{{Short description|Jet stream}} The '''Tropical Easterly Jet''' ([[jet stream]]) is the [[meteorological]] term referring to an upper level easterly [[wind]] that forms in late June and continues until early September. This strong flow of air that develops in the upper [[atmosphere]] during [[Monsoon of South Asia|the Asian monsoon]] is centred on 15°N, 50-80°E and extends from [[South-East Asia]] to [[Africa]].
A much weaker easterly jet exists in the northeast Pacific associated with the [[North American Monsoon]].<ref name="Dramatic"/>
==Meteorology== The strongest development of the jet is at about {{convert|15|km|mi|0}} above the Earth's surface with wind speeds of up to {{convert|40|m/s|km/h mph knots}} over the [[Indian Ocean]].<ref>{{cite book |title=Climate and Circulation of the Tropics |last=Hastenrath |first=Stefan |year=1985|publisher=Kluwer Academic Publishers |isbn=90-277-2026-6 |pages=464}}</ref> This jet subsides at the [[Somalia]] coast with the Mascarene High,<ref>{{cite journal|journal=Journal of the Atmospheric Sciences|last1=Krishnamurte|last2=Bhalme|first1=T.N.|first2=H.N.|title=Oscillations of a Monsoon System. Part 1. Observational Aspects|date=October 1976|pages=1937–1954|volume=33|issue=10 |doi=10.1175/1520-0469(1976)033<1937:OOAMSP>2.0.CO;2 |bibcode=1976JAtS...33.1937K }}</ref> and in the [[Sahara Desert]]. It has been suggested that the subsidence in the northwestern quadrant of the Tropical Easterly Jet is an important factor producing the exceptional hyperaridity of the Sahara<ref>{{cite encyclopedia|last1=Webster|first1=Peter J.|last2=Fasullo|first2=John|year=2003|title=Monsoon: Dynamical Theory|encyclopedia=Encyclopedia of Atmospheric Sciences|editor-last1=Holton|editor-first1=James R.|editor-last2=Curry|editor-first2=Judith A.|pages=1370–1385|location=London|publisher=Academic Press}}</ref> – drier than any other desert outside of coastal [[upwelling]] zones.<ref>{{cite book|title=Dryland Climatology|isbn=9781139500241|page=303|date=27 October 2011|publisher=Cambridge University Press|first=Sharon E.|last=Nicholson}}</ref><ref>{{cite book|last=Besla|first=Helga|chapter=The Tropical Easterly Jet as a Cause for Intensified Aridity in the Sahara|title=Palaeoecology of Africa, volume 16|isbn=9780203744512|editor-link1=Johanna Alida Coetzee|editor-first1=Johanna Alida|editor-last1=Coetzee|editor-first2=Eduard Meine|editor-last2=van Zinderen-Bakker|pages=163–172}}</ref>
The easterly jet induces significant vertical wind shear during the monsoonal months, especially from July to September, which suppresses any [[North Indian Ocean tropical cyclone|tropical cyclone]] activity.<ref group="note">Monsoonal depressions, characteristic of the monsoon season in India, generally do not intensify into cyclones</ref>
==History of study== Although the upper-level easterly wind flow over India during the summer monsoon was established as early as the 1930s, the term "easterly jet" originated with Indian researcher P.R. Krishna Rao in 1952.<ref>{{cite journal|journal=Current Science|publisher=Current Science Association|location=[[Bangalore]]|volume=21|last=Krishna Rao|first=P.R.|title=Probable Regions of Jet Streams in the Upper Air over India|year=1952}}</ref> P. Koteswaram later in the 1950s demonstrated that the easterly flow is a jet stream in the strict sense of the term.<ref>{{cite conference|chapter=The Indian Summer Monsoon and the General Circulation Over the Tropics|last=Koteswaram|first=P.|title=Symposium on Monsoons of the World|location=Meteorological Office, New Delhi|date=19–21 February 1958}}</ref> During the 1960s and 1970s it became recognised that the Tropical Easterly Jet arises due to the intense solar heating of the [[Tibetan Plateau]],<ref>{{cite journal |last1=Mason |first1=Ralph B. |last2=Anderson |first2=Calvin E. |title=The Development and Decay of the 100 mb. Summertime Anticyclone over Southern Asia |journal=[[Monthly Weather Review]] |date=January 1963 |volume=91 |issue=1 |pages=3–12|doi=10.1175/1520-0493(1963)091<0003:TDADOT>2.3.CO;2 |bibcode=1963MWRv...91....3M }}</ref> and that it was closely linked to the Asian monsoon in both day-to-day fluctuations and overall seasonal strength.
==Relationship with Asian monsoon== The Tropical Easterly Jet's outflow from the Tibetan Plateau is deeply connected to the low-level monsoonal flow over India. Studies have demonstrated that when the high-level easterly jet is weak, monsoonal rainfall over India<ref>{{cite journal |last1=Pattanaik |first1=D. R. |last2=Satyan |first2=V. |title=Fluctuations of Tropical Easterly Jet during contrasting monsoons over India: A GCM study |journal=Meteorology and Atmospheric Physics |year=2000 |volume=75 |issue=1–2 |pages=51–60|doi=10.1007/s007030070015 |bibcode=2000MAP....75...51P }}</ref> and seasonal rainfall as far east as [[Micronesia]]<ref>{{cite journal |author=Minoru Tanaka |title=Internal Fluctuations of the Tropical Easterly Jet and the Summer Monsoon in the Asian Region |journal=Journal of the Meteorological Society of Japan |date=June 1982 |volume=60 |issue=3 |pages=865–875}}</ref> will be deficient.
It has been more recently suggested that variations in surface rainfall can actually drive variations in the strength of the jet,<ref>{{cite journal|volume=75|issue=1–2|date=January 2011|pages=21–30|journal= Global and Planetary Change|title=Variations of Indian summer monsoon rainfall induce the weakening of easterly jet stream in the warming environment?|first1=C.V.|last1=Naidu|first2=K. Muni|last2=Krishna|first3=S.|last3=Ramalingeswara Rao|first4=O.S.R.U.|last4=Bhanu Kumar|first5=K.|last5=Durgalakshmi|first6=S.S.V.S.|last6=Ramakrishna|doi=10.1016/j.gloplacha.2010.10.001 |bibcode=2011GPC....75...21N }}</ref> contradicting earlier suggestions that the surface southwesterly monsoon flow is actually a return flow from the upper-level easterlies.
==Recent weakening== Since 1950, the Tropical Easterly Jet has weakened by as much as 25 percent, so that the contour of {{convert|30|m/s|km/h mph knots}} has disappeared.<ref>{{cite journal|volume=31|year=2004|journal=Geophysical Research Letters|title=Decreasing trend in the strength of Tropical Easterly Jet during the Asian summer monsoon season and the number of tropical cyclonic systems over Bay of Bengal|first1=B. R.|last1=Srinivasa Rao|first2=D. V.|last2=Bhaskar Rao|first3=V.|last3=Brahmananda Rao|publisher=Department of Meteorology and Oceanography, [[Andhra University]]|location=