org/web/20160324105254/http://www.bom.gov.au/watl/about-weather-and-climate/australian-climate-influences.shtml |url-status=live}}</ref><ref name="ENSO:Nutshell" /> as upwelling of cold water occurs less or not at all offshore.<ref name="Climate Change 2007" />

This warming causes a shift in the atmospheric circulation, leading to higher air pressure in the western Pacific and lower in the eastern Pacific,<ref>{{cite web|url=http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3-7.html|title=Climate Change 2007: Working Group I: The Physical Science Basis: 3.7 Changes in the Tropics and Subtropics, and the Monsoons|author=Intergovernmental Panel on Climate Change|date=2007|access-date=2014-07-01|publisher=World Meteorological Organization|archive-url=https://web.archive.org/web/20140714132803/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3-7.html|archive-date=2014-07-14}}</ref> with rainfall reducing over Indonesia, India and northern Australia, while rainfall and tropical cyclone formation increases over the tropical Pacific Ocean.<ref name="What is an El Niño?3">{{cite web |title=What is El Niño and what might it mean for Australia? |url=http://www.bom.gov.au/climate/updates/articles/a008-el-nino-and-australia.shtml |url-status=live |archive-url=https://web.archive.org/web/20160318103614/http://www.bom.gov.au/climate/updates/articles/a008-el-nino-and-australia.shtml |archive-date=18 March 2016 |access-date=10 April 2016 |publisher=Australian Bureau of Meteorology}}</ref> The low-level surface trade winds, which normally blow from east to west along the equator, either weaken or start blowing from the other direction.<ref name="ENSO:Nutshell" />

El Niño phases are known to happen at irregular intervals of two to seven years, and lasts nine months to two years.<ref name="autogenerated2005">{{cite web |author=Climate Prediction Center |date=19 December 2005 |title=ENSO FAQ: How often do El Niño and La Niña typically occur? |url=http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#HOWOFTEN |archive-url=https://web.archive.org/web/20090827143632/http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#HOWOFTEN |archive-date=27 August 2009 |access-date=26 July 2009 |publisher=National Centers for Environmental Prediction}}</ref> The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Niño "conditions"; when its duration is longer, it is classified as an El Niño "episode".<ref name="administration1">{{cite web |author=National Climatic Data Center |author-link=National Climatic Data Center |date=June 2009 |title=El Niño / Southern Oscillation (ENSO) June 2009 |url=http://www.ncdc.noaa.gov/oa/climate/research/enso/?year=2009&month=6&submitted=true |access-date=26 July 2009 |publisher=National Oceanic and Atmospheric Administration}}</ref>

<div class="center"><timeline> ImageSize = width:1050 height:70 PlotArea = left:50 bottom:20 width:700 height:40 Period = from:1900 till:2030 DateFormat = yyyy TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:5 start:1900 PlotData = bar:elniño width:30 color:red mark:(line,white) from:1902 till:1903 from:1905 till:1906 from:1911 till:1912 from:1913 till:1915 from:1919 till:1920 from:1925 till:1926 from:1940 till:1942 from:1946 till:1947 from:1951 till:1952 from:1953 till:1954 from:1957 till:1958 from:1963 till:1964 from:1965 till:1966 from:1968 till:1969 from:1969 till:1970 from:1972 till:1973 from:1976 till:1977 from:1977 till:1978 from:1979 till:1980 from:1982 till:1983 from:1986 till:1988 from:1991 till:1992 from:1992 till:1993 from:1994 till:1995 from:1997 till:1998 from:2002 till:2003 from:2004 till:2005 from:2006 till:2007 from:2009 till:2010 from:2015 till:2016 from:2018 till:2019 from:2023 till:2024

</timeline></div>{{center|Timeline of El Niño episodes between 1900 and 2024.<ref name="ENSO ONIs">{{cite web|title=Historical El Niño/La Niña episodes (1950–present)|url=http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml|publisher=United States Climate Prediction Center|date=1 February 2019|access-date=15 March 2019|archive-date=29 November 2014|archive-url=https://web.archive.org/web/20141129084421/http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml|url-status=live}}</ref><ref name="BoM El Nino">{{cite web |url=http://www.bom.gov.au/climate/enso/enlist/index.shtml |title=El Niño - Detailed Australian Analysis |access-date=3 April 2016 |publisher=Australian Bureau of Meteorology |archive-date=3 May 2021 |archive-url=https://web.archive.org/web/20210503211501/http://www.bom.gov.au/climate/enso/enlist/index.shtml |url-status=live}}</ref>}}It is thought that there have been at least 30 El Niño events between 1900 and 2024, with the 1982–83, 1997–98 and 2014–16 events among the strongest on record.<ref>{{cite web |title=El Niño in Australia |url=http://www.bom.gov.au/climate/enso/images/El-Nino-in-Australia.pdf |access-date=1 March 2022 |website=Bom.gov.au |archive-date=7 March 2022 |archive-url=https://web.archive.org/web/20220307003732/http://www.bom.gov.au/climate/enso/images/El-Nino-in-Australia.pdf |url-status=live}}</ref> Since 2000, El Niño events have been observed in 2002–03, 2004–05, 2006–07, 2009–10, 2014–16, 2018–19,<ref>{{cite web |author=Brian Donegan |date=14 March 2019 |title=El Niño Conditions Strengthen, Could Last Through Summer |url=https://weather.com/news/weather/news/2019-03-14-el-nino-conditions-strengthen-could-last-through-summer |access-date=15 March 2019 |publisher=The Weather Company |archive-date=15 March 2019 |archive-url=https://web.archive.org/web/20190315020623/https://weather.com/news/weather/news/2019-03-14-el-nino-conditions-strengthen-could-last-through-summer |url-status=live}}</ref><ref>{{cite web |date=8 August 2019 |title=El Nino is over, NOAA says |url=https://www.al.com/hurricane/2019/08/el-nino-is-over-noaa-says.html |access-date=5 September 2019 |website=Al.com |archive-date=5 September 2019 |archive-url=https://web.archive.org/web/20190905080221/https://www.al.com/hurricane/2019/08/el-nino-is-over-noaa-says.html |url-status=live}}</ref><ref>{{Cite web |date=2023-06-08 |title=Here comes El Nino: It's early, likely to be big, sloppy and add even more heat to a warming world |url=https://www.independent.co.uk/news/el-nino-ap-la-nina-national-oceanic-and-atmospheric-administration-atlantic-b2353918.html |access-date=2023-06-23 |website=The Independent |language=en |archive-date=2023-06-10 |archive-url=https://web.archive.org/web/20230610020101/https://www.independent.co.uk/news/el-nino-ap-la-nina-national-oceanic-and-atmospheric-administration-atlantic-b2353918.html |url-status=live}}</ref> and 2023–24.<ref>{{cite web |last=Henson |first=Bob |date=9 June 2023 |title=NOAA makes it official: El Niño is here |url=https://yaleclimateconnections.org/2023/06/noaa-makes-it-official-el-nino-is-here/ |access-date=11 June 2023 |publisher=Yale Climate Connections |archive-date=10 June 2023 |archive-url=https://web.archive.org/web/20230610195030/https://yaleclimateconnections.org/2023/06/noaa-makes-it-official-el-nino-is-here/ |url-status=live}}</ref><ref>{{cite web |date=9 June 2023 |title=El Niño Outlook ( June 2023 - December 2023 ) |url=https://ds.data.jma.go.jp/tcc/tcc/products/elnino/outlook.html |access-date=12 June 2023 |work=Climate Prediction Division |publisher=Japan Meteorological Agency |quote=El Niño conditions are considered to be present in the equatorial Pacific. |archive-date=2 May 2023 |archive-url=https://web.archive.org/web/20230502140814/https://ds.data.jma.go.jp/tcc/tcc/products/elnino/outlook.html |url-status=live}}</ref>

Major ENSO events were recorded in the years 1790–93, 1828, 1876–78, 1891, 1925–26, 1972–73, 1982–83, 1997–98, 2014–16, and 2023–24.<ref name="Davis20012">{{cite book |last=Davis |first=Mike |url=https://archive.org/details/latevictorianhol00dav_wbr/page/271 |title=Late Victorian Holocausts: El Niño Famines and the Making of the Third World |publisher=Verso |year=2001 |isbn=978-1-85984-739-8 |location=London |page=[https://archive.org/details/latevictorianhol00dav_wbr/page/271 271] |author-link=Mike Davis (scholar)}}</ref><ref>{{cite web |title=Very strong 1997-98 Pacific warm episode (El Niño) |url=http://www.cpc.ncep.noaa.gov/products/assessments/assess_97/enso.html |access-date=28 July 2015 |archive-date=3 May 2021 |archive-url=https://web.archive.org/web/20210503211516/https://www.cpc.ncep.noaa.gov/products/assessments/assess_97/enso.html |url-status=live}}</ref><ref>{{cite news |last=Sutherland |first=Scott |date=16 February 2017 |title=La Niña calls it quits. Is El Niño paying us a return visit? |url=https://www.theweathernetwork.com/us/news/articles/la-nina-calls-it-quits-is-el-nino-paying-us-a-return-visit/79424 |access-date=17 February 2017 |work=The Weather Network |archive-date=18 February 2017 |archive-url=https://web.archive.org/web/20170218143231/https://www.theweathernetwork.com/us/news/articles/la-nina-calls-it-quits-is-el-nino-paying-us-a-return-visit/79424 |url-status=dead}}</ref> During strong El Niño episodes, a secondary peak in sea surface temperature across the far eastern equatorial Pacific Ocean sometimes follows the initial peak.<ref>{{cite journal |last1=Kim |first1=WonMoo |author2=Wenju Cai |year=2013 |title=Second peak in the far eastern Pacific sea surface temperature anomaly following strong El Niño events |journal=Geophys. Res. Lett. |volume=40 |issue=17 |pages=4751–4755 |bibcode=2013GeoRL..40.4751K |doi=10.1002/grl.50697 |s2cid=129885922 |doi-access=free}}</ref>

=== La Niña phase === {{redirect|La Niña|the ship|Niña (ship)|other uses|Niña (disambiguation)}} [[File:Sea_Surface_Temperature_-_November_2007.jpg|thumb|300x300px|Sea surface temperature anomalies in November 2007, showing La Niña conditions]] An especially strong Walker circulation causes La Niña, which is considered to be the cold oceanic and positive atmospheric phase of the broader El Niño–Southern Oscillation (ENSO) weather phenomenon, as well as the opposite of {{nowrap|El Niño}} weather pattern,<ref name="USNOAA-NOS-NinoNina" /> where sea surface temperature across the eastern equatorial part of the central Pacific Ocean will be lower than normal by 3��5 °C (5.4–9 °F). The phenomenon occurs as strong winds blow warm water at the ocean's surface away from South America, across the Pacific Ocean towards Indonesia.<ref name="USNOAA-NOS-NinoNina" /> As this warm water moves west, cold water from the deep sea rises to the surface near South America.<ref name="USNOAA-NOS-NinoNina" />

The movement of so much heat across a quarter of the planet, and particularly in the form of temperature at the ocean surface, can have a significant effect on weather across the entire planet. Tropical instability waves visible on sea surface temperature maps, showing a tongue of colder water, are often present during neutral or La Niña conditions.<ref>{{cite web |title=August 2016 ENSO update;Wavy Gravy |url=https://www.climate.gov/news-features/blogs/enso/august-2016-enso-update-wavy-gravy |access-date=16 October 2021 |publisher=Climate.gov.uk |archive-date=11 December 2022 |archive-url=https://web.archive.org/web/20221211121517/https://www.climate.gov/news-features/blogs/enso/august-2016-enso-update-wavy-gravy |url-status=dead}}</ref>

La Niña is a complex weather pattern that occurs every few years,<ref name="USNOAA-NOS-NinoNina" /> often persisting for longer than five months. El Niño and La Niña can be indicators of weather changes across the globe. Atlantic and Pacific hurricanes can have different characteristics due to lower or higher wind shear and cooler or warmer sea surface temperatures.

<div class="center"><timeline> ImageSize = width:800 height:70 PlotArea = left:50 bottom:20 width:700 height:40 Period = from:1900 till:2025 DateFormat = yyyy TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:5 start:1900 PlotData = bar:laniña width:30 color:blue mark:(line,white) from:1903 till:1904 from:1906 till:1907 from:1909 till:1911 from:1916 till:1918 from:1924 till:1925 from:1928 till:1930 from:1938 till:1939 from:1942 till:1943 from:1949 till:1951 from:1954 till:1957 from:1964 till:1965 from:1967 till:1968 from:1970 till:1972 from:1973 till:1974 from:1974 till:1976 from:1983 till:1984 from:1984 till:1985 from:1988 till:1989 from:1995 till:1996 from:1998 till:2001 from:2005 till:2006 from:2007 till:2008 from:2008 till:2009 from:2010 till:2012 from:2016 till:2017 from:2017 till:2018 from:2020 till:2023 </timeline>A timeline of all La Niña episodes between 1900 and 2023.<ref name="Cold & Warm Episodes">{{cite report |url=https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml |title=Cold and warm episodes by season |publisher=National Oceanic and Atmospheric Administration |access-date=September 11, 2020 |department=Climate Prediction Center |archive-date=September 26, 2023 |archive-url=https://web.archive.org/web/20230926120733/https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml |url-status=live}}</ref><ref>{{cite report |url=http://www.bom.gov.au/climate/enso/lnlist/index.shtml |title=La Niña – Detailed Australian analysis |publisher=Australian Bureau of Meteorology |access-date=3 April 2016 |archive-date=28 December 2017 |archive-url=https://web.archive.org/web/20171228165657/http://www.bom.gov.au/climate/enso/lnlist/index.shtml |url-status=live}}</ref> Note that each forecast agency has a different criteria for what constitutes a La Niña event, which is tailored to their specific interests. </div>

La Niña events have been observed for hundreds of years, and occurred on a regular basis during the early parts of both the 17th and 19th centuries.<ref>{{cite journal |last1=Druffel |first1=Ellen R. M. |last2=Griffin |first2=Sheila |last3=Vetter |first3=Desiree |last4=Dunbar |first4=Robert B. |last5=Mucciarone |first5=David M. |date=16 March 2015 |title=Identification of frequent La Niña events during the early 1800s in the east equatorial Pacific |url=https://escholarship.org/uc/item/3xt6x5fb |journal=Geophysical Research Letters |volume=42 |issue=5 |pages=1512–1519 |bibcode=2015GeoRL..42.1512D |doi=10.1002/2014GL062997 |s2cid=129644802 |access-date=26 February 2022 |doi-access=free |archive-date=15 January 2023 |archive-url=https://web.archive.org/web/20230115072041/https://escholarship.org/uc/item/3xt6x5fb |url-status=live}}</ref> Since the start of the 20th century, La Niña events have occurred during the following years:{{refn|name=La-Nina-declaration-announcement-authorities}} {{plainlist}} {{columns-list|# 1903–04 # 1906–07 # 1909–11 # 1916–18 # 1924–25 # 1928–30 # 1938–39 # 1942–43 # 1949–51 # 1954–57 # 1964–65 # 1970–72 # 1973–76 # 1983–85 # 1988–89 # 1995–96 # 1998–2001 # 2005–06 # 2007–08 # 2008–09 # 2010–12 # 2016 # 2017–18 # 2020–23|colwidth=10em|style=margin: 0 1.6em;}} {{endplainlist}}

=== Transitional phases === Transitional phases at the onset or departure of El Niño or La Niña can also be important factors on global weather by affecting teleconnections. Significant episodes, known as Trans-Niño, are measured by the Trans-Niño index (TNI).<ref name="TNI">{{cite journal |last1=Trenberth |first1=Kevin E. |last2=Stepaniak |first2=David P. |title=Indices of El Niño Evolution |journal=Journal of Climate |date=15 April 2001 |volume=14 |issue=8 |pages=1697–1701 |doi=10.1175/1520-0442(2001)014<1697:LIOENO>2.0.CO;2 |bibcode=2001JCli...14.1697T |url=https://zenodo.org/record/1234669 |doi-access=free |access-date=27 August 2019 |archive-date=23 December 2019 |archive-url=https://web.archive.org/web/20191223101002/https://zenodo.org/record/1234669 |url-status=live}}</ref> Examples of affected short-time climate in North America include precipitation in the Northwest US<ref>{{cite journal |last1=Kennedy |first1=Adam M. |author2=D. C. Garen |author3=R. W. Koch |title=The association between climate teleconnection indices and Upper Klamath seasonal streamflow: Trans-Niño Index |journal=Hydrol. Process. |volume=23 |issue=7 |pages=973–84 |year=2009 |doi=10.1002/hyp.7200 |bibcode=2009HyPr...23..973K|citeseerx=10.1.1.177.2614 |s2cid=16514830}}</ref> and intense tornado activity in the contiguous US.<ref>{{cite journal |last1=Lee |first1=Sang-Ki |author2=R. Atlas |author3=D. Enfield |author4=C. Wang |author5=H. Liu |title=Is there an optimal ENSO pattern that enhances large-scale atmospheric processes conducive to tornado outbreaks in the U.S? |journal=J. Climate |volume=26 |issue=5 |pages=1626–1642 |year=2013 |doi=10.1175/JCLI-D-12-00128.1 |bibcode=2013JCli...26.1626L|doi-access=free}}</ref>

==Variations==

===ENSO Modoki=== thumb|300px|A map showing Niño/Niña 1 to 4 regions, 3 and 4 being west and far west and much larger than 1 and 2. A coastal Peruvian/Ecuadorian zone differs subtly from north–south The first ENSO pattern to be recognised, called Eastern Pacific (EP) ENSO, to distinguish it from others,<ref name="Yu2009">{{cite journal|journal=J. Climate|author1=Kao, Hsun-Ying |author2=Jin-Yi Yu|title=Contrasting Eastern-Pacific and Central-Pacific Types of ENSO|volume=22|pages=615–632|year=2009|doi=10.1175/2008JCLI2309.1|issue=3|bibcode=2009JCli...22..615K|citeseerx=10.1.1.467.457}}</ref> involves temperature anomalies in the eastern Pacific. In the 1990s and 2000s, variations of ENSO conditions were observed, in which the usual place of the temperature anomaly (Niño 1 and 2) is not affected, but an anomaly also arises in the central Pacific (Niño 3.4).<ref name="Larkin2007">{{Cite journal |doi=10.1029/2005GL022738 |title=On the definition of El Niño and associated seasonal average U.S. Weather anomalies |year=2005 |last1=Larkin |first1=N. K. |journal=Geophysical Research Letters |volume=32 |issue=13 |pages=L13705 |last2=Harrison |first2=D. E. |bibcode=2005GeoRL..3213705L |doi-access=free}}</ref> The phenomenon is called Central Pacific (CP) ENSO,<ref name="Yu2009" /> "dateline" ENSO (because the anomaly arises near the dateline), or ENSO "Modoki" (Modoki is Japanese for "similar, but different").<ref name="link.springer.com">{{cite journal |title=Different types of La Niña events and different responses of the tropical atmosphere |journal=Chinese Science Bulletin |volume=58 |issue=3 |pages=406–415 |year=2012 |doi=10.1007/s11434-012-5423-5 |author1=Yuan Yuan |author2=HongMing Yan|bibcode=2013ChSBu..58..406Y |doi-access=free}}</ref><ref name="auto">{{cite journal |last1=Cai |first1=W. |last2=Cowan |first2=T. |title=La Niña Modoki impacts Australia autumn rainfall variability |journal=Geophysical Research Letters |date=17 June 2009 |volume=36 |issue=12 |pages=L12805 |doi=10.1029/2009GL037885 |bibcode=2009GeoRL..3612805C |doi-access=free}}</ref> There are variations of ENSO additional to the EP and CP types, and some scientists argue that ENSO exists as a continuum, often with hybrid types.<ref>{{cite journal |last1=Johnson |first1=Nathaniel C. |title=How Many ENSO Flavors Can We Distinguish? |journal=Journal of Climate |date=1 July 2013 |volume=26 |issue=13 |pages=4816–4827 |doi=10.1175/JCLI-D-12-00649.1 |bibcode=2013JCli...26.4816J |s2cid=55416945 |doi-access=free}}</ref>

The effects of the CP ENSO are different from those of the EP ENSO. The El Niño Modoki is associated with more hurricanes more frequently making landfall in the Atlantic.<ref name="Kim2009">{{cite journal |last1=Kim |first1=Hye-Mi |last2=Webster |first2=Peter J. |last3=Curry |first3=Judith A. |title=Impact of Shifting Patterns of Pacific Ocean Warming on North Atlantic Tropical Cyclones |journal=Science |date=3 July 2009 |volume=325 |issue=5936 |pages=77–80 |doi=10.1126/science.1174062 |pmid=19574388 |bibcode=2009Sci...325...77K |s2cid=13250045}}</ref> La Niña Modoki leads to a rainfall increase over northwestern Australia and northern Murray–Darling basin, rather than over the eastern portion of the country as in a conventional EP La Niña.<ref name="LNModoki">{{cite journal |last1=Cai |first1=W. |last2=Cowan |first2=T. |year=2009 |title=La Niña Modoki impacts Australia autumn rainfall variability |journal=Geophysical Research Letters |volume=36 |issue=12 |pages=L12805 |doi=10.1029/2009GL037885 |issn=0094-8276 |bibcode=2009GeoRL..3612805C|doi-access=free}}</ref> Also, La Niña Modoki increases the frequency of cyclonic storms over Bay of Bengal, but decreases the occurrence of severe storms in the Indian Ocean overall.<ref name="LNM Indian">{{cite web |author=M R Ramesh Kumar |publisher=Society for Environmental Communications |title=El Nino, La Nina and the Indian sub-continent |url=http://www.downtoearth.org.in/content/el-nino-la-nina-and-indian-sub-continent |date=2014-04-23 |access-date=2014-07-25 |archive-date=2014-07-21 |archive-url=https://web.archive.org/web/20140721030933/http://www.downtoearth.org.in/content/el-nino-la-nina-and-indian-sub-continent |url-status=dead}}</ref>

The first recorded El Niño that originated in the central Pacific and moved toward the east was in 1986.<ref>{{cite book |author=S. George Philander |url=https://archive.org/details/ouraffairwitheln00phil |title=Our Affair with El Niño: How We Transformed an Enchanting Peruvian Current Into a Global Climate Hazard |publisher=Princeton University Press |year=2004 |isbn=978-0-691-11335-7}}</ref> Recent Central Pacific El Niños happened in 1986–87, 1991–92, 1994–95, 2002–03, 2004–05 and 2009–10.<ref>{{cite web |title=Study Finds El Niños are Growing Stronger |url=http://www.nasa.gov/topics/earth/features/elnino20100825.html |access-date=3 August 2014 |publisher=NASA |archive-date=17 November 2022 |archive-url=https://web.archive.org/web/20221117152009/https://www.nasa.gov/topics/earth/features/elnino20100825.html |url-status=dead}}</ref> There were "Modoki" events in 1957–59,<ref>{{cite journal |last1=Takahashi |first1=K. |last2=Montecinos |first2=A. |last3=Goubanova |first3=K. |last4=Dewitte |first4=B. |year=2011 |title=Reinterpreting the Canonical and Modoki El Nino |url=https://hal.archives-ouvertes.fr/hal-00994266/file/grl28063.pdf |journal=Geophysical Research Letters |volume=38 |issue=10 |pages=n/a |bibcode=2011GeoRL..3810704T |doi=10.1029/2011GL047364 |s2cid=55675672 |hdl=10533/132105 |access-date=2019-08-12 |archive-date=2019-05-03 |archive-url=https://web.archive.org/web/20190503065117/https://hal.archives-ouvertes.fr/hal-00994266/file/grl28063.pdf |url-status=live}}</ref> 1963–64, 1965–66, 1968–70, 1977–78 and 1979–80.<ref>{{cite report |url=http://www.aoml.noaa.gov/phod/docs/Rev_Manuscript_CD.pdf |title=Different Impacts of Various El Niño Events |publisher=NOAA |access-date=2024-01-18 |archive-date=2023-07-25 |archive-url=https://web.archive.org/web/20230725133830/https://www.aoml.noaa.gov/phod/docs/Rev_Manuscript_CD.pdf |url-status=live}}</ref><ref>{{cite report |url=http://iopscience.iop.org/1748-9326/8/1/014019/article |title=The Enhanced Drying Effect of Central Pacific El Niño on US Winters |publisher=IOP Science |access-date=5 February 2023 |archive-date=3 September 2015 |archive-url=https://web.archive.org/web/20150903223640/http://iopscience.iop.org/1748-9326/8/1/014019/article |url-status=live}}.</ref> Some sources say that the El Niños of 2006-07 and 2014-16 were also Central Pacific El Niños.<ref>{{cite report |title=Monitoring the Pendulum |publisher=IOP Science |doi=10.1088/1748-9326/aac53f |doi-access=free}}</ref><ref>{{cite news |title=El Nino's Bark is Worse than its Bite |url=https://www.producer.com/2015/09/el-ninos-bark-is-worse-than-its-bite |access-date=11 January 2019 |publisher=The Western Producer |archive-date=14 January 2019 |archive-url=https://web.archive.org/web/20190114102132/https://www.producer.com/2015/09/el-ninos-bark-is-worse-than-its-bite/ |url-status=live}}</ref> Recent years when La Niña Modoki events occurred include 1973–1974, 1975–1976, 1983–1984, 1988–1989, 1998–1999, 2000–2001, 2008–2009, 2010–2011, and 2016–2017.<ref name="LNM">{{cite journal |last1=Yuan |first1=Yuan |last2=Yan |first2=HongMing |title=Different types of La Niña events and different responses of the tropical atmosphere |journal=Chinese Science Bulletin |volume=58 |issue=3 |pages=406–415 |year=2012 |bibcode=2013ChSBu..58..406Y |doi=10.1007/s11434-012-5423-5 |doi-access=free}}</ref><ref name="ENSO SA">{{cite web |last1=Tedeschi |first1=Renata G. |last2=Cavalcanti |first2=Iracema F. A. |title=Influência dos ENOS Canônico e Modoki na precipitação da América do Sul |url=http://cbmet2010.web437.uni5.net/anais/artigos/354_23512.pdf |url-status=dead |publisher=Instituto Nacional de Pesquisas Espaciais/Centro de Previsão de Tempo e Estudos Climáticos |date=23 April 2014 |access-date=27 September 2014 |archive-url=https://web.archive.org/web/20141023061148/http://cbmet2010.web437.uni5.net/anais/artigos/354_23512.pdf |archive-date=23 October 2014 |language=pt |df=dmy-all}}</ref><ref name=Modoki-authorities/>

The recent discovery of ENSO Modoki has some scientists believing it to be linked to global warming.<ref name="Yeh2009">{{cite journal |last1=Yeh |first1=Sang-Wook |last2=Kug |first2=Jong-Seong |last3=Dewitte |first3=Boris |last4=Kwon |first4=Min-Ho |last5=Kirtman |first5=Ben P. |last6=Jin |first6=Fei-Fei |date=September 2009 |title=El Niño in a changing climate |journal=Nature |volume=461 |issue=7263 |pages=511–4 |doi=10.1038/nature08316 |bibcode=2009Natur.461..511Y |pmid=19779449|s2cid=4423723}}</ref> However, comprehensive satellite data go back only to 1979. More research must be done to find the correlation and study past El Niño episodes. More generally, there is no scientific consensus on how/if climate change might affect ENSO.<ref name="Collins102" />

There is also a scientific debate on the very existence of this "new" ENSO. A number of studies dispute the reality of this statistical distinction or its increasing occurrence, or both, either arguing the reliable record is too short to detect such a distinction,<ref>{{cite journal |last1=Nicholls |first1=N. |title=Recent trends in the seasonal and temporal behaviour of the El Niño Southern Oscillation |journal=Geophys. Res. Lett. |volume=35 |issue=19|pages=L19703 |year=2008 |article-number=2008GL034499 |doi=10.1029/2008GL034499|bibcode=2008GeoRL..3519703N|s2cid=129372366}}</ref><ref>{{cite journal |last1=McPhaden |first1=M.J. |first2=T. |last2=Lee |first3=D. |last3=McClurg |title=El Niño and its relationship to changing background conditions in the tropical Pacific Ocean |journal=Geophys. Res. Lett. |volume=38 |issue=15|pages=L15709 |year=2011 |doi=10.1029/2011GL048275 |bibcode=2011GeoRL..3815709M|s2cid=9168925 |doi-access=free}}</ref> finding no distinction or trend using other statistical approaches,<ref>{{cite journal |last1=Giese |first1=B.S. |first2=S. |last2=Ray |s2cid=85504316 |title=El Niño variability in simple ocean data assimilation (SODA), 1871–2008 |journal=J. Geophys. Res. |volume=116 |issue=C2|pages=C02024 |year=2011 |doi=10.1029/2010JC006695|bibcode=2011JGRC..116.2024G|doi-access=free}}</ref><ref>{{cite journal |last1=Newman |first1=M. |first2=S.-I. |last2=Shin |first3=M.A. |last3=Alexander |title=Natural variation in ENSO flavors |journal=Geophys. Res. Lett. |volume=38 |issue=14 |pages=L14705 |year=2011 |doi=10.1029/2011GL047658 |url=http://www.esrl.noaa.gov/psd/people/michael.alexander/Newman.et_al.2011_GRL.pdf |bibcode=2011GeoRL..3814705N |doi-access=free |access-date=2019-08-27 |archive-date=2020-01-24 |archive-url=https://web.archive.org/web/20200124085041/https://www.esrl.noaa.gov/psd/people/michael.alexander/Newman.et_al.2011_GRL.pdf |url-status=live}}</ref><ref>{{cite journal |last1=Yeh |first1=S.-W. |first2=B.P. |last2=Kirtman |first3=J.-S. |last3=Kug |first4=W. |last4=Park |first5=M. |last5=Latif |title=Natural variability of the central Pacific El Niño event on multi-centennial timescales |journal=Geophys. Res. Lett. |volume=38 |issue=2 |pages=L02704 |year=2011 |doi=10.1029/2010GL045886 |url=http://oceanrep.geomar.de/10452/1/2010GL045886.pdf |bibcode=2011GeoRL..38.2704Y |doi-access=free |access-date=2019-08-27 |archive-date=2019-12-03 |archive-url=https://web.archive.org/web/20191203002441/http://oceanrep.geomar.de/10452/1/2010GL045886.pdf |url-status=live}}</ref><ref>{{cite journal |title=Statistical simulations of the future 50-year statistics of cold-tongue El Niño and warm-pool El Niño |journal=Asia-Pacific J. Atmos. Sci. |volume=47 |issue=3 |pages=223–233 |year=2011 |doi=10.1007/s13143-011-0011-1 |bibcode=2011APJAS..47..223N |author1=Hanna Na |author2=Bong-Geun Jang |author3=Won-Moon Choi |author4=Kwang-Yul Kim|s2cid=120649138}}</ref><ref>{{cite journal |last1=L'Heureux |first1=M. |last2=Collins |first2=D. |last3=Hu |first3=Z.-Z. |title=Linear trends in sea surface temperature of the tropical Pacific Ocean and implications for the El Niño-Southern Oscillation |journal=Climate Dynamics |volume=40 |issue=5–6 |pages=1–14 |year=2012 |doi=10.1007/s00382-012-1331-2 |bibcode=2013ClDy...40.1223L |doi-access=free}}</ref> or that other types should be distinguished, such as standard and extreme ENSO.<ref>{{cite journal |last1=Lengaigne |first1=M. |last2=Vecchi |first2=G. |title=Contrasting the termination of moderate and extreme El Niño events in coupled general circulation models |journal=Climate Dynamics |volume=35 |issue=2–3 |pages=299–313 |year=2010 |doi=10.1007/s00382-009-0562-3 |url=https://hal.archives-ouvertes.fr/hal-00758929 |bibcode=2010ClDy...35..299L |s2cid=14423113 |access-date=2019-01-10 |archive-date=2019-12-03 |archive-url=https://web.archive.org/web/20191203002627/https://hal.archives-ouvertes.fr/hal-00758929 |url-status=live|url-access=subscription }}</ref><ref>{{cite journal |last1=Takahashi |first1=K. |first2=A. |last2=Montecinos |first3=K. |last3=Goubanova |first4=B. |last4=Dewitte |title=ENSO regimes: Reinterpreting the canonical and Modoki El Niño |journal=Geophys. Res. Lett. |volume=38 |issue=10 |pages=L10704 |year=2011 |doi=10.1029/2011GL047364 |url=https://hal.archives-ouvertes.fr/hal-00994266/file/grl28063.pdf |bibcode=2011GeoRL..3810704T |hdl=10533/132105 |s2cid=55675672 |access-date=2019-08-12 |archive-date=2019-05-03 |archive-url=https://web.archive.org/web/20190503065117/https://hal.archives-ouvertes.fr/hal-00994266/file/grl28063.pdf |url-status=live}}</ref>

Likewise, following the asymmetric nature of the warm and cold phases of ENSO, some studies could not identify similar variations for La Niña, both in observations and in the climate models,<ref>{{cite journal |last1=Kug |first1=J.-S. |first2=F.-F. |last2=Jin |first3=S.-I. |last3=An |s2cid=6708133 |title=Two types of El Niño events: Cold Tongue El Niño and Warm Pool El Niño |journal=J. Climate |volume=22 |issue=6 |pages=1499–1515 |year=2009 |doi=10.1175/2008JCLI2624.1 |bibcode=2009JCli...22.1499K|doi-access=free}}</ref> but some sources could identify variations on La Niña with cooler waters on central Pacific and average or warmer water temperatures on both eastern and western Pacific, also showing eastern Pacific Ocean currents going to the opposite direction compared to the currents in traditional La Niñas.<ref name="link.springer.com" /><ref name="auto" /><ref>{{cite journal |last1=Shinoda |first1=Toshiaki |first2=Harley E. |last2=Hurlburt |first3=E. Joseph |last3=Metzger |title=Anomalous tropical ocean circulation associated with La Niña Modoki |journal=Journal of Geophysical Research: Oceans |volume=115 |issue=12 |pages=C12001 |year=2011 |doi=10.1029/2011JC007304 |bibcode=2011JGRC..11612001S|doi-access=free}}</ref>

===ENSO Costero=== Coined by the Peruvian {{ill|Comité Multisectorial Encargado del Estudio Nacional del Fenómeno El Niño|es}} (ENFEN),<ref name=INOCAR>{{cite web |title=El Niño, La Niña, ENSO, ENOS, El Niño Modoki, El Niño Canónico, El Niño Extraordinario, El Niño Godzilla, El Niño Costero, El Niño Oriental ¿En qué consisten realmente y cómo afectan al Ecuador? |url=https://www.inocar.mil.ec/web/index.php/articulos/770-el-nino-la-nina-enso-enos-el-nino-modoki-el-nino-canonico-el-nino-extraordinario-el-nino-godzilla-el-nino-costero-el-nino-oriental-en-que-consisten-realmente-y-como-afectan-al-ecuador |access-date=11 February 2024 |work=Instituto Oceanográfico de la Armada del Ecuador |language=es}}</ref> ENSO Costero, or ENSO Oriental, is the name given to the phenomenon where the sea-surface temperature anomalies are mostly focused on the South American coastline, especially from Peru and Ecuador.<ref name="IMARPE">{{cite web |title=Antecedentes De 'El Niño Costero' |url=https://www.imarpe.gob.pe/imarpe/index2.php?id_seccion=I0175040100000000000000 |access-date=11 February 2024 |publisher=Instituto Del Mar Del Peru |language=es}}</ref> Studies point many factors that can lead to its occurrence,<ref name=Coastalvar>{{Cite journal |last1=Hu |first1=Zeng-Zhen |first2=Bohua |last2=Huang |first3=Jieshun |last3=Zhu |first4=Arun |last4=Kumar |first5=Michael J. |last5=McPhaden |date=6 June 2018 |title=On the variety of coastal El Niño events |url=https://doi.org/10.1007/s00382-018-4290-4 |journal=Climate Dynamics |language=en |volume=52 |issue=12 |pages=7537–7552 |doi=10.1007/s00382-018-4290-4 |s2cid=135045763 |access-date=11 February 2024|url-access=subscription }}</ref> sometimes accompanying, or being accompanied, by a larger EP ENSO occurrence,<ref name="IMARPE" /> or even displaying opposite conditions from the observed ones in the other Niño regions when accompanied by Modoki variations.<ref name=PhD>{{cite thesis |last=Zenteno |first=Hermogenes Edgard Gonzales |date=2022 |title=Predicción Del Fenómeno El Niño Mediante Índices Oceánicos E Influencia De La Zona De Convergencia Intertropical En El Norte Peruano |type=PhD |institution=National Agrarian University |lang=es |url=https://repositorio.lamolina.edu.pe/bitstream/handle/20.500.12996/5289/gonzales-centeno-hermogenes-edgard.pdf?sequence=1&isAllowed=y |access-date=11 February 2024}}</ref>

ENSO Costero events usually present more localized effects, with warm phases leading to increased rainfall over the coast of Ecuador, northern Peru and the Amazon rainforest, and increased temperatures over the northern Chilean coast,<ref name=INOCAR/><ref name=Chile>{{cite web |last1=Aste |first1=Fiorella |title=Cómo afecta El Niño costero a Chile, el fenómeno que ha dejado a más de 60 mil damnificados en Perú |url=https://www.latercera.com/noticia/afecta-nino-costero-chile-fenomeno-ha-dejado-mas-60-mil-damnificados-peru/ |date=17 March 2017 |access-date=11 February 2024 |publisher=La Tercera |language=es}}</ref> and cold phases leading to droughts on the Peruvian coast, and increased rainfall and decreased temperatures on its mountainous and jungle regions.<ref>{{cite web |last1=Blume |first1=Daniela Valdivia |title=¿Se viene La Niña en Perú? Enfen explica lo que podría suceder en los siguientes meses |url=https://www.infobae.com/peru/2024/01/17/se-viene-la-nina-en-peru-enfen-senala-lo-que-ocurrira-en-el-pais-en-los-proximos-meses/ |date=19 January 2024 |access-date=11 February 2024 |publisher=Infobae |language=es}}</ref>

Because they don't influence the global climate as much as the other types, these events present lesser and weaker correlations to other significant ENSO features, neither always being triggered by Kelvin waves,<ref name=INOCAR/> nor always being accompanied by proportional Southern Oscillation responses.<ref>{{Cite journal |last1=Takahashi |first1=Ken |last2=Martínez |first2=Alejandra G. |date=2019-06-01 |title=The very strong coastal El Niño in 1925 in the far-eastern Pacific |url=https://doi.org/10.1007/s00382-017-3702-1 |journal=Climate Dynamics |language=en |volume=52 |issue=12 |pages=7389–7415 |doi=10.1007/s00382-017-3702-1 |bibcode=2019ClDy...52.7389T |s2cid=134011107 |issn=1432-0894|hdl=20.500.12816/738 |hdl-access=free}}</ref> According to the Coastal Niño Index (ICEN), strong El Niño Costero events include 1957, 1982–83, 1997–98 and 2015–16, and La Niña Costera ones include 1950, 1954–56, 1962, 1964, 1966, 1967–68, 1970–71, 1975–76 and 2013.<ref name=ICEN>{{cite web |title=Eventos El Niño y La Niña Costeros |url=http://met.igp.gob.pe/elnino/lista_eventos.html |access-date=11 February 2024 |publisher=Comité Multisectorial Encargado del Estudio Nacional del Fenómeno El Niño |language=es}}</ref>

== Monitoring and declaration of conditions == thumb|The "Niño regions" where sea surface temperatures are monitored to determine the current ENSO phase (warm or cold)

Currently, each country has a different threshold for what constitutes an El Niño event, which is tailored to their specific interests, for example:<ref name="December 2014 EU2">{{cite web |author-last1=Becker |author-first1=Emily |date=4 December 2014 |title=December's ENSO Update: Close, but no cigar |url=https://www.climate.gov/news-features/blogs/enso/decembers-enso-update-close-no-cigar |archive-url=https://web.archive.org/web/20160322155232/https://www.climate.gov/news-features/blogs/enso/decembers-enso-update-close-no-cigar |archive-date=22 March 2016 |website=ENSO Blog}}</ref>

* In the United States, an El Niño is declared when the Climate Prediction Center, which monitors the sea surface temperatures in the ''Niño 3.4 region'' and the tropical Pacific, forecasts that the sea surface temperature will be {{cvt|.5|C-change||||}} above average or more for the next several seasons.<ref name="How?">{{cite web |author-last1=Becker |author-first1=Emily |date=27 May 2014 |title=How will we know when an El Niño has arrived? |url=https://www.climate.gov/news-features/blogs/enso/march-2015-enso-discussion-el-ni%C3%B1o-here |url-status=dead |archive-url=https://web.archive.org/web/20160322155232/https://www.climate.gov/news-features/blogs/enso/decembers-enso-update-close-no-cigar |archive-date=22 March 2016 |website=ENSO Blog}}</ref> The ''Niño 3.4 region'' stretches from the 120th to 170th meridians west longitude astride the equator five degrees of latitude on either side, are monitored. It is approximately {{convert|3000|km|mi}} to the southeast of Hawaii. The most recent three-month average for the area is computed, and if the region is more than 0.5 °C (0.9 °F) above (or below) normal for that period, then an El Niño (or La Niña) is considered in progress.<ref>{{cite web |author=Climate Prediction Center |author-link=Climate Prediction Center |date=2014-06-30 |title=ENSO: Recent Evolution, Current Status and Predictions |url=http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf |access-date=2014-06-30 |publisher=National Oceanic and Atmospheric Administration |pages=5, 19–20 |archive-date=2005-03-05 |archive-url=https://web.archive.org/web/20050305231546/http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf |url-status=live}}</ref> In February 2026, anomalous increases in tropical sea surface temperatures caused NOAA to revise the threshold distinguishing La Niña and El Niño from each other.<ref name=NOAA_20260212/> The new method replaces a dependency on a 30-year climate base period with the Relative Oceanic Niño Index (RONI): a comparison of the ENSO region to the global tropics.<ref name=NOAA_20260212>{{cite web |title=CPC adopts Relative Oceanic Niño Index (RONI) for reliable, responsive monitoring and tracking of ENSO |url=https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/announcement.php |publisher=National Oceanic and Atmospheric Administration (NOAA) |archive-url=https://web.archive.org/web/20260224062958/https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/announcement.php |archive-date=24 February 2026 |date=12 February 2026 |url-status=live}}</ref> * The Australian Bureau of Meteorology looks at the trade winds, Southern Oscillation Index, weather models and sea surface temperatures in the Niño 3 and 3.4 regions, before declaring an ENSO event.<ref>{{cite web |title=ENSO Tracker: About ENSO and the Tracker |url=http://www.bom.gov.au/climate/enso/tracker/#tabs=About-ENSO-and-the-Tracker |access-date=4 April 2016 |publisher=Australian Bureau of Meteorology |archive-date=15 January 2023 |archive-url=https://web.archive.org/web/20230115070614/http://www.bom.gov.au/climate/enso/tracker/#tabs=About-ENSO-and-the-Tracker |url-status=live}}</ref> * The Japan Meteorological Agency declares that an ENSO event has started when the average five month sea surface temperature deviation for the Niño 3 region is over {{convert|0.5|C-change}} for six consecutive months or longer.<ref>{{cite web |title=Historical El Niño and La Niña Events |url=http://ds.data.jma.go.jp/tcc/tcc/products/elnino/ensoevents.html |access-date=4 April 2016 |publisher=Japan Meteorological Agency |archive-date=14 July 2022 |archive-url=https://web.archive.org/web/20220714144456/https://ds.data.jma.go.jp/tcc/tcc/products/elnino/ensoevents.html |url-status=live}}</ref> * The Peruvian government declares that an ENSO Costero is under way if the sea surface temperature deviation in the Niño 1+2 regions equal or exceed {{cvt|0.4|C-change||||}} for at least three months.<ref name=ICEN/> * The United Kingdom's Met Office also uses a several month period to determine ENSO state.<ref>{{cite web |author=Met Office |date=2012-10-11 |title=El Niño, La Niña and the Southern Oscillation |url=http://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina/enso-description |access-date=2014-06-30 |publisher=United Kingdom |archive-date=2023-10-27 |archive-url=https://web.archive.org/web/20231027063920/https://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina/enso-description |url-status=live}}</ref> When this warming or cooling occurs for only seven to nine months, it is classified as El Niño/La Niña "conditions"; when it occurs for more than that period, it is classified as El Niño/La Niña "episodes".<ref>{{cite web |author=National Climatic Data Center |author-link=National Climatic Data Center |date=June 2009 |title=El Niño / Southern Oscillation (ENSO) June 2009 |url=http://www.ncdc.noaa.gov/oa/climate/research/enso/?year=2009&month=6&submitted=true |access-date=2009-07-26 |publisher=National Oceanic and Atmospheric Administration}}</ref>

== Effects of ENSO on global climate == In climate change science, ENSO is known as one of the internal climate variability phenomena. The other two main ones are Pacific decadal oscillation and Atlantic multidecadal oscillation.<ref name="IPCC2021" />{{rp|23}}

La Niña impacts the global climate and disrupts normal weather patterns, which can lead to intense storms in some places and droughts in others.<ref name="NIWA NZ">{{cite web |date=2007-02-27 |title=El Niño and La Niña |url=https://www.niwa.co.nz/climate/information-and-resources/elnino |url-status=live |archive-url=https://web.archive.org/web/20160319114842/http://www.niwa.co.nz/climate/information-and-resources/elnino |archive-date=19 March 2016 |access-date=11 April 2016 |publisher=National Institute of Water and Atmospheric Research |place=New Zealand}}</ref> El Niño events cause short-term (approximately 1 year in length) spikes in global average surface temperature while La Niña events cause short term cooling.<ref name=":1">{{cite journal |last1=Brown |first1=Patrick T. |last2=Li |first2=Wenhong |last3=Xie |first3=Shang-Ping |author-link3=Shang-Ping Xie |date=27 January 2015 |title=Regions of significant influence on unforced global mean surface air temperature variability in climate models: Origin of global temperature variability |journal=Journal of Geophysical Research: Atmospheres |volume=120 |issue=2 |pages=480–494 |doi=10.1002/2014JD022576 |doi-access=free |hdl-access=free |hdl=10161/9564}}</ref> Therefore, the relative frequency of El Niño compared to La Niña events can affect global temperature trends on decadal timescales.<ref name=":2">{{cite journal |last1=Trenberth |first1=Kevin E. |last2=Fasullo |first2=John T. |date=December 2013 |title=An apparent hiatus in global warming? |journal=Earth's Future |volume=1 |issue=1 |pages=19–32 |bibcode=2013EaFut...1...19T |doi=10.1002/2013EF000165 |doi-access=free}}</ref>

=== Climate change === {{multiple image | total_width = 650 | image1 = 20210827 Global surface temperature bar chart - bars color-coded by El Niño and La Niña intensity.svg | caption1 = Colored bars show how El Niño years (red, regional warming) and La Niña years (blue, regional cooling) relate to overall global warming. The El Niño–Southern Oscillation has been linked to variability in longer-term global average temperature increase, with El Niño years usually corresponding to annual global temperature increases. | image2 = 1994- Global average temperature during June-July-August.svg | caption2 = 2023's June–July–August season was the warmest on record globally by a large margin, as El Niño conditions continued to develop.<ref name=Copernicus_202309>{{cite web |title=August Climate Bulletins / Summer 2023: the hottest on record |url=https://climate.copernicus.eu/summer-2023-hottest-record |publisher=Copernicus Programme |archive-url=https://web.archive.org/web/20230908232623/https://climate.copernicus.eu/summer-2023-hottest-record |archive-date=8 September 2023 |date=6 September 2023 |url-status=live}}</ref> 1998—a very strong El Niño year—also experienced a global temperature spike. | image3 = 1850- 01 January each year - Global average temperature changes.svg | caption3 = Global average surface temperatures, shown here for each January, reached a record temperature change of 1.75{{nbsp}}°C over pre-industrial levels on one day in January 2025—despite the Earth being in a La Niña (regional cooling) phase.<ref name=Copernicus_January2025>{{cite web |title=Copernicus: January 2025 was the warmest on record globally, despite an emerging La Niña |url=https://climate.copernicus.eu/copernicus-january-2025-was-warmest-record-globally-despite-emerging-la-nina |publisher=The Copernicus Programm |archive-url=https://web.archive.org/web/20250210195120/https://climate.copernicus.eu/copernicus-january-2025-was-warmest-record-globally-despite-emerging-la-nina |archive-date=10 February 2025 |date=February 2025 |url-status=live}} Click on "Download data" button and extract January values.</ref> }}

The consequences of ENSO in terms of the temperature anomalies and precipitation and weather extremes around the world are clearly increasing and associated with climate change. For example, recent scholarship (since about 2019) has found that climate change is increasing the frequency of extreme El Niño events.<ref>{{Cite web |title=Climate Change is Making El Niños More Intense, Study Finds |url=https://e360.yale.edu/digest/climate-change-is-making-el-ninos-more-intense-study-finds |access-date=2022-04-19 |website=Yale E360 |language=en-US |archive-date=2022-04-25 |archive-url=https://web.archive.org/web/20220425165841/https://e360.yale.edu/digest/climate-change-is-making-el-ninos-more-intense-study-finds |url-status=live}}</ref><ref>{{Cite journal |last1=Wang |first1=Bin |last2=Luo |first2=Xiao |last3=Yang |first3=Young-Min |last4=Sun |first4=Weiyi |last5=Cane |first5=Mark A. |last6=Cai |first6=Wenju |last7=Yeh |first7=Sang-Wook |last8=Liu |first8=Jian |date=2019-11-05 |title=Historical change of El Niño properties sheds light on future changes of extreme El Niño |journal=Proceedings of the National Academy of Sciences |language=en |volume=116 |issue=45 |pages=22512–22517 |bibcode=2019PNAS..11622512W |doi=10.1073/pnas.1911130116 |issn=0027-8424 |pmc=6842589 |pmid=31636177 |doi-access=free}}</ref><ref name="Liu2022">{{cite journal |author1=Jiu, Liping |author2=Song, Mirong |author3=Zhu, Zhu |author4=Horton, Radley M |author5=Hu, Yongyun |author6=Xie, Shang-Ping |author-link6=Shang-Ping Xie |date=23 Aug 2022 |title=Arctic sea-ice loss is projected to lead to more frequent strong El Niño events |journal=Nature Communications |volume=13 |issue=1 |page=4952 |bibcode=2022NatCo..13.4952L |doi=10.1038/s41467-022-32705-2 |pmc=9399112 |pmid=35999238 |doi-access=free}}</ref> Previously there was no consensus on whether climate change will have any influence on the strength or duration of El Niño events, as research alternately supported El Niño events becoming stronger and weaker, longer and shorter.<ref name="ENSO + Climate Change">{{cite web |author1=Di Liberto, Tom |date=11 September 2014 |title=ENSO + Climate Change=Headache |url=https://www.climate.gov/news-features/blogs/enso/enso-climate-change-headache |url-status=dead |archive-url=https://web.archive.org/web/20160418015833/https://www.climate.gov/news-features/blogs/enso/enso-climate-change-headache |archive-date=18 April 2016 |website=ENSO Blog}}</ref><ref name="Collins102"/>

While much longer observation of ENSO is needed to robustly detect changes,<ref>{{cite journal |last1=Wittenberg |first1=A.T. |year=2009 |title=Are historical records sufficient to constrain ENSO simulations? |journal=Geophys. Res. Lett. |volume=36 |issue=12 |pages=L12702 |article-number=2009GL038710 |bibcode=2009GeoRL..3612702W |doi=10.1029/2009GL038710 |s2cid=16619392 |doi-access=free}}</ref> a large ensemble experiment with multiple climate models shows an increase of approximately 10% in eastern Pacific ENSO amplitude between the 1901–1960 and 1961–2020 periods of greenhouse-gas increase. Compared to centuries-long runs with pre-industrial GHG concentrations, the ensemble of 1961–2020 results shows twice the likelihood of strong eastern Pacific El Niño events and nine times the likelihood of strong central Pacific La Niña events.<ref>{{cite journal |last1=Cai |first1=Wenju |last2=Ng |first2=Benjamin |last3=Geng |first3=Tao |last4=Jia |first4=Fan |last5=Wu |first5=Lixin |last6=Wang |first6=Guojian |last7=Liu |first7=Yu |last8=Gan |first8=Bolan |last9=Yang |first9=Kai |last10=Santoso |first10=Agus |last11=Lin |first11=Xiaopei |last12=Li |first12=Ziguang |last13=Liu |first13=Yi |last14=Yang |first14=Yun |last15=Jin |first15=Fei-Fei |date=June 2023 |title=Antropogenic impacts on twentieth - century ENSO variability changes |url=https://www.nature.com/articles/s43017-023-00427-8.epdf?sharing_token=xOI9atsuOvGT9TmJei7L0dRgN0jAjWel9jnR3ZoTv0Pte_xqACNGqtGKpHCToNiPgSeUDJPls2PSAP6Pcf5JGQ1pdQKtWF7BvtG6Gcl82ASZ3YPNjGKikkxEkzLO8YzpIXT_q2adEox-V40kvMawQ1xNtDVQwjquYdaOlw6E-MVsgs1GvL8eHxbdN7zo03AUZmhj-GRQGbg_RUrw--1Y8g3iD9kpof0c06A9WEr5G-y8ORdWo6odtUlbF5X2Juy2hAbvxcaJaiMP50Ei49oROnn7IEJuPAOBMS1C7eQO2QXcxd7GRq-8lZ21CiPUiC6H&tracking_referrer=www.abc.net.au |journal=Nature Reviews Earth & Environment |volume=4 |issue=6 |pages=407–418 |bibcode=2023NRvEE...4..407C |doi=10.1038/s43017-023-00427-8 |s2cid=258793531 |access-date=17 July 2023 |last16=Collins |first16=Mat |last17=McPhaden |first17=Michael J. |hdl=1959.4/unsworks_83673 |archive-date=17 July 2023 |archive-url=https://web.archive.org/web/20230717075336/https://www.nature.com/articles/s43017-023-00427-8.epdf?sharing_token=xOI9atsuOvGT9TmJei7L0dRgN0jAjWel9jnR3ZoTv0Pte_xqACNGqtGKpHCToNiPgSeUDJPls2PSAP6Pcf5JGQ1pdQKtWF7BvtG6Gcl82ASZ3YPNjGKikkxEkzLO8YzpIXT_q2adEox-V40kvMawQ1xNtDVQwjquYdaOlw6E-MVsgs1GvL8eHxbdN7zo03AUZmhj-GRQGbg_RUrw--1Y8g3iD9kpof0c06A9WEr5G-y8ORdWo6odtUlbF5X2Juy2hAbvxcaJaiMP50Ei49oROnn7IEJuPAOBMS1C7eQO2QXcxd7GRq-8lZ21CiPUiC6H&tracking_referrer=www.abc.net.au |url-status=live|hdl-access=free }}</ref>

The IPCC Sixth Assessment Report summarized the state of the art of research in 2021 into the future of ENSO as follows:

* "In the long term, it is very likely that the precipitation variance related to El Niño–Southern Oscillation will increase"<ref name="IPCC2021" />{{rp|113}} and * "It is very likely that rainfall variability related to changes in the strength and spatial extent of ENSO teleconnections will lead to significant changes at regional scale".<ref name="IPCC2021" />{{rp|114}} and * "There is medium confidence that both ENSO amplitude and the frequency of high-magnitude events since 1950 are higher than over the period from 1850 and possibly as far back as 1400".<ref name="IPCC2021" />{{rp|373}}

===Investigations regarding tipping points=== The ENSO is considered to be a potential tipping element in Earth's climate.<ref>{{cite journal |last1=Lenton |first1=T. M. |last2=Held |first2=H. |last3=Kriegler |first3=E. |last4=Hall |first4=J. W. |last5=Lucht |first5=W. |last6=Rahmstorf |first6=S. |last7=Schellnhuber |first7=H. J. |date=12 February 2008 |title=Tipping elements in the Earth's climate system |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=6 |pages=1786–1793 |doi=10.1073/pnas.0705414105 |pmc=2538841 |pmid=18258748 |doi-access=free}}</ref> Global warming can strengthen the ENSO teleconnection and resulting extreme weather events.<ref>{{cite journal |last1=Simon Wang |first1=S.-Y. |last2=Huang |first2=Wan-Ru |last3=Hsu |first3=Huang-Hsiung |last4=Gillies |first4=Robert R. |date=16 October 2015 |title=Role of the strengthened El Niño teleconnection in the May 2015 floods over the southern Great Plains |journal=Geophysical Research Letters |volume=42 |issue=19 |pages=8140–8146 |bibcode=2015GeoRL..42.8140S |doi=10.1002/2015GL065211 |doi-access=free}}</ref> For example, an increase in the frequency and magnitude of El Niño events have triggered warmer than usual temperatures over the Indian Ocean, by modulating the Walker circulation.<ref>{{cite journal |last1=Roxy |first1=Mathew Koll |last2=Ritika |first2=Kapoor |last3=Terray |first3=Pascal |last4=Masson |first4=Sébastien |date=15 November 2014 |title=The Curious Case of Indian Ocean Warming*,+ |url=https://hal.archives-ouvertes.fr/hal-01141647/file/jclim2015_roxy_etal.pdf |journal=Journal of Climate |volume=27 |issue=22 |pages=8501–8509 |bibcode=2014JCli...27.8501R |doi=10.1175/JCLI-D-14-00471.1 |s2cid=42480067 |access-date=10 January 2019 |archive-date=3 September 2019 |archive-url=https://web.archive.org/web/20190903093421/https://hal.archives-ouvertes.fr/hal-01141647/file/jclim2015_roxy_etal.pdf |url-status=live}}</ref> This has resulted in a rapid warming of the Indian Ocean, and consequently a weakening of the Asian Monsoon.<ref>{{cite journal |last1=Roxy |first1=Mathew Koll |last2=Ritika |first2=Kapoor |last3=Terray |first3=Pascal |last4=Murtugudde |first4=Raghu |last5=Ashok |first5=Karumuri |last6=Goswami |first6=B. N. |date=November 2015 |title=Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient |journal=Nature Communications |volume=6 |issue=1 |page=7423 |bibcode=2015NatCo...6.7423R |doi=10.1038/ncomms8423 |pmid=26077934 |doi-access=free}}</ref>

{{excerpt|Tipping points in the climate system#Formerly considered tipping elements|paragraphs=1}}

== Effects of ENSO on weather patterns == El Niño affects the global climate and disrupts normal weather patterns, which can lead to intense storms in some places and droughts in others.<ref name="NIWA El Niño/La Niña">{{cite web |date=27 February 2007 |title=El Niño and La Niña |url=https://www.niwa.co.nz/climate/information-and-resources/elnino |url-status=live |archive-url=https://web.archive.org/web/20160319114842/http://www.niwa.co.nz/climate/information-and-resources/elnino |archive-date=19 March 2016 |access-date=11 April 2016 |publisher=New Zealand's National Institute of Water and Atmospheric Research}}</ref><ref name="How Much Do El Niño">{{cite journal |author-last1=Becker |author-first1=Emily |year=2016 |title=How Much Do El Niño and La Niña Affect Our Weather? This fickle and influential climate pattern often gets blamed for extreme weather. A closer look at the most recent cycle shows that the truth is more subtle |journal=Scientific American |volume=315 |issue=4 |pages=68–75 |doi=10.1038/scientificamerican1016-68 |pmid=27798565}}</ref>

=== Tropical cyclones === {{See also|Hurricanes and climate change}}

Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies.<ref>{{cite web |author=Joint Typhoon Warning Center |date=2006 |title=3.3 JTWC Forecasting Philosophies |url=http://www.nrlmry.navy.mil/forecaster_handbooks/Philippines2/Forecasters%20Handbook%20for%20the%20Philippine%20Islands%20and%20Surrounding%20Waters%20Typhoon%20Forecasting.3.pdf |access-date=11 February 2007 |archive-date=5 July 2012 |archive-url=https://web.archive.org/web/20120705161830/http://www.nrlmry.navy.mil/forecaster_handbooks/Philippines2/Forecasters%20Handbook%20for%20the%20Philippine%20Islands%20and%20Surrounding%20Waters%20Typhoon%20Forecasting.3.pdf |url-status=dead}}</ref> Areas west of Japan and Korea tend to experience many fewer September–November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break{{clarify|what is this break?|date=January 2024}} in the subtropical ridge tends to lie near 130°E, which would favor the Japanese archipelago.<ref name="China2">{{cite journal |last1=Wu |first1=M. C. |last2=Chang |first2=W. L. |last3=Leung |first3=W. M. |year=2004 |title=Impacts of El Niño–Southern Oscillation Events on Tropical Cyclone Landfalling Activity in the Western North Pacific |journal=Journal of Climate |volume=17 |issue=6 |pages=1419–28 |bibcode=2004JCli...17.1419W |citeseerx=10.1.1.461.2391 |doi=10.1175/1520-0442(2004)017<1419:ioenoe>2.0.co;2}}</ref> thumb|El Niño influence on hurricane season activity thumb|La Niña influence on hurricane season activity Based on modeled and observed accumulated cyclone energy (ACE), El Niño years usually result in less active hurricane seasons in the Atlantic Ocean, but instead favor a shift to tropical cyclone activity in the Pacific Ocean, compared to La Niña years favoring above average hurricane development in the Atlantic and less so in the Pacific basin.<ref>{{cite journal |last1=Patricola |first1=Christina M. |last2=Saravanan |first2=R. |last3=Chang |first3=Ping |date=15 July 2014 |title=The Impact of the El Niño–Southern Oscillation and Atlantic Meridional Mode on Seasonal Atlantic Tropical Cyclone Activity |journal=Journal of Climate |volume=27 |issue=14 |pages=5311–5328 |bibcode=2014JCli...27.5311P |doi=10.1175/JCLI-D-13-00687.1 |doi-access=free}}</ref>

Over the Atlantic Ocean, vertical wind shear is increased, which inhibits tropical cyclone genesis and intensification, by causing the westerly winds to be stronger.<ref name="TC FAQ G2">{{cite book |author=Landsea, Christopher W |title=Tropical Cyclone Frequently Asked Question |author2=Dorst, Neal M |date=1 June 2014 |publisher=United States National Oceanic and Atmospheric Administration's Hurricane Research Division |chapter=Subject: G2) How does El Niño-Southern Oscillation affect tropical cyclone activity around the globe? |chapter-url=http://www.aoml.noaa.gov/hrd/tcfaq/G2.html |archive-url=https://web.archive.org/web/20141009155501/http://www.aoml.noaa.gov/hrd/tcfaq/G2.html |archive-date=9 October 2014}}</ref> The atmosphere over the Atlantic Ocean can also be drier and more stable during El Niño events, which can inhibit tropical cyclone genesis and intensification.<ref name="TC FAQ G2" /> Within the Eastern Pacific basin: El Niño events contribute to decreased easterly vertical wind shear and favor above-normal hurricane activity.<ref name="EPAC Background">{{cite web |date=27 May 2015 |title=Background Information: East Pacific Hurricane Outlook |url=http://www.cpc.ncep.noaa.gov/products/Epac_hurr/background_information.html |access-date=7 April 2016 |publisher=United States Climate Prediction Center |archive-date=9 May 2009 |archive-url=https://web.archive.org/web/20090509063106/http://www.cpc.ncep.noaa.gov/products/Epac_hurr/background_information.html |url-status=live}}</ref> However, the impacts of the ENSO state in this region can vary and are strongly influenced by background climate patterns.<ref name="EPAC Background" />

The Western Pacific basin experiences a change in the location of where tropical cyclones form during El Niño events, with tropical cyclone formation shifting eastward, without a major change in how many develop each year.<ref name="TC FAQ G2" /> As a result of this change, Micronesia is more likely, and China less likely, to be affected by tropical cyclones.<ref name="China2" /> A change in the location of where tropical cyclones form also occurs within the Southern Pacific Ocean between 135°E and 120°W, with tropical cyclones more likely to occur within the Southern Pacific basin than the Australian region.<ref name="What is an El Niño?3"/><ref name="TC FAQ G2" /> As a result of this change tropical cyclones are 50% less likely to make landfall on Queensland, while the risk of a tropical cyclone is elevated for island nations like Niue, French Polynesia, Tonga, Tuvalu, and the Cook Islands.<ref name="What is an El Niño?3"/><ref name="NIWA">{{cite press release |publisher=New Zealand National Institute of Water and Atmospheric Research |url=https://www.niwa.co.nz/news/el-ni%C3%B1o-expected-to-produce-severe-tropical-storms-in-the-southwest-pacific |title=Southwest Pacific Tropical Cyclone Outlook: El Niño expected to produce severe tropical storms in the Southwest Pacific |date=14 October 2015 |url-status=live |access-date=22 October 2014 |archive-url=https://web.archive.org/web/20151212004320/http://www.niwa.co.nz/news/el-ni%C3%B1o-expected-to-produce-severe-tropical-storms-in-the-southwest-pacific |archive-date=12 December 2015 |type=Press release}}</ref><ref>{{cite press release |title=El Nino is here! |url=http://www.mic.gov.to/news-today/press-releases/5667-el-nino-is-here- |publisher=Tonga Ministry of Information and Communications |url-status=live |access-date=8 May 2016 |archive-url=https://web.archive.org/web/20171025003626/http://www.mic.gov.to/news-today/press-releases/5667-el-nino-is-here- |archive-date=25 October 2017 |date=11 November 2015}}</ref>

=== Remote influence on tropical Atlantic Ocean === A study of climate records has shown that El Niño events in the equatorial Pacific are generally associated with a warm tropical North Atlantic in the following spring and summer.<ref>{{cite journal |last1=Enfield |first1=David B. |last2=Mayer |first2=Dennis A. |year=1997 |title=Tropical Atlantic sea surface temperature variability and its relation to El Niño–Southern Oscillation |journal=Journal of Geophysical Research |volume=102 |issue=C1 |pages=929–945 |bibcode=1997JGR...102..929E |doi=10.1029/96JC03296 |doi-access=free}}</ref> About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool to become unusually large in summer.<ref>{{cite journal |author1=Lee, Sang-Ki |author2=Chunzai Wang |year=2008 |editor3=David B. Enfield |title=Why do some El Niños have no impact on tropical North Atlantic SST? |journal=Geophysical Research Letters |volume=35 |issue=L16705 |pages=L16705 |bibcode=2008GeoRL..3516705L |doi=10.1029/2008GL034734 |doi-access=free}}</ref> Occasionally, El Niño's effect on the Atlantic Walker circulation over South America strengthens the easterly trade winds in the western equatorial Atlantic region. As a result, an unusual cooling may occur in the eastern equatorial Atlantic in spring and summer following El Niño peaks in winter.<ref>{{cite journal |last1=Latif |first1=M. |last2=Grötzner |first2=A. |year=2000 |title=The equatorial Atlantic oscillation and its response to ENSO |journal=Climate Dynamics |volume=16 |issue=2–3 |pages=213–218 |bibcode=2000ClDy...16..213L |doi=10.1007/s003820050014 |s2cid=129356060}}</ref> Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.<ref name="Davis20012"/>

==Impacts on humans and ecosystems== === Economic impacts === [[File:ElNinoBloom_1998JanJul.gif|thumb|El Niño has the most direct impacts on life in the equatorial Pacific, its effects propagate north and south along the coast of the Americas, affecting marine life all around the Pacific. Changes in chlorophyll-a concentrations are visible in this animation, which compares phytoplankton in January and July 1998. Since then, scientists have improved both the collection and presentation of chlorophyll data.{{clarify|which image is January and which is July?|date=January 2024}}]] When El Niño conditions last for many months, extensive ocean warming and the reduction in easterly trade winds limits upwelling of cold nutrient-rich deep water, and its economic effect on local fishing for an international market can be serious.<ref name="deadfish2">{{cite web |author=((WW2010)) |date=28 April 1998 |title=El Niño |url=http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/eln/home.rxml |access-date=17 July 2009 |publisher=University of Illinois at Urbana-Champaign |archive-date=19 September 2023 |archive-url=https://web.archive.org/web/20230919005554/http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/eln/home.rxml |url-status=live}}</ref> Developing countries that depend on their own agriculture and fishing, particularly those bordering the Pacific Ocean, are usually most affected by El Niño conditions. In this phase of the Oscillation, the pool of warm water in the Pacific near South America is often at its warmest in late December.<ref name="DFG2">{{cite web |title=El Niño Information |url=https://wildlife.ca.gov/Conservation/Marine/El-Nino |work=California Department of Fish and Game, Marine Region |access-date=2024-01-18 |archive-date=2021-11-22 |archive-url=https://web.archive.org/web/20211122235130/https://www.wildlife.ca.gov/Conservation/Marine/El-Nino |url-status=live}}</ref>

The 2023 El Niño also disrupted weather patterns globally, with drought reducing maize production by up to 70% in some of Zimbabwe, Zambia and Malawi. The reduced harvests forced countries to declare states of disaster and appeal for international assistance to prevent widespread hunger. It also brought excessive rainfall to East Africa, causing flooding that destroyed crops and infrastructure in areas still recovering from consecutive years of drought.<ref>{{Cite book |last=FAO |url=https://openknowledge.fao.org/handle/20.500.14283/cd7185en |title=The Impact of Disasters on Agriculture and Food Security 2025 |date=2025 |publisher=FAO |isbn=978-92-5-140180-4 |language=English |doi=10.4060/cd7185en}}</ref>

thumb|A map showing the risk of El Niño to different areas of Zimbabwe. More generally, El Niño can affect commodity prices and the macroeconomy of different countries. It can constrain the supply of rain-driven agricultural commodities; reduce agricultural output, construction, and services activities; increase food prices; and may trigger social unrest in commodity-dependent poor countries that primarily rely on imported food.<ref name="University of Cambridge">{{cite web |date=11 July 2014 |title=Study reveals economic impact of El Niño |url=http://www.cam.ac.uk/research/news/study-reveals-economic-impact-of-el-nino |access-date=25 July 2014 |publisher=University of Cambridge |archive-date=28 July 2014 |archive-url=https://web.archive.org/web/20140728200450/http://www.cam.ac.uk/research/news/study-reveals-economic-impact-of-el-nino |url-status=live}}</ref>

A 2014 University of Cambridge Working Paper shows that while Australia, Chile, Indonesia, India, Japan, New Zealand and South Africa face a short-lived fall in economic activity in response to an El Niño shock, other countries may actually benefit from an El Niño weather shock (either directly or indirectly through positive spillovers from major trading partners), for instance, Argentina, Canada, Mexico and the United States. Most countries experience short-run inflationary pressures following an El Niño shock, while global energy and non-fuel commodity prices increase.<ref>{{cite journal |last1=Cashin |first1=Paul |last2=Mohaddes |first2=Kamiar |last3=Raissi |first3=Mehdi |name-list-style=amp |year=2014 |title=Fair Weather or Foul? The Macroeconomic Effects of El Niño |url=https://www.econ.cam.ac.uk/sites/default/files/publication-cwpe-pdfs/cwpe1418.pdf |journal=Cambridge Working Papers in Economics |archive-url=https://web.archive.org/web/20140728201219/http://www.econ.cam.ac.uk/research/repec/cam/pdf/cwpe1418.pdf |archive-date=28 July 2014}}</ref>

The IMF has traced El Niño events to macroeconomic performance. In a one-year time horizon, an El Niño shock is associated with changes in real GDP growth of -1.01% in Indonesia, -0.72% in South Africa, +0.5% in the United States, +1.57% in Mexico, and +1.81% in Thailand.<ref>{{Cite web |title=International Monetary Fund |url=https://www.imf.org/external/error.htm?URL=https://www.imf.org/en/Publications/WP/Issues/2016/12/31/Fair-Weather-or-Foul-The-Macroeconomic-Effects-of-El-Ni%c3%83%c2%b1o-42841 |url-status=live |archive-url=https://web.archive.org/web/20220301233510/https://www.imf.org/external/error.htm?URL=https://www.imf.org/en/Publications/WP/Issues/2016/12/31/Fair-Weather-or-Foul-The-Macroeconomic-Effects-of-El-Ni%C3%83%C2%B1o-42841 |archive-date=1 March 2022 |access-date=1 March 2022 |website=Imf.org |page=17}}</ref>

=== Health and social impacts === Extreme weather conditions related to the ENSO cycle correlate with changes in the incidence of epidemic diseases like Influenza pandemic, though studies have come to differing conclusions as to whether they are associated with the El Niño or the La Niña phase of the cycle.<ref>{{Cite journal |last1=Mazzarella |first1=A. |last2=Giuliacci |first2=A. |last3=Fabrizio |first3=P. |date=2011 |title=Hypothesis on a possible role of El Niño in the occurrence of influenza pandemics |url=https://doi.org/10.1007/s00704-010-0375-7 |journal=Theoretical and Applied Climatology |volume=105 |issue=1–2 |pages=65–69 |doi=10.1007/s00704-010-0375-7 |bibcode=2011ThApC.105...65M |s2cid=54777457 |via=Springer Link (DOI)|url-access=subscription }}</ref><ref>{{Cite journal |last1=Shaman |first1=J. |last2=Lipsitch |first2=M. |date=26 February 2013 |title=The El Niño–Southern Oscillation (ENSO)–pandemic Influenza connection: Coincident or causal? |journal=Proceedings of the National Academy of Sciences |volume=110 |issue=Suppl 1 |pages=3689–3691 |doi=10.1073/pnas.1107485109 |doi-access=free |pmc=3586607 |pmid=22308322}}</ref>

El Niño cycles are associated with increased risks of some of the diseases transmitted by mosquitoes, such as malaria, dengue fever, and Rift Valley fever,<ref>{{Cite web |title=El Niño and its health impact |url=http://www.allcountries.org/health/el_nino_and_its_health_impact.html |access-date=10 October 2017 |website=allcountries.org |archive-date=20 January 2011 |archive-url=https://web.archive.org/web/20110120092706/http://www.allcountries.org/health/el_nino_and_its_health_impact.html |url-status=live}}</ref> with the latter having a severe outbreak after extreme rainfall in north-eastern Kenya and southern Somalia during the 1997–98 El Niño.<ref>{{cite web |title=El Niño and its health impact |url=http://www.allcountries.org/health/el_nino_and_its_health_impact.html |access-date=1 January 2011 |work=Health Topics A to Z |archive-date=20 January 2011 |archive-url=https://web.archive.org/web/20110120092706/http://www.allcountries.org/health/el_nino_and_its_health_impact.html |url-status=live}}</ref> Cycles of malaria in India, Venezuela, Brazil, and Colombia have now been linked to El Niño. Outbreaks of another mosquito-transmitted disease, Australian encephalitis (Murray Valley encephalitis—MVE), occur in temperate south-east Australia after heavy rainfall and flooding, which are associated with La Niña events.

ENSO conditions have also been related to Kawasaki disease incidence in Japan and the west coast of the United States,<ref name="Ballester">{{Cite journal |last1=Ballester |first1=Joan |author2=Jane C. Burns |author3=Dan Cayan |author4=Yosikazu Nakamura |author5=Ritei Uehara |author6=Xavier Rodó |year=2013 |title=Kawasaki disease and ENSO-driven wind circulation |url=https://authors.library.caltech.edu/43425/1/grl50388.pdf |journal=Geophysical Research Letters |volume=40 |issue=10 |pages=2284–2289 |bibcode=2013GeoRL..40.2284B |doi=10.1002/grl.50388 |doi-access=free |access-date=2024-01-18 |archive-date=2020-11-22 |archive-url=https://web.archive.org/web/20201122065428/https://authors.library.caltech.edu/43425/1/grl50388.pdf |url-status=live}}</ref> via the linkage to tropospheric winds across the north Pacific Ocean.<ref name="Rod">{{Cite journal |last1=Rodó |first1=Xavier |author2=Joan Ballester |author3=Dan Cayan |author4=Marian E. Melish |author5=Yoshikazu Nakamura |author6=Ritei Uehara |author7=Jane C. Burns |date=10 November 2011 |title=Association of Kawasaki disease with tropospheric wind patterns |journal=Scientific Reports |volume=1 |article-number=152 |bibcode=2011NatSR...1..152R |doi=10.1038/srep00152 |issn=2045-2322 |pmc=3240972 |pmid=22355668}}</ref>

ENSO may be linked to civil conflicts. Scientists at The Earth Institute of Columbia University, having analyzed data from 1950 to 2004, suggest ENSO may have had a role in 21% of all civil conflicts since 1950, with the risk of annual civil conflict doubling from 3% to 6% in countries affected by ENSO during El Niño years relative to La Niña years.<ref>{{cite journal |author1=Hsiang, S. M. |author2=Meng, K. C. |author3=Cane, M. A. |year=2011 |title=Civil conflicts are associated with the global climate |journal=Nature |volume=476 |issue=7361 |pages=438–441 |bibcode=2011Natur.476..438H |doi=10.1038/nature10311 |pmid=21866157 |s2cid=4406478}}</ref><ref>{{cite journal |author=Quirin Schiermeier |year=2011 |title=Climate cycles drive civil war |journal=Nature |volume=476 |pages=406–407 |doi=10.1038/news.2011.501 |pmid=}}</ref>

=== Ecological consequences === During the 1982–83, 1997–98 and 2015–16 ENSO events, large extensions of tropical forests experienced a prolonged dry period that resulted in widespread fires, and drastic changes in forest structure and tree species composition in Amazonian and Bornean forests. Their impacts do not restrict only vegetation, since declines in insect populations were observed after extreme drought and terrible fires during El Niño 2015–16.<ref>{{Cite journal |last1=França |first1=Filipe |last2=Ferreira |first2=J |last3=Vaz-de-Mello |first3=FZ |last4=Maia |first4=LF |last5=Berenguer |first5=E |last6=Palmeira |first6=A |last7=Fadini |first7=R |last8=Louzada |first8=J |last9=Braga |first9=R |last10=Oliveira |first10=VH |last11=Barlow |first11=J |date=10 February 2020 |title=El Niño impacts on human-modified tropical forests: Consequences for dung beetle diversity and associated ecological processes |journal=Biotropica |volume=52 |issue=1 |pages=252–262 |doi=10.1111/btp.12756 |doi-access=free|bibcode=2020Biotr..52..252F}}</ref> Declines in habitat-specialist and disturbance-sensitive bird species and in large-frugivorous mammals were also observed in Amazonian burned forests, while temporary extirpation of more than 100 lowland butterfly species occurred at a burned forest site in Borneo.

In seasonally dry tropical forests, which are more drought tolerant, researchers found that El Niño induced drought increased seedling mortality. In a research published in October 2022, researchers studied seasonally dry tropical forests in a national park in Chiang Mai in Thailand for 7 years and observed that El Niño increased seedling mortality even in seasonally dry tropical forests and may impact entire forests in long run.<ref>{{Cite web |title=El Niño increases seedling mortality even in drought-tolerant forests |url=https://www.sciencedaily.com/releases/2022/10/221028111627.htm |access-date=2022-11-01 |website=ScienceDaily |language=en |archive-date=2022-11-01 |archive-url=https://web.archive.org/web/20221101181110/https://www.sciencedaily.com/releases/2022/10/221028111627.htm |url-status=live}}</ref>

==== Coral bleaching ==== The Pacific Marine Environmental Laboratory attributes the first large-scale coral bleaching event in 1997–1998 to the warming waters of the concurrent El Niño event with possible contribution from anthropogenic climate change.<ref>{{Cite web |title=FAQs {{!}} El Nino Theme Page – A comprehensive Resource |url=http://www.pmel.noaa.gov/elnino/faq |access-date=2025-09-23|website=www.pmel.noaa.gov |df=dmy-all |archive-date=2016-11-13 |archive-url=https://web.archive.org/web/20161113114526/http://www.pmel.noaa.gov/elnino/faq |url-status=live}}</ref>

Most critically, global mass bleaching events were recorded in 1997–98 and 2015–16, when around 75–99% losses of live coral were registered across the world. Considerable attention was also given to the collapse of Peruvian and Chilean anchovy populations that led to a severe fishery crisis following the ENSO events in 1972–73, 1982–83, 1997–98 and, more recently, in 2015–16. In particular, increased surface seawater temperatures in 1982-83 also lead to the probable extinction of two hydrocoral species in Panamá, and to a massive mortality of kelp beds along 600 km of coastline in Chile, from which kelps and associated biodiversity slowly recovered in the most affected areas even after 20 years. All these findings enlarge the role of ENSO events as a strong climatic force driving ecological changes all around the world – particularly in tropical forests and coral reefs.<ref>{{Cite journal |last1=França |first1=FM |last2=Benkwitt |first2=CE |last3=Peralta |first3=G |last4=Robinson |first4=JPW |last5=Graham |first5=NAJ |last6=Tylianakis |first6=JM |last7=Berenguer |first7=E |last8=Lees |first8=AC |last9=Ferreira |first9=J |last10=Louzada |first10=J |last11=Barlow |first11=J |year=2020 |title=Climatic and local stressor interactions threaten tropical forests and coral reefs |journal=Philosophical Transactions of the Royal Society B |volume=375 |issue=1794 |id=Art. No. 20190116 |doi=10.1098/rstb.2019.0116 |pmc=7017775 |pmid=31983328}}</ref>

== Impacts by region == Observations of ENSO events since 1950 show that impacts associated with such events depend on the time of year.<ref name="Cascade2">{{cite web |author1=Barnston, Anthony |date=19 May 2014 |title=How ENSO leads to a cascade of global impacts |url=https://www.climate.gov/news-features/blogs/enso/united-states-el-ni%C3%B1o-impacts-0 |url-status=dead |archive-url=https://web.archive.org/web/20160526193526/https://www.climate.gov/news-features/blogs/enso/united-states-el-ni%C3%B1o-impacts-0 |archive-date=26 May 2016 |website=ENSO Blog}}</ref> While some events and impacts are expected to occur, it is not certain that they will happen.<ref name="Cascade2" /> The impacts that generally do occur during most El Niño events include below-average rainfall over Indonesia and northern South America, and above average rainfall in southeastern South America, eastern equatorial Africa, and the southern United States.<ref name="Cascade2" />

=== Africa === [[File:Oxfam_East_Africa_-_A_family_gathers_sticks_and_branches_for_firewood.jpg|thumb|Between 50,000 and 100,000 people died during the 2011 East Africa drought.<ref>{{cite news |date=18 January 2012 |title=Slow response to East Africa famine 'cost lives' |url=https://www.bbc.co.uk/news/world-africa-16606021 |access-date=27 February 2022 |publisher=BBC News |archive-date=4 April 2022 |archive-url=https://web.archive.org/web/20220404002707/https://www.bbc.co.uk/news/world-africa-16606021 |url-status=live}}</ref>]] La Niña results in wetter-than-normal conditions in southern Africa from December to February, and drier-than-normal conditions over equatorial east Africa over the same period.<ref>{{cite news |date=12 October 2010 |title=La Niña weather likely to last for months |url=http://www.scoop.co.nz/stories/WO1010/S00173/la-nina-weather-likely-to-last-for-months.htm |access-date=27 February 2022 |website=Scoop News (Scoop.co.nz) |archive-date=28 June 2011 |archive-url=https://web.archive.org/web/20110628183246/http://www.scoop.co.nz/stories/WO1010/S00173/la-nina-weather-likely-to-last-for-months.htm |url-status=live}}</ref>

The effects of El Niño on rainfall in southern Africa differ between the summer and winter rainfall areas. Winter rainfall areas tend to get higher rainfall than normal and summer rainfall areas tend to get less rain. The effect on the summer rainfall areas is stronger and has led to severe drought in strong El Niño events.<ref>{{cite web |url=https://reliefweb.int/report/madagascar/southern-africa-el-nino-positive-indian-ocean-dipole-forecast-and-humanitarian-impact-october-2023 |title=Southern Africa: El Niño, Positive Indian Ocean Dipole Forecast and Humanitarian Impact (October 2023) |date=16 October 2023 |access-date=20 January 2024 |publisher=OCHA |website=reliefweb.int}}</ref><ref>{{cite journal |title=South African extreme weather during the 1877–1878 El Niño |first1=Yuri |last1=Brugnara |first2=Stefan |last2=Brönnimann |first3=Stefan |last3=Grab |first4=Jessica |last4=Steinkopf |first5=Angela-Maria |last5=Burgdorf |first6=Clive |last6=Wilkinson |first7=Rob |last7=Allan |journal=Weather |doi=10.1002/wea.4468 |volume=78 |issue=10 |date=October 2023 |pages=286–293 |bibcode=2023Wthr...78..286B |doi-access=free}}</ref>

Sea surface temperatures off the west and south coasts of South Africa are affected by ENSO via changes in surface wind strength.<ref name="Nhesvure 2020" >Nhesvure, B. (2020). Impacts of ENSO on coastal South African sea surface temperatures. Faculty of Science, Department of Oceanography. Retrieved from http://hdl.handle.net/11427/32954/</ref> During El Niño the south-easterly winds driving upwelling are weaker which results in warmer coastal waters than normal, while during La Niña the same winds are stronger and cause colder coastal waters. These effects on the winds are part of large scale influences on the tropical Atlantic and the South Atlantic High-pressure system, and changes to the pattern of westerly winds further south. There are other influences not known to be related to ENSO of similar importance. Some ENSO events do not lead to the expected changes.<ref name="Nhesvure 2020" />

=== Antarctica === Many ENSO linkages exist in the high southern latitudes around Antarctica.<ref name="Turner2004">{{cite journal |last=Turner |first=John |year=2004 |title=The El Niño–Southern Oscillation and Antarctica |journal=International Journal of Climatology |volume=24 |issue=1 |pages=1–31 |bibcode=2004IJCli..24....1T |doi=10.1002/joc.965 |s2cid=129117190}}</ref> Specifically, El Niño conditions result in high-pressure anomalies over the Amundsen and Bellingshausen Seas, causing reduced sea ice and increased poleward heat fluxes in these sectors, as well as the Ross Sea. The Weddell Sea, conversely, tends to become colder with more sea ice during El Niño. The exact opposite heating and atmospheric pressure anomalies occur during La Niña.<ref name="Yuan2004">{{cite journal |last=Yuan |first=Xiaojun |year=2004 |title=ENSO-related impacts on Antarctic sea ice: a synthesis of phenomenon and mechanisms |journal=Antarctic Science |volume=16 |issue=4 |pages=415–425 |bibcode=2004AntSc..16..415Y |doi=10.1017/S0954102004002238 |s2cid=128831185}}</ref> This pattern of variability is known as the Antarctic dipole mode, although the Antarctic response to ENSO forcing is not ubiquitous.<ref name="Yuan2004" />

=== Asia === In Western Asia, during the region's November–April rainy season, there is increased precipitation in the El Niño phase and reduced precipitation in the La Niña phase on average.<ref>Barlow, M., H. Cullen, and B. Lyon, 2002: Drought in central and southwest Asia: La Niña, the warm pool, and Indian Ocean precipitation. J. Climate, 15, 697–700</ref><ref>Nazemosadat, M. J., and A. R. Ghasemi, 2004: Quantifying the ENSO-related shifts in the intensity and probability of drought and wet periods in Iran. J. Climate, 17, 4005–4018</ref>

During El Niño years: As warm water spreads from the west Pacific and the Indian Ocean to the east Pacific, it takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific. Singapore experienced the driest February in 2010 since records began in 1869, with only 6.3 mm of rain falling in the month. The years 1968 and 2005 had the next driest Februaries, when 8.4 mm of rain fell.<ref>{{cite web |date=3 March 2010 |title=channelnewsasia.com - February 2010 is driest month for S'pore since records began in 1869 |url=http://www.channelnewsasia.com/stories/singaporelocalnews/view/1040778/1/.html |archive-url=https://web.archive.org/web/20100303222328/http://www.channelnewsasia.com/stories/singaporelocalnews/view/1040778/1/.html |archive-date=3 March 2010}}</ref>

During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific Ocean, which increases the landfall threat in China.<ref name="China">{{cite journal |last1=Wu |first1=M. C. |last2=Chang |first2=W. L. |last3=Leung |first3=W. M. |year=2004 |title=Impacts of El Niño–Southern Oscillation events on tropical cyclone landfalling activity in the western north Pacific |journal=Journal of Climate |volume=17 |issue=6 |pages=1419–1428 |bibcode=2004JCli...17.1419W |citeseerx=10.1.1.461.2391 |doi=10.1175/1520-0442(2004)017<1419:ioenoe>2.0.co;2}}</ref> In March 2008, La Niña caused a drop in sea surface temperatures over Southeast Asia by {{cvt|2|°C-change|°F-change||}}. It also caused heavy rains over the Philippines, Indonesia, and Malaysia.<ref>{{cite news |last=Hong |first=Lynda |date=13 March 2008 |title=Recent heavy rain not caused by global warming |url=http://www.channelnewsasia.com/stories/singaporelocalnews/view/334735/1/.html |url-status=dead |archive-url=https://web.archive.org/web/20080514073949/http://www.channelnewsasia.com/stories/singaporelocalnews/view/334735/1/.html |archive-date=14 May 2008 |access-date=22 June 2008 |publisher=Channel News Asia}}</ref>

=== Australia === Across most of the continent, El Niño and La Niña have more impact on climate variability than any other factor. There is a strong correlation between the strength of La Niña and rainfall: the greater the sea surface temperature and Southern Oscillation difference from normal, the larger the rainfall change.<ref>{{cite journal |last1=Power |first1=Scott |last2=Haylock |first2=Malcolm |last3=Colman |first3=Rob |last4=Wang |first4=Xiangdong |date=1 October 2006 |title=The Predictability of Interdecadal Changes in ENSO Activity and ENSO Teleconnections |journal=Journal of Climate |volume=19 |issue=19 |pages=4755–4771 |bibcode=2006JCli...19.4755P |doi=10.1175/JCLI3868.1 |issn=0894-8755 |s2cid=55572677 |doi-access=free}}</ref>

During El Niño events, the shift in rainfall away from the Western Pacific may mean that rainfall across Australia is reduced.<ref name="What is an El Niño?3"/> Over the southern part of the continent, warmer than average temperatures can be recorded as weather systems are more mobile and fewer blocking areas of high pressure occur.<ref name="What is an El Niño?3"/> The onset of the Indo-Australian Monsoon in tropical Australia is delayed by two to six weeks, which as a consequence means that rainfall is reduced over the northern tropics.<ref name="What is an El Niño?3"/> The risk of a significant bushfire season in south-eastern Australia is higher following an El Niño event, especially when it is combined with a positive Indian Ocean Dipole event.<ref name="What is an El Niño?3"/>

{{excerpt|Effects of the El Niño–Southern Oscillation in Australia|file=no}}

=== Europe === El Niño's effects on Europe are controversial, complex and difficult to analyze, as it is one of several factors that influence the weather over the continent and other factors can overwhelm the signal.<ref>{{cite web |date=29 October 2015 |title=What are the prospects for the weather in the coming winter? |url=https://blog.metoffice.gov.uk/2015/10/29/what-are-the-prospects-for-the-weather-in-the-coming-winter/ |url-status=live |archive-url=https://web.archive.org/web/20160420184138/https://blog.metoffice.gov.uk/2015/10/29/what-are-the-prospects-for-the-weather-in-the-coming-winter/ |archive-date=20 April 2016 |website=Met Office News Blog |publisher=United Kingdom Met Office}}</ref><ref>{{cite journal |author1=Ineson, S. |author2=Scaife, A. A. |date=7 December 2008 |title=The role of the stratosphere in the European climate response to El Niño |journal=Nature Geoscience |volume=2 |issue=1 |pages=32–36 |bibcode=2009NatGe...2...32I |doi=10.1038/ngeo381}}</ref>

=== North America === {{See also|Effects of the El Niño–Southern Oscillation in the United States}} La Niña causes mostly the opposite effects of El Niño: above-average precipitation across the northern Midwest, the northern Rockies, Northern California, and the Pacific Northwest's southern and eastern regions.<ref>{{cite web |date=22 October 2021 |title=La Niña is coming. Here's what that means for winter weather in the U.S. |url=https://www.npr.org/2021/10/15/1046313870/la-nina-winter-weather-us-temperatures-rainfall |access-date=21 December 2021 |publisher=NPR |archive-date=20 December 2021 |archive-url=https://web.archive.org/web/20211220231740/https://www.npr.org/2021/10/15/1046313870/la-nina-winter-weather-us-temperatures-rainfall |url-status=live}}</ref> Meanwhile, precipitation in the southwestern and southeastern states, as well as southern California, is below average.<ref>{{cite web |date=5 June 2014 |title=ENSO Diagnostic Discussion |url=https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html |url-status=dead |archive-url=https://web.archive.org/web/20140626200119/https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html |archive-date=26 June 2014 |publisher=National Oceanic and Atmospheric Administration |department=Climate Prediction Center}}</ref> This also allows{{clarify|What is "this" and how does it "allow for" the development of stronger hurricanes?|date=March 2024}} for the development of many stronger-than-average hurricanes in the Atlantic and fewer in the Pacific.

ENSO is linked to rainfall over Puerto Rico.{{clarify|linked in what way?|date=March 2024}}<ref>{{cite web |author=San Juan, Puerto Rico Weather Forecast Office |date=2010-09-02 |title=The Local Impacts of ENSO across the Northeastern Caribbean |url=http://www.srh.noaa.gov/sju/?n=enso2010 |access-date=2014-07-01 |publisher=National Weather Service Southern Region Headquarters |archive-date=2014-07-14 |archive-url=https://web.archive.org/web/20140714225041/http://www.srh.noaa.gov/sju/?n=enso2010 |url-status=live}}</ref> During an El Niño, snowfall is greater than average across the southern Rockies and Sierra Nevada mountain range, and is well-below normal across the Upper Midwest and Great Lakes states. During a La Niña, snowfall is above normal across the Pacific Northwest and western Great Lakes.<ref>Climate Prediction Center. [http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/us_impacts/ustp_impacts.shtml ENSO Impacts on United States Winter Precipitation and Temperature.] {{Webarchive|url=https://web.archive.org/web/20080412152736/http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/us_impacts/ustp_impacts.shtml |date=2008-04-12}} Retrieved on 2008-04-16.</ref>

In Canada, La Niña will, in general, cause a cooler, snowier winter, such as the near-record-breaking amounts of snow recorded in the La Niña winter of 2007–2008 in eastern Canada.<ref>{{cite web |date=2008-12-29 |title=A never-ending winter |url=http://www.ec.gc.ca/doc/smc-msc/2008/s3_eng.html |url-status=dead |archive-url=https://web.archive.org/web/20110807170704/http://www.ec.gc.ca/doc/smc-msc/2008/s3_eng.html |archive-date=7 August 2011 |series=Canada's top ten weather stories for 2008 |publisher=Environment Canada |id=number 3}}</ref><ref name="cpc.noaa.gov">{{cite report |url=https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf |title=ENSO evolution, status, and forecasts |date=2005-02-28 |publisher=National Oceanic and Atmospheric Administration |edition=update |archive-url=https://web.archive.org/web/20050515171707/https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf |archive-date=2005-05-15 |department=Climate Prediction Center}}</ref>

In the spring of 2022, La Niña caused above-average precipitation and below-average temperatures in the state of Oregon. April was one of the wettest months on record, and La Niña effects, while less severe, were expected to continue into the summer.<ref>{{cite web |date=29 April 2022 |title=If la Niña continues, what does that mean for Oregon this summer? |url=https://www.kgw.com/article/news/local/la-nina-impact-oregon-summer-weather/283-b75a0df3-6b27-4210-a4b3-cebfa8279882 |access-date=17 January 2024 |archive-date=26 April 2023 |archive-url=https://web.archive.org/web/20230426134552/https://www.kgw.com/article/news/local/la-nina-impact-oregon-summer-weather/283-b75a0df3-6b27-4210-a4b3-cebfa8279882 |url-status=live}}</ref>

Over North America, the main temperature and precipitation impacts of El Niño generally occur in the six months between October and March.<ref name="ENSO:Impacts">{{cite web |author1=Halpert, Mike |date=12 June 2014 |title=United States El Niño Impacts |url=https://www.climate.gov/news-features/blogs/enso/united-states-el-ni%C3%B1o-impacts-0 |url-status=dead |archive-url=https://web.archive.org/web/20160526193526/https://www.climate.gov/news-features/blogs/enso/united-states-el-ni%C3%B1o-impacts-0 |archive-date=26 May 2016 |website=ENSO Blog}}</ref><ref name="climate impacts">{{cite web |author1=Barnston, Anthony |date=12 June 2014 |title=With El Niño likely, what climate impacts are favored for this summer? |url=https://www.climate.gov/news-features/blogs/enso/el-ni%C3%B1o-likely-what-climate-impacts-are-favored-summer |url-status=dead |archive-url=https://web.archive.org/web/20160330112702/https://www.climate.gov/news-features/blogs/enso/el-ni%C3%B1o-likely-what-climate-impacts-are-favored-summer |archive-date=30 March 2016 |website=ENSO Blog}}</ref> In particular, the majority of Canada generally has milder than normal winters and springs, with the exception of eastern Canada where no significant impacts occur.<ref>{{cite web |date=2 December 2015 |title=El Niño: What are the El Niño impacts in Canada? |url=https://ec.gc.ca/meteo-weather/default.asp?lang=En&n=1C524B98-1 |url-status=live |archive-url=https://web.archive.org/web/20160322024048/http://ec.gc.ca/meteo-weather/default.asp?lang=En&n=1C524B98-1 |archive-date=22 March 2016 |publisher=Environment and Climate Change Canada}}</ref> Within the United States, the impacts generally observed during the six-month period include wetter-than-average conditions along the Gulf Coast between Texas and Florida, while drier conditions are observed in Hawaii, the Ohio Valley, Pacific Northwest and the Rocky Mountains.<ref name="ENSO:Impacts" />

Study of more recent weather events over California and the southwestern United States indicate that there is a variable relationship between El Niño and above-average precipitation, as it strongly depends on the strength of the El Niño event and other factors.<ref name="ENSO:Impacts" /> Though it has been historically associated with high rainfall in California, the effects of El Niño depend more strongly on the "flavor"{{clarify|what are the "flavors" of El Nino?|date=March 2024}} of El Niño than its presence or absence, as only "persistent El Niño" events lead to consistently high rainfall.<ref>{{Cite web |last=Oetting |first=Jeremiah |date=11 May 2018 |title=El Nino "flavors" affect California rainfall |url=https://www.earthmagazine.org/article/el-nino-flavors-affect-california-rainfall/ |access-date=2022-04-18 |website=www.earthmagazine.org |archive-date=2022-06-17 |archive-url=https://web.archive.org/web/20220617104205/https://www.earthmagazine.org/article/el-nino-flavors-affect-california-rainfall/ |url-status=live}}</ref><ref>{{Cite journal |last1=Lee |first1=Sang-Ki |last2=Lopez |first2=Hosmay |last3=Chung |first3=Eui-Seok |last4=DiNezio |first4=Pedro |last5=Yeh |first5=Sang-Wook |last6=Wittenberg |first6=Andrew T. |date=2018-01-28 |title=On the Fragile Relationship Between El Niño and California Rainfall |journal=Geophysical Research Letters |language=en |volume=45 |issue=2 |pages=907–915 |bibcode=2018GeoRL..45..907L |doi=10.1002/2017GL076197 |issn=0094-8276 |s2cid=35504261 |doi-access=free}}</ref>

To the north across Alaska, La Niña events lead to drier than normal conditions, while El Niño events do not have a correlation towards dry or wet conditions. During El Niño events, increased precipitation is expected in California due to a more southerly, zonal, storm track.<ref>Monteverdi, John and Jan Null. [http://tornado.sfsu.edu/geosciences/elnino/elnino.html Western Region Technical Attachment No. 97-37 November 21, 1997: El Niño and California Precipitation.] {{Webarchive|url=https://web.archive.org/web/20091227155828/http://tornado.sfsu.edu/geosciences/elnino/elnino.html |date=December 27, 2009}} Retrieved on 2008-02-28.</ref> During La Niña, increased precipitation is diverted into the Pacific Northwest due to a more northerly storm track.<ref>Mantua, Nathan. [http://www.ccb.ucar.edu/lanina/report/mantua.html La Niña Impacts in the Pacific Northwest.] {{webarchive|url=https://web.archive.org/web/20071022030057/http://www.ccb.ucar.edu/lanina/report/mantua.html|date=2007-10-22}} Retrieved on 2008-02-29.</ref> During La Niña events, the storm track shifts far enough northward to bring wetter than normal winter conditions (in the form of increased snowfall) to the Midwestern states, as well as hot and dry summers.<ref>Reuters. [https://www.reuters.com/article/domesticNews/idUSN1619766420070216 La Nina could mean dry summer in Midwest and Plains.] {{Webarchive|url=https://web.archive.org/web/20080421224855/http://www.reuters.com/article/domesticNews/idUSN1619766420070216 |date=2008-04-21}} Retrieved on 2008-02-29.</ref> During the El Niño portion of ENSO, increased precipitation falls along the Gulf coast and Southeast due to a stronger than normal, and more southerly, polar jet stream.<ref>Climate Prediction Center. [http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/ensorain.shtml El Niño (ENSO) Related Rainfall Patterns Over the Tropical Pacific.] {{Webarchive|url=https://web.archive.org/web/20100528035733/http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/ensorain.shtml|date=2010-05-28}} Retrieved on 2008-02-28.</ref>

==== Isthmus of Tehuantepec ==== {{Main|Tehuantepecer}}

The synoptic condition for the Tehuantepecer, a violent mountain-gap wind in between the mountains of Mexico and Guatemala, is associated with high-pressure system forming in Sierra Madre of Mexico in the wake of an advancing cold front, which causes winds to accelerate through the Isthmus of Tehuantepec. Tehuantepecers primarily occur during the cold season months for the region in the wake of cold fronts, between October and February, with a summer maximum in July caused by the westward extension of the Azores-Bermuda high pressure system.<ref>{{cite journal |last1=Romero-Centeno |first1=Rosario |last2=Zavala-Hidalgo |first2=Jorge |last3=Gallegos |first3=Artemio |last4=O'Brien |first4=James J. |date=1 August 2003 |title=Isthmus of Tehuantepec Wind Climatology and ENSO Signal |journal=Journal of Climate |volume=16 |issue=15 |pages=2628–2639 |bibcode=2003JCli...16.2628R |doi=10.1175/1520-0442(2003)016<2628:iotwca>2.0.co;2 |s2cid=53654865 |doi-access=free}}</ref>

Wind magnitude is greater during El Niño years than during La Niña years, due to the more frequent cold frontal incursions during El Niño winters.<ref>{{cite journal |last1=Romero-Centeno |first1=Rosario |last2=Zavala-Hidalgo |first2=Jorge |last3=Gallegos |first3=Artemio |last4=O'Brien |first4=James J. |date=1 August 2003 |title=Isthmus of Tehuantepec Wind Climatology and ENSO Signal |journal=Journal of Climate |volume=16 |issue=15 |pages=2628–2639 |bibcode=2003JCli...16.2628R |doi=10.1175/1520-0442(2003)016<2628:iotwca>2.0.co;2 |s2cid=53654865 |doi-access=free}}</ref> Tehuantepec winds reach {{convert|20|kn|km/h|-1}} to {{convert|45|kn|km/h|-1}}, and on rare occasions {{convert|100|kn|km/h|-1}}. The wind's direction is from the north to north-northeast.<ref name="gloss">{{cite web |author=American Meteorological Society |author-link=American Meteorological Society |date=2012-01-26 |title=Tehuantepecer |url=http://glossary.ametsoc.org/wiki/Tehuantepecer |access-date=2013-05-16 |work=Glossary of Meteorology |archive-date=2014-01-11 |archive-url=https://web.archive.org/web/20140111100511/http://glossary.ametsoc.org/wiki/Tehuantepecer |url-status=live}}</ref> It leads to a localized acceleration of the trade winds in the region, and can enhance thunderstorm activity when it interacts with the Intertropical Convergence Zone.<ref>{{cite web |author=Fett, Bob |date=2002-12-09 |title=World Wind Regimes – Central America Gap Wind Tutorial |url=http://www.nrlmry.navy.mil/sat_training/world_wind_regimes/tehantepecer/index.html |access-date=2013-05-16 |publisher=United States Naval Research Laboratory Monterey, Marine Meteorology Division |archive-date=2013-02-18 |archive-url=https://web.archive.org/web/20130218030859/http://www.nrlmry.navy.mil/sat_training/world_wind_regimes/tehantepecer/index.html |url-status=dead}}</ref> The effects can last from a few hours to six days.<ref name="MWL1971">{{cite journal |author=Arnerich, Paul A. |title=Tehuantepecer Winds of the West Coast of Mexico |journal=Mariners Weather Log |volume=15 |pages=63–67 |number=2}}</ref> Between 1942 and 1957, La Niña had an impact that caused isotope changes in the plants of Baja California, and that had helped scientists to study his impact.<ref>{{cite journal |last1=Martínez-Ballesté |first1=Andrea |last2=Ezcurra |first2=Exequiel |year=2018 |title=Reconstruction of past climatic events using oxygen isotopes in ''Washingtonia robusta'' growing in three anthropic oases in Baja California |journal=Boletín de la Sociedad Geológica Mexicana |volume=70 |issue=1 |pages=79–94 |doi=10.18268/BSGM2018v70n1a5 |doi-access=free|bibcode=2018BoSGM..70...79M }}</ref>

=== Pacific islands === During an El Niño event, New Zealand tends to experience stronger or more frequent westerly winds during their summer, which leads to an elevated risk of drier than normal conditions along the east coast.<ref name="New Zealand El Niño's impacts">{{cite web |date=19 October 2015 |title=El Niño's impacts on New Zealand's climate |url=https://www.niwa.co.nz/climate/information-and-resources/elnino/elnino-impacts-on-newzealand |url-status=live |archive-url=https://web.archive.org/web/20160319170708/http://www.niwa.co.nz/climate/information-and-resources/elnino/elnino-impacts-on-newzealand |archive-date=19 March 2016 |access-date=11 April 2016 |publisher=New Zealand's National Institute of Water and Atmospheric Research}}</ref> There is more rain than usual though on New Zealand's West Coast, because of the barrier effect of the North Island mountain ranges and the Southern Alps.<ref name="New Zealand El Niño's impacts" />

Fiji generally experiences drier than normal conditions during an El Niño, which can lead to drought becoming established over the Islands.<ref name="Fiji ENSO Update">{{Cite web |title=ENSO Update, Weak La Nina Conditions Favoured |url=http://www.met.gov.fj/ENSO_Update.pdf |archive-url=https://web.archive.org/web/20171107082128/http://www.met.gov.fj/ENSO_Update.pdf |archive-date=7 November 2017 |website=Fiji Meteorological Service}}</ref> However, the main impacts on the island nation is felt about a year after the event becomes established.<ref name="Fiji ENSO Update" /> Within the Samoan Islands, below average rainfall and higher than normal temperatures are recorded during El Niño events, which can lead to droughts and forest fires on the islands.<ref name="Clim Jan 2016">{{Cite web |date=January 2016 |title=Climate Summary January 2016 |url=http://www.samet.gov.ws/images/Climate_Services/CS/CSJAN2016.pdf |url-status=live |archive-url=https://web.archive.org/web/20170410141912/http://www.samet.gov.ws/images/Climate_Services/CS/CSJAN2016.pdf |archive-date=10 April 2017 |access-date=2021-05-02 |website=Samoa Meteorology Division, Ministry of Natural Resources and Environment}}</ref> Other impacts include a decrease in the sea level, possibility of coral bleaching in the marine environment and an increased risk of a tropical cyclone affecting Samoa.<ref name="Clim Jan 2016" />

In the late winter and spring during El Niño events, drier than average conditions can be expected in Hawaii.<ref>Chu, Pao-Shin. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0442(1995)008%3C1697%3AHRAAEN%3E2.0.CO%3B2 Hawaii Rainfall Anomalies and El Niño.]{{Dead link|date=August 2025 |bot=InternetArchiveBot |fix-attempted=yes }} Retrieved on 2008-03-19.</ref> On Guam during El Niño years, dry season precipitation averages below normal, but the probability of a tropical cyclone is more than triple what is normal, so extreme short duration rainfall events are possible.<ref>Pacific ENSO Applications Climate Center. [http://www.soest.hawaii.edu/MET/Enso/peu/2006_4th/guam_cnmi.htm Pacific ENSO Update: 4th Quarter, 2006. Vol. 12 No. 4.] {{Webarchive|url=https://web.archive.org/web/20121022060201/http://www.soest.hawaii.edu/MET/Enso/peu/2006_4th/guam_cnmi.htm |date=2012-10-22}} Retrieved on 2008-03-19.</ref> On American Samoa during El Niño events, precipitation averages about 10 percent above normal, while La Niña events are associated with precipitation averaging about 10 percent below normal.<ref>Pacific ENSO Applications Climate Center. [http://www.soest.hawaii.edu/MET/Enso/enso/rain/asamoa.html RAINFALL VARIATIONS DURING ENSO.] {{webarchive|url=https://web.archive.org/web/20080421210239/http://www.soest.hawaii.edu/MET/Enso/enso/rain/asamoa.html|date=2008-04-21}} Retrieved on 2008-03-19.</ref>

=== South America === The effects of El Niño in South America are direct and strong. An El Niño is associated with warm and very wet weather months in April–October along the coasts of northern Peru and Ecuador, causing major flooding whenever the event is strong or extreme.<ref>{{cite web |title=Atmospheric Consequences of El Niño |url=http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/eln/atms.rxml |access-date=2010-05-31 |publisher=University of Illinois |archive-date=2014-10-06 |archive-url=https://web.archive.org/web/20141006102340/http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/eln/atms.rxml |url-status=live}}</ref>

Because El Niño's warm pool feeds thunderstorms above, it creates increased rainfall across the east-central and eastern Pacific Ocean, including several portions of the South American west coast. The effects of El Niño in South America are direct and stronger than in North America. An El Niño is associated with warm and very wet weather months in April–October along the coasts of northern Peru and Ecuador, causing major flooding whenever the event is strong or extreme.<ref>{{cite web |title=Atmospheric Consequences of El Niño |url=http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/eln/atms.rxml |access-date=31 May 2010 |publisher=University of Illinois |archive-date=6 October 2014 |archive-url=https://web.archive.org/web/20141006102340/http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/eln/atms.rxml |url-status=live}}</ref> The effects during the months of February, March, and April may become critical along the west coast of South America, El Niño reduces the upwelling of cold, nutrient-rich water that sustains large fish populations, which in turn sustain abundant sea birds, whose droppings support the fertilizer industry. The reduction in upwelling leads to fish kills off the shore of Peru.<ref name="deadfish2"/>

The local fishing industry along the affected coastline can suffer during long-lasting El Niño events. Peruvian fisheries collapsed during the 1970s due to overfishing following the 1972 El Niño Peruvian anchoveta reduction.<ref name="perufish">{{Cite web |title=An El Niño Fish Tale |url=https://scied.ucar.edu/learning-zone/how-climate-works/el-nino-fish-tale |access-date=2023-11-26 |website=scied.ucar.edu |archive-date=2023-12-14 |archive-url=https://web.archive.org/web/20231214060657/https://scied.ucar.edu/learning-zone/how-climate-works/el-nino-fish-tale |url-status=live}}</ref> The fisheries were previously the world's largest, however, this collapse led to the decline of these fisheries. During the 1982–83 event, jack mackerel and anchoveta populations were reduced, scallops increased in warmer water, but hake followed cooler water down the continental slope, while shrimp and sardines moved southward, so some catches decreased while others increased.<ref name="Pearcy1987">{{cite journal |last1=Pearcy |first1=W. G. |author2=Schoener, A. |year=1987 |title=Changes in the marine biota coincident with the 1982-83 El Niño in the northeastern subarctic Pacific Ocean |url=http://www.agu.org/pubs/crossref/1987/JC092iC13p14417.shtml |url-status=dead |journal=Journal of Geophysical Research |volume=92 |issue=C13 |pages=14417–28 |bibcode=1987JGR....9214417P |doi=10.1029/JC092iC13p14417 |archive-url=https://web.archive.org/web/20120922000729/http://www.agu.org/pubs/crossref/1987/JC092iC13p14417.shtml |archive-date=22 September 2012 |access-date=22 June 2008|url-access=subscription }}</ref>

Horse mackerel have increased in the region during warm events. Shifting locations and types of fish due to changing conditions create challenges for the fishing industry. Peruvian sardines have moved during El Niño events to Chilean areas. Other conditions provide further complications, such as the government of Chile in 1991 creating restrictions on the fishing areas for self-employed fishermen and industrial fleets.

Southern Brazil and northern Argentina also experience wetter than normal conditions during El Niño years, but mainly during the spring and early summer. Central Chile receives a mild winter with large rainfall, and the Peruvian-Bolivian Altiplano receives more precipitation during its rainy season,<ref name=aceituno97>{{Cite book |title=El Altiplano. Ciencia y conciencia en los Andes |last=Aceituno |first=Patricio |year=1997 |pages=63–69 |language=es |chapter=Aspectos generales del clima en el altiplano sudamericano |chapter-url=https://bibliotecadigital.ciren.cl/server/api/core/bitstreams/d4693a12-cdc8-400c-99bd-b0f6c4696c48/content}}</ref> sometimes being exposed to unusual winter snowfall events. Drier and hotter weather occurs in parts of the Amazon River Basin, Colombia, and Central America.<ref>{{Cite book |last1=Sharma |first1=P. D. |url=https://books.google.com/books?id=fjmhn4g5VHkC&q=Southern+Brazil+and+northern+Argentina+also+experience+wetter+than+normal+conditions%2C+but+mainly+during+the+spring+and+early+summer.+Central+Chile+receives+a+mild+winter+with+large+rainfall%2C+and+the+Peruvian-Bolivian+Altiplano+is+sometimes+exposed+to+unusual+winter+snowfall+events.+Drier+and+hotter+weather+occurs+in+parts+of+the+Amazon+River+Basin%2C+Colombia%2C+and+Central+America&pg=PA392 |title=Ecology And Environment |last2=P.D |first2=Sharma |date=2012 |publisher=Rastogi Publications |isbn=978-81-7133-905-1 |language=en |access-date=2024-01-18 |archive-date=2024-01-20 |archive-url=https://web.archive.org/web/20240120022012/https://books.google.com/books?id=fjmhn4g5VHkC&q=Southern+Brazil+and+northern+Argentina+also+experience+wetter+than+normal+conditions%2C+but+mainly+during+the+spring+and+early+summer.+Central+Chile+receives+a+mild+winter+with+large+rainfall%2C+and+the+Peruvian-Bolivian+Altiplano+is+sometimes+exposed+to+unusual+winter+snowfall+events.+Drier+and+hotter+weather+occurs+in+parts+of+the+Amazon+River+Basin%2C+Colombia%2C+and+Central+America&pg=PA392#v=snippet&q=Southern%20Brazil%20and%20northern%20Argentina%20also%20experience%20wetter%20than%20normal%20conditions%2C%20but%20mainly%20during%20the%20spring%20and%20early%20summer.%20Central%20Chile%20receives%20a%20mild%20winter%20with%20large%20rainfall%2C%20and%20the%20Peruvian-Bolivian%20Altiplano%20is%20sometimes%20exposed%20to%20unusual%20winter%20snowfall%20events.%20Drier%20and%20hotter%20weather%20occurs%20in%20parts%20of%20the%20Amazon%20River%20Basin%2C%20Colombia%2C%20and%20Central%20America&f=false |url-status=live}}</ref>

During a time of La Niña, drought affects the coastal regions of Peru and Chile.<ref name="GLOBE">{{cite web |title=La Niña follows El Niño, the GLOBE El Niño Experiment continues |url=http://classic.globe.gov/fsl/html/templ.cgi?butler_lanina&lang=en |url-status=dead |archive-url=https://web.archive.org/web/20111015125212/http://classic.globe.gov/fsl/html/templ.cgi?butler_lanina&lang=en |archive-date=15 October 2011 |access-date=2010-05-31 |df=dmy-all}}</ref> From December to February, northern Brazil is wetter than normal.<ref name="GLOBE" /> La Niña causes higher than normal rainfall in the central Andes, which in turn causes catastrophic flooding on the Llanos de Mojos of Beni Department, Bolivia. Such flooding is documented from 1853, 1865, 1872, 1873, 1886, 1895, 1896, 1907, 1921, 1928, 1929 and 1931.<ref>{{cite book |last=van Valen |first=Gary |title=Indigenous Agency in the Amazon |publisher=University of Arizona Press |year=2013 |place=Tucson, Arizona |page=10}}</ref>

==== Galápagos Islands ==== The Galápagos Islands are a chain of volcanic islands, nearly 600 miles west of Ecuador, South America.<ref>{{cite web |title=Biodiversity |url=https://www.galapagos.org/about_galapagos/biodiversity/ |access-date=June 24, 2022 |website=Galapagos Conservancy |archive-date=June 28, 2022 |archive-url=https://web.archive.org/web/20220628114326/https://www.galapagos.org/about_galapagos/biodiversity/ |url-status=live}}</ref> in the Eastern Pacific Ocean. These islands support a wide diversity of terrestrial and marine species.<ref>{{cite web |last1=Karnauskas |first1=Kris |title=El Niño and the Galapagos |url=https://www.climate.gov/news-features/blogs/enso/el-ni%C3%B1o-and-gal%C3%A1pagos |access-date=Nov 17, 2022 |website=Climate.gov |archive-date=November 29, 2022 |archive-url=https://web.archive.org/web/20221129202333/https://www.climate.gov/news-features/blogs/enso/el-ni%C3%B1o-and-gal%C3%A1pagos |url-status=dead}}</ref> The ecosystem is based on the normal trade winds which influence upwelling of cold, nutrient rich waters to the islands.<ref>{{cite journal |last1=Vargas |date=2006 |title=Biological effects of El Niño on the Galápagos penguin |journal=Biological Conservation |volume=127 |issue=1 |pages=107–114 |doi=10.1016/j.biocon.2005.08.001|bibcode=2006BCons.127..107V}}</ref> During an El Niño event the trade winds weaken and sometimes blow from west to east, which causes the Equatorial current to weaken, raising surface water temperatures and decreasing nutrients in waters surrounding the Galápagos.<ref>{{cite journal |last1=Edgar |date=2010 |title=El Niño, grazers and fisheries interact to greatly elevate extinction risk for Galapagos marine species |journal=Global Change Biology |volume=16 |issue=10 |pages=2876–2890 |bibcode=2010GCBio..16.2876E |doi=10.1111/j.1365-2486.2009.02117.x |s2cid=83795836 |url=https://figshare.com/articles/journal_contribution/22879289 }}</ref>

El Niño causes a trophic cascade which impacts entire ecosystems starting with primary producers and ending with critical animals such as sharks, penguins, and seals.<ref>{{cite journal |last1=Edgar |date=2010 |title=El Niño, grazers and fisheries interact to greatly elevate extinction risk for Galapagos marine species |journal=Global Change Biology |volume=16 |issue=10 |pages=2876–2890 |bibcode=2010GCBio..16.2876E |doi=10.1111/j.1365-2486.2009.02117.x |s2cid=83795836 |url=https://figshare.com/articles/journal_contribution/22879289 }}</ref> The effects of El Niño can become detrimental to populations that often starve and die back during these years. Rapid evolutionary adaptations are displayed amongst animal groups during El Niño years to mitigate El Niño conditions.<ref>{{cite journal |last1=Holmgren |date=2001 |title=El Niño effects on the dynamics of terrestrial ecosystems |journal=Trends in Ecology and Evolution |volume=16 |issue=2 |pages=89–94 |doi=10.1016/S0169-5347(00)02052-8 |pmid=11165707 |url=http://americanae.aecid.es/americanae/es/registros/registro.do?tipoRegistro=MTD&idBib=3265581 |hdl=10533/172455 |hdl-access=free }}</ref>

== History == === In geologic timescales === Evidence is also strong for El Niño events during the early Holocene epoch 10,000 years ago.<ref name=":0" /> Different modes of ENSO-like events have been registered in paleoclimatic archives, showing different triggering methods, feedbacks and environmental responses to the geological, atmospheric and oceanographic characteristics of the time. These paleorecords can be used to provide a qualitative basis for conservation practices.<ref>{{cite journal |last1=Willis |first1=Katherine J |last2=Araújo |first2=Miguel B |last3=Bennett |first3=Keith D |last4=Figueroa-Rangel |first4=Blanca |last5=Froyd |first5=Cynthia A |last6=Myers |first6=Norman |title=How can a knowledge of the past help to conserve the future? Biodiversity conservation and the relevance of long-term ecological studies |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=28 February 2007 |volume=362 |issue=1478 |pages=175–187 |doi=10.1098/rstb.2006.1977 |pmid=17255027 |pmc=2311423}}</ref>

Scientists have also found chemical signatures of warmer sea surface temperatures and increased rainfall caused by El Niño in coral specimens that are around 13,000 years old.<ref name="Weather Underground">{{cite web |date=6 October 2015 |title=El Niño 2016 |url=https://wunderground.atavist.com/el-nino-forecast |work=Atavist |access-date=18 January 2024 |archive-date=26 February 2018 |archive-url=https://web.archive.org/web/20180226192006/https://wunderground.atavist.com/el-nino-forecast |url-status=dead}}</ref>

In a paleoclimate study published in 2024, the authors suggest that El Niños had a strong influence on Earth's hothouse climate during the Permian-Triassic extinction event. The increasing intensity and duration of El Niño events were associated with active volcanism, which resulted in the dieback of vegetation, an increase in the amount of carbon dioxide in the atmosphere, a significant warming and disturbances in the circulation of air masses.<ref name=Sun2024>{{cite journal|author1=Yadong Sun|author2=Alexander Farnsworth|author3=Michael M. Joachimski|author4=Paul B. Wignall|author5=Leopold Krystyn|author6=David P. G. Bond|author7=Domenico C. G. Ravidà|author8=Paul J. Valdes|date=September 12, 2024|title=Mega El Niño instigated the end-Permian mass extinction|journal=Science|volume=385|issue=6714|pages=1189–1195 |doi=10.1126/science.ado2030|pmid=39265011 |bibcode=2024Sci...385.1189S |url=https://hull-repository.worktribe.com/output/4785016 |language=en}}</ref>

{| class="wikitable" !Series/ epoch !Age of archive / Location / Type of archive or proxy !Description and references |- |Mid Holocene |4150 ya / Vanuatu Islands / Coral core |Coral bleaching in Vanuatu coral records, indication of shoaling of thermocline, is analyzed for Sr/Ca and U/Ca content, from which temperature is regressed. The temperature variability shows that during the mid-Holocene, changes in the position of the anticyclonic gyre produced average to cold (La Niña) conditions, which were probably interrupted by strong warm events (El Niño), which might have produced the bleaching, associated to decadal variability.<ref>{{cite journal |last1=Corrège |first1=Thierry |last2=Delcroix |first2=Thierry |last3=Récy |first3=Jacques |last4=Beck |first4=Warren |last5=Cabioch |first5=Guy |last6=Le Cornec |first6=Florence |title=Evidence for stronger El Niño-Southern Oscillation (ENSO) Events in a Mid-Holocene massive coral |journal=Paleoceanography |date=August 2000 |volume=15 |issue=4 |pages=465–470 |doi=10.1029/1999pa000409 |bibcode=2000PalOc..15..465C}}</ref> |- |Holocene |12000ya / Bay of Guayaquil, Ecuador / Pollen content of marine core |Pollen records show changes in precipitation, possibly related to variability of the position of the ITCZ, as well as the latitudinal maxima of the Humboldt Current, which both depend on ENSO frequency and amplitude variability. Three different regimes of ENSO influence are found in the marine core.<ref>{{cite journal |last1=Seillès |first1=Brice |last2=Sánchez Goñi |first2=Maria Fernanda |last3=Ledru |first3=Marie-Pierre |last4=Urrego |first4=Dunia H |last5=Martinez |first5=Philippe |last6=Hanquiez |first6=Vincent |last7=Schneider |first7=Ralph |title=Holocene land–sea climatic links on the equatorial Pacific coast (Bay of Guayaquil, Ecuador) |journal=The Holocene |date=April 2016 |volume=26 |issue=4 |pages=567–577 |doi=10.1177/0959683615612566 |bibcode=2016Holoc..26..567S |s2cid=130306658 |hdl=10871/18307 |hdl-access=free}}</ref> |- |Holocene |12000ya / Pallcacocha Lake, Ecuador / Sediment core |Core shows warm events with periodicities of 2–8 years, which become more frequent over the Holocene until about 1,200 years ago, and then decline, on top of which there are periods of low and high ENSO-related events, possibly due to changes in insolation.<ref>{{cite journal |last1=Rodbell |first1=Donald T. |last2=Seltzer |first2=Geoffrey O. |last3=Anderson |first3=David M. |last4=Abbott |first4=Mark B. |last5=Enfield |first5=David B. |last6=Newman |first6=Jeremy H. |title=An ~15,000-Year Record of El Niño-Driven Alluviation in Southwestern Ecuador |journal=Science |date=22 January 1999 |volume=283 |issue=5401 |pages=516–520 |doi=10.1126/science.283.5401.516 |pmid=9915694 |bibcode=1999Sci...283..516R |s2cid=13714632}}</ref><ref>{{Cite journal|last1=Moy|first1=Christopher M.|last2=Seltzer|first2=Geoffrey O.|last3=Rodbell|first3=Donald T.|last4=Anderson|first4=David M.|title=Variability of El Niño/Southern Oscillation activity at millennial timescales during the Holocene epoch|journal=Nature|volume=420|issue=6912|pages=162–165|doi=10.1038/nature01194|pmid=12432388|bibcode=2002Natur.420..162M|year=2002|s2cid=4395030}}</ref> |- |LGM |45000ya / Australia / Peat core |Moisture variability in the Australian core shows dry periods related to frequent warm events (El Niño), correlated to DO events. Although no strong correlation was found with the Atlantic Ocean, it is suggested that the insolation influence probably affected both oceans, although the Pacific Ocean seems to have the most influence on teleconnection in annual, millennial and semi-precessional timescales.<ref>{{Cite journal|last1=Turney|first1=Chris S. M.|last2=Kershaw|first2=A. Peter|last3=Clemens|first3=Steven C.|last4=Branch|first4=Nick|last5=Moss|first5=Patrick T.|last6=Fifield|first6=L. Keith|title=Millennial and orbital variations of El Niño/Southern Oscillation and high-latitude climate in the last glacial period|journal=Nature|volume=428|issue=6980|pages=306–310|doi=10.1038/nature02386|pmid=15029193|bibcode=2004Natur.428..306T|year=2004|s2cid=4303100}}</ref> |- |Pleistocene |240 Kya / Indian and Pacific oceans / Coccolithophore in 9 deep sea cores |9 deep cores in the equatorial Indian and Pacific show variations in primary productivity, related to glacial-interglacial variability and precessional periods (23 ky) related to changes in the thermocline. There is also indication that the equatorial areas can be early responders to insolation forcing.<ref>{{cite journal |last1=Beaufort |first1=Luc |last2=Garidel-Thoron |first2=Thibault de |last3=Mix |first3=Alan C. |last4=Pisias |first4=Nicklas G. |title=ENSO-like Forcing on Oceanic Primary Production During the Late Pleistocene |journal=Science |date=28 September 2001 |volume=293 |issue=5539 |pages=2440–2444 |doi=10.1126/science.293.5539.2440 |pmid=11577233 |bibcode=2001Sci...293.2440B}}</ref> |- |Pliocene |2.8 Mya / Spain / Lacustrine laminated sediments core |The basin core shows light and dark layers, related to summer/autumn transition where more/less productivity is expected. The core shows thicker or thinner layers, with periodicities of 12, 6–7 and 2–3 years, related to ENSO, North Atlantic Oscillation (NAO) and Quasi-biennial Oscillation (QBO), and possibly also insolation variability (sunspots).<ref>{{cite journal |last1=Muñoz |first1=Arsenio |last2=Ojeda |first2=Jorge |last3=Sánchez-Valverde |first3=Belén |title=Sunspot-like and ENSO/NAO-like periodicities in lacustrinelaminated sediments of the Pliocene Villarroya Basin (La Rioja, Spain)|journal=Journal of Paleolimnology |date=2002 |volume=27 |issue=4 |pages=453–463 |doi=10.1023/a:1020319923164 |bibcode=2002JPall..27..453M |s2cid=127610981}}</ref> |- |Pliocene |5.3 Mya / Equatorial Pacific / Foraminifera in deep sea cores |Deep sea cores at ODP site 847 and 806 show that the Pliocene warm period presented permanent El Niño-like conditions, possibly related to changes in the mean state of extratropical regions<ref>{{cite journal |last1=Wara |first1=Michael W. |last2=Ravelo |first2=Ana Christina|author-link2=Ana Christina Ravelo|last3=Delaney |first3=Margaret L. |title=Permanent El Niño-Like Conditions During the Pliocene Warm Period |journal=Science |date=29 July 2005 |volume=309 |issue=5735 |pages=758–761 |doi=10.1126/science.1112596 |pmid=15976271 |bibcode=2005Sci...309..758W |citeseerx=10.1.1.400.7297 |s2cid=37042990}}</ref> or changes in ocean heat transport resulting from increased tropical cyclone activity.<ref>{{cite journal |last1=Fedorov |first1=Alexey V. |last2=Brierley |first2=Christopher M. |last3=Emanuel |first3=Kerry |title=Tropical cyclones and permanent El Niño in the early Pliocene epoch |journal=Nature |date=February 2010 |volume=463 |issue=7284 |pages=1066–1070 |doi=10.1038/nature08831 |pmid=20182509 |hdl=1721.1/63099 |bibcode=2010Natur.463.1066F |s2cid=4330367 |hdl-access=free}}</ref> |- |Miocene |5.92-5.32 Mya / Italy / Evaporite varve thickness |The varve close to the Mediterranean shows 2–7 year variability, closely related to ENSO periodicity. Model simulations show that there is more correlation with ENSO than NAO, and that there is a strong teleconnection with the Mediterranean due to lower gradients of temperature.<ref>{{cite journal |last1=Galeotti |first1=Simone |last2=von der Heydt |first2=Anna |last3=Huber |first3=Matthew |last4=Bice |first4=David |last5=Dijkstra |first5=Henk |last6=Jilbert |first6=Tom |last7=Lanci |first7=Luca |last8=Reichart |first8=Gert-Jan |title=Evidence for active El Niño Southern Oscillation variability in the Late Miocene greenhouse climate |journal=Geology |date=May 2010 |volume=38 |issue=5 |pages=419–422 |doi=10.1130/g30629.1 |bibcode=2010Geo....38..419G |s2cid=140682002 |url=https://epic.awi.de/id/eprint/24732/1/Gal2010b.pdf }}</ref> |} {{Clear}}

=== During human history === thumb|299x299px|Average equatorial Pacific temperatures, published in 2009. ENSO conditions have occurred at two- to seven-year intervals for at least the past 300 years, but most of them have been weak.<ref name=":0">{{cite journal |last1=Carrè |first1=Matthieu |last2=Bentaleb |first2=Ilhem |last3=Fontugne |first3=Michel |last4=Lavallée |first4=Danièle |display-authors=1 |year=2005 |title=Strong El Niño events during the early Holocene: stable isotope evidence from Peruvian sea shells |journal=The Holocene |volume=15 |issue=1 |pages=42–7 |bibcode=2005Holoc..15...42C |doi=10.1191/0959683605h1782rp |s2cid=128967433 |url=http://americanae.aecid.es/americanae/es/registros/registro.do?tipoRegistro=MTD&idBib=565069 }}</ref>

El Niño may have led to the demise of the Moche c. 700 AD and other pre-Columbian Peruvian cultures.<ref>{{cite book |author=Brian Fagan |url=https://archive.org/details/floodsfaminesemp00faga/page/119 |title=Floods, Famines and Emperors: El Niño and the Fate of Civilizations |publisher=Basic Books |year=1999 |isbn=978-0-465-01120-9 |pages=[https://archive.org/details/floodsfaminesemp00faga/page/119 119–138]}}</ref> Around 1525, when Francisco Pizarro made landfall in Peru, he noted rainfall in the deserts, the first written record of the impacts of El Niño.<ref name="Weather Underground" /> A recent study suggests a strong El Niño effect between 1789 and 1793 caused poor crop yields in Europe, which in turn helped touch off the French Revolution.<ref name="Grove1998">{{cite journal |last=Grove |first=Richard H. |year=1998 |title=Global Impact of the 1789–93 El Niño |journal=Nature |volume=393 |issue=6683 |pages=318–9 |bibcode=1998Natur.393..318G |doi=10.1038/30636 |s2cid=205000683}}</ref> The extreme weather produced by El Niño in 1876–77 gave rise to the most deadly famines of the 19th century.<ref>{{cite book |last=Ó Gráda |first=C. |title=Famine: A Short History |publisher=Princeton University Press |year=2009 |isbn=9780691147970 |chapter=Ch. 1: The Third Horseman |access-date=3 March 2010 |chapter-url=http://press.princeton.edu/chapters/s8857.html |archive-url=https://web.archive.org/web/20160112061115/http://press.princeton.edu/chapters/s8857.html |archive-date=12 January 2016}}</ref> The 1876 famine alone in northern China killed up to 13 million people.<ref>{{cite web |title=Dimensions of need - People and populations at risk |url=http://www.fao.org/docrep/U8480E/U8480E05.htm |access-date=28 July 2015 |publisher=Fao.org |archive-date=10 October 2017 |archive-url=https://web.archive.org/web/20171010151615/http://www.fao.org/docrep/U8480E/U8480E05.htm |url-status=live}}</ref>

The phenomenon had long been of interest because of its effects on the guano industry and other enterprises that depend on biological productivity of the sea. It is recorded that as early as 1822, cartographer Joseph Lartigue, of the French frigate ''La Clorinde'' under Baron Mackau, noted the "counter-current" and its usefulness for traveling southward along the Peruvian coast.<ref>{{cite book |last1=Lartigue |url=https://books.google.com/books?id=us33ALotzJcC&pg=PA22 |title=Description de la Côte Du Pérou, Entre 19° et 16° 20' de Latitude Sud, ... |date=1827 |publisher=L'Imprimerie Royale |location=Paris, France |pages=22–23 |language=fr |trans-title=Description of the Coast of Peru, Between 19° and 16° 20' South Latitude, ... |access-date=2024-01-18 |archive-date=2024-01-20 |archive-url=https://web.archive.org/web/20240120022004/https://books.google.com/books?id=us33ALotzJcC&pg=PA22#v=onepage&q&f=false |url-status=live}} From pp. 22–23: ''"Il est néanmoins nécessaire, au sujet de cette règle générale, de faire part d'une exception ... dépassé le port de sa destination de plus de 2 ou 3 lieues; ... "'' (It is nevertheless necessary, with regard to this general rule, to announce an exception which, in some circumstances, might shorten the sailing. One said above that the breeze was sometimes quite fresh [i.e., strong], and that then the counter-current, which bore southward along the land, stretched some miles in length; it is obvious that one will have to tack in this counter-current, whenever the wind's force will permit it and whenever one will not have gone past the port of one's destination by more than 2 or 3 leagues; ...)</ref><ref name="pezet">{{citation |last=Pezet |first=Federico Alfonso |title=Report of the Sixth International Geographical Congress: Held in London, 1895, Volume 6 |pages=603–606 |year=1896 |contribution=The Counter-Current "El Niño," on the Coast of Northern Peru |contribution-url=https://archive.org/stream/reportsixthinte00unkngoog#page/n651/mode/2up}}</ref><ref>{{cite book |last=Findlay |first=Alexander G. |url=https://archive.org/details/adirectoryforna03findgoog |title=A Directory for the Navigation of the Pacific Ocean -- Part II. The Islands, Etc., of the Pacific Ocean |publisher=R. H. Laurie |year=1851 |location=London |page=[https://archive.org/details/adirectoryforna03findgoog/page/n621 1233] |quote=M. Lartigue is among the first who noticed a counter or southerly current.}}</ref>

The strongest El Niño on record as of early 2026 occurred in 1877 to 1878, and led to a global famine that killed more than 50 million people—about 3-4 percent of the estimated global population.<ref name="WashPost_20260512">{{cite news |last1=Noll |first1=Ben |title=A super El Niño wiped out millions of people in 1877. Are we better prepared now? |url=https://www.washingtonpost.com/weather/2026/05/12/super-el-nino-1877-population-impacts/ |work=The Washington Post |date=12 May 2026}}</ref> In 1888, Charles Todd suggested droughts in India and Australia tended to occur at the same time;<ref>"Droughts in Australia: Their causes, duration, and effect: The views of three government astronomers [R. L. J. Ellery, H. C. Russell, and C. Todd]," ''The Australasian'' (Melbourne, Victoria), 29 December 1888, pp. 1455–1456. [http://trove.nla.gov.au/newspaper/page/15068512 From p. 1456:] {{Webarchive|url=https://web.archive.org/web/20170916052707/http://trove.nla.gov.au/newspaper/page/15068512|date=16 September 2017}} "Australian and Indian Weather" : "Comparing our records with those of India, I find a close correspondence or similarity of seasons with regard to the prevalence of drought, and there can be little or no doubt that severe droughts occur as a rule simultaneously over the two countries."</ref> Norman Lockyer noted the same in 1904.<ref>Lockyer, N. and Lockyer, W.J.S. (1904) [https://babel.hathitrust.org/cgi/pt?id=uva.x002014109;view=1up;seq=525 "The behavior of the short-period atmospheric pressure variation over the Earth's surface,"] {{Webarchive|url=https://web.archive.org/web/20230403174054/https://babel.hathitrust.org/cgi/pt?id=uva.x002014109;view=1up;seq=525 |date=2023-04-03}} ''Proceedings of the Royal Society of London'', '''73''' : 457–470.</ref> An El Niño connection with flooding was reported in 1894 by Victor Eguiguren (1852–1919) and in 1895 by Federico Alfonso Pezet (1859–1929).<ref>Eguiguren, D. Victor (1894) [https://www.biodiversitylibrary.org/item/183713#page/263/mode/1up "Las lluvias de Piura"] {{Webarchive|url=https://web.archive.org/web/20231030150744/https://www.biodiversitylibrary.org/item/183713#page/263/mode/1up |date=2023-10-30}} (The rains of Piura), ''Boletín de la Sociedad Geográfica de Lima'', '''4''' : 241–258. [in Spanish] [https://www.biodiversitylibrary.org/item/183713#page/279/mode/1up From p. 257:] ''"Finalmente, la época en que se presenta la corriente de Niño, es la misma de las lluvias en aquella región."'' (Finally, the period in which the El Niño current is present is the same as that of the rains in that region [i.e., the city of Piura, Peru].)</ref><ref name="pezet" /><ref>Pezet, Federico Alfonso (1896) [https://www.biodiversitylibrary.org/item/183555#page/491/mode/1up "La contra-corriente "El Niño", en la costa norte de Perú"] {{Webarchive|url=https://web.archive.org/web/20231030150743/https://www.biodiversitylibrary.org/item/183555#page/491/mode/1up |date=2023-10-30}} (The counter-current "El Niño", on the northern coast of Peru), ''Boletín de la Sociedad Geográfica de Lima'', '''5''' : 457-461. [in Spanish]</ref> In 1924, Gilbert Walker (for whom the Walker circulation is named) coined the term "Southern Oscillation".<ref>Walker, G. T. (1924) "Correlation in seasonal variations of weather. IX. A further study of world weather," ''Memoirs of the Indian Meteorological Department'', '''24''' : 275–332. From p. 283: "There is also a slight tendency two quarters later towards an increase of pressure in S. America and of Peninsula [i.e., Indian] rainfall, and a decrease of pressure in Australia : this is part of the main oscillation described in the previous paper* which will in future be called the 'southern' oscillation." Available at: [https://www.rmets.org/sites/default/files/classicindia2.pdf Royal Meteorological Society] {{Webarchive|url=https://web.archive.org/web/20170318173334/https://www.rmets.org/sites/default/files/classicindia2.pdf|date=18 March 2017}}</ref> He and others (including Norwegian-American meteorologist Jacob Bjerknes) are generally credited with identifying the El Niño effect.<ref>{{cite web |last1=Cushman |first1=Gregory T |title=Who Discovered the El Niño-Southern Oscillation? |url=https://ams.confex.com/ams/annual2003/techprogram/paper_58909.htm |archive-url=https://web.archive.org/web/20151201002510/https://ams.confex.com/ams/annual2003/techprogram/paper_58909.htm |archive-date=1 December 2015 |access-date=18 December 2015 |website=Presidential Symposium on the History of the Atmospheric Sciences: People, Discoveries, and Technologies |publisher=American Meteorological Society (AMS)}}</ref>

The major 1982–83 El Niño led to an upsurge of interest from the scientific community. The period from 1991–1994 was unusual in that El Niños have rarely occurred in such rapid succession.<ref>{{cite journal |title=The El Niño Phenomenon Returns |url=http://www.wildsingapore.com/news/20060910/060918-6.htm |journal=Wild Singapore |access-date=May 8, 2022 |archive-date=April 3, 2023 |archive-url=https://web.archive.org/web/20230403174053/http://www.wildsingapore.com/news/20060910/060918-6.htm |url-status=live}}</ref><ref>{{cite thesis |author=Sinamaw Zeleke Wallie |title=Economic Impact from El Niños |date=January 2019 |publisher=Debark University |via=Academia.Edu |url=https://www.academia.edu/40687434 |access-date=May 8, 2022 |archive-date=April 3, 2023 |archive-url=https://web.archive.org/web/20230403174046/https://www.academia.edu/40687434 |url-status=live}}</ref>{{unreliable source?|date=March 2023}}<ref name="Trenberth19962">{{cite journal |last1=Trenberth |first1=Kevin E. |last2=Hoar |first2=Timothy J. |date=January 1996 |title=The 1990–95 El Niño–Southern Oscillation event: Longest on record |journal=Geophysical Research Letters |volume=23 |issue=1 |pages=57–60 |bibcode=1996GeoRL..23...57T |citeseerx=10.1.1.54.3115 |doi=10.1029/95GL03602}}</ref> An especially intense El Niño event in 1998 caused an estimated 16% of the world's reef systems to die. The event temporarily warmed air temperature by 1.5 °C, compared to the usual increase of 0.25 °C associated with El Niño events.<ref name="Trenberth">{{cite journal |last1=Trenberth |first1=K. E. |last2=Stepaniak |first2=David P. |last3=Worley |first3=Steve |display-authors=1  |year=2002 |title=Evolution of El Niño – Southern Oscillation and global atmospheric surface temperatures |journal=Journal of Geophysical Research |volume=107 |issue=D8 |page=4065 |bibcode=2002JGRD..107.4065T |citeseerx=10.1.1.167.1208 |doi=10.1029/2000JD000298}}</ref> Since then, mass coral bleaching has become common worldwide, with all regions having suffered "severe bleaching".<ref name="Marshall2006">{{cite book |last1=Marshall |first1=Paul |url=http://coris.noaa.gov/activities/reef_managers_guide/ |title=A reef manager's guide to coral bleaching |author2=Schuttenberg, Heidi |publisher=Great Barrier Reef Marine Park Authority |year=2006 |isbn=978-1-876945-40-4 |location=Townsville, Qld. |access-date=2024-01-18 |archive-date=2023-07-30 |archive-url=https://web.archive.org/web/20230730122801/https://www.coris.noaa.gov/activities/reef_managers_guide/ |url-status=live}}</ref>

== Related patterns ==

=== Madden–Julian oscillation === {{excerpt|Madden–Julian oscillation|paragraphs=1}}

==== Link to the El Niño-Southern oscillation==== {{excerpt|Madden–Julian oscillation#Link to the El Niño-Southern oscillation|paragraphs=1|hat=no}}

=== Pacific decadal oscillation === {{excerpt|Pacific decadal oscillation|paragraphs=1}}

====Mechanisms==== {{excerpt|Pacific decadal oscillation#Mechanisms|paragraphs=4|only=paragraph}}

=== Pacific Meridional Mode === {{excerpt|Pacific Meridional Mode|paragraphs=1,3,4}}

== See also == * {{annotated link|Ocean dynamical thermostat}} * {{annotated link|Recharge oscillator}}

'''For La Niña:'''

* 2000 Mozambique flood * 2010 Pakistan floods * 2010–2011 Queensland floods * 2010–2012 La Niña event * 2010–2011 Southern Africa floods * 2010–2013 Southern United States and Mexico drought * 2011 East Africa drought * 2020 Atlantic hurricane season * 2021 eastern Australia floods * 2022 Suriname floods * 2023 Auckland Anniversary Weekend floods * 2020–2023 La Niña event

'''For El Niño:''' *1789-1790 influenza epidemic *1982–83 El Niño event *1997 Pacific hurricane season *1997–98 El Niño event *2014–2016 El Niño event *2015 Pacific hurricane season *2023–2024 El Niño event

== Sources == {{Free-content attribution | title = The Impact of Disasters on Agriculture and Food Security 2025 | publisher = The Food and Agriculture Organization of the United Nations | documentURL = https://openknowledge.fao.org/bitstreams/d18d9cb5-3e78-469a-b268-0aed1799d147/download | license statement URL = https://openknowledge.fao.org/items/74d08f97-306a-4653-8ffe-140a2fc4d783 | license = CC BY 4.0 }}

== References == {{reflist|25em |refs=

<ref name=La-Nina-declaration-announcement-authorities>The following sources identified the listed "La Niña years": * {{cite news |title=La Niña years |url=http://www.noaanews.noaa.gov/stories/s300c.htm |newspaper=The NOAA News |publisher=National Oceanic and Atmospheric Administration |access-date=20 April 2016 |archive-date=20 December 2016 |archive-url=https://web.archive.org/web/20161220164305/http://www.noaanews.noaa.gov/stories/s300c.htm |url-status=live}} * {{cite report |title=La Niña and winter weather |url=http://www.nws.noaa.gov/om/winter/la_nina-q-a.shtml |publisher=National Oceanic and Atmospheric Administration |access-date=14 March 2014 |archive-date=14 March 2014 |archive-url=https://web.archive.org/web/20140314060746/http://www.nws.noaa.gov/om/winter/la_nina-q-a.shtml |url-status=live}} * {{cite report |title=ENSO Impacts on the U.S. – Previous Events |series=Monitoring & Data |website=cpc.noaa.gov |department=Climate Prediction Center |publisher=U.S. National Oceanic and Atmospheric Administration |url=https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml |url-status=dead |date=4 November 2015 |access-date=3 January 2017 |archive-url=https://web.archive.org/web/20101206141055/https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml |archive-date=6 December 2010}} * {{cite web |title=La Niña Information |department=Public Affairs |publisher=U.S. National Oceanic and Atmospheric Administration |url=http://www.publicaffairs.noaa.gov/lanina.html |access-date=31 May 2010 |url-status=dead |archive-url=https://web.archive.org/web/20140812112241/http://www.publicaffairs.noaa.gov/lanina.html |archive-date=12 August 2014 }} * {{cite news |last=Sutherland |first=Scott |date=16 February 2017 |title=La Niña calls it quits. Is El Niño paying us a return visit? |website=The Weather Network |url=https://www.theweathernetwork.com/us/news/articles/la-nina-calls-it-quits-is-el-nino-paying-us-a-return-visit/79424 |access-date=17 February 2017 |archive-date=18 February 2017 |archive-url=https://web.archive.org/web/20170218143231/https://www.theweathernetwork.com/us/news/articles/la-nina-calls-it-quits-is-el-nino-paying-us-a-return-visit/79424 |url-status=dead }} * {{cite web |title=El Niño/Southern Oscillation (ENSO) diagnostic discussion |date=10 September 2020 |publisher=Climate Prediction Center / NCEP / NWS and the International Research Institute for Climate and Society |url=https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf |access-date=10 September 2020 |archive-date=17 September 2020 |archive-url=https://web.archive.org/web/20200917151935/https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf |url-status=live }}</ref><ref name=Modoki-authorities>For evidence of La Niña Modoki, and identification of La Niña Modoki year: * {{cite web |last1=Platonov |first1=V. |last2=Semenov |first2=E. |last3=Sokolikhina |first3=E. |title=Extreme La Niña 2010/11 and the vigorous flood at the north-east of Australia |url=http://meetingorganizer.copernicus.org/EGU2012/EGU2012-313-2.pdf |publisher=EGU General Assembly / Geophysical Research |date=13 February 2014 |access-date=15 October 2014 |archive-date=16 July 2015 |archive-url=https://web.archive.org/web/20150716174413/http://meetingorganizer.copernicus.org/EGU2012/EGU2012-313-2.pdf |url-status=live}} * {{cite journal |last1=Shinoda |first1=Toshiaki |last2=Hurlburt |first2=Harley E. |last3=Metzger |first3=E. Joseph |title=Anomalous tropical ocean circulation associated with La Niña Modoki |journal=Journal of Geophysical Research: Oceans |volume=116 |issue=12 |pages=C12001 |year=2011 |bibcode=2011JGRC..11612001S |doi=10.1029/2011JC007304 |doi-access=free}} * {{cite magazine |last=Welsh |first=Jon |title=Introducing La Niña Modoki: She's 'similar but different' ... |url=http://www.crdc.com.au/sites/default/files/pdf/SpotSpring16_sc2.pdf |magazine=Spotlight Magazine |pages=34–35 |publisher=Australian Government Cotton Research and Development Corporation |date=Spring 2016 |access-date=18 February 2017 |archive-date=19 February 2017 |archive-url=https://web.archive.org/web/20170219130102/http://www.crdc.com.au/sites/default/files/pdf/SpotSpring16_sc2.pdf |url-status=live}} * {{cite web |last=Welsh |first=Jon |title=Are we heading for a La Niña Modoki? |url=http://www.graincentral.com/weather/are-we-heading-for-a-la-nina-modoki/ |website=Grain Central |date=6 October 2016 |access-date=18 February 2017 |archive-date=19 February 2017 |archive-url=https://web.archive.org/web/20170219095752/http://www.graincentral.com/weather/are-we-heading-for-a-la-nina-modoki/ |url-status=live}}</ref> }}

==External links== {{Commons category|El Niño/La Niña–Southern Oscillation}} {{Commons category|El Niño}} * {{cite web |title=Current map of sea surface temperature anomalies in the Pacific Ocean |url=https://earth.nullschool.net/#current/ocean/surface/currents/overlay=sea_surface_temp_anomaly/orthographic=-140.00,00.001/loc=-140.000,00.001 |website=earth.nullschool.net}} * {{cite web |title=Southern Oscillation diagnostic discussion |url=http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ |publisher=National Oceanic and Atmospheric Administration |department=Climate Prediction Center}} * {{cite web |title=ENSO Outlook – An alert system for the El Niño–Southern Oscillation |url=http://www.bom.gov.au/climate/enso/outlook/ |publisher=Australian Bureau of Meteorology}} Provides current phase of ENSO according to the Australian interpretation. * {{cite web |title=Cold & Warm Episodes by Season / Relative Oceanic Niño Index (RONI): Historical El Niño / La Niña episodes (1950–Present) |url=https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ |publisher=National Oceanic and Atmospheric Administration (NOAA) }}

{{DEFAULTSORT:El Nino-Southern Oscillation}} Category:Tropical meteorology Category:Physical oceanography Category:Natural history of the Americas Category:Natural history of Oceania Category:Effects of climate change Category:Regional climate effects Category:Weather hazards Category:Spanish words and phrases Category:El Niño–Southern Oscillation events Category:Climate oscillations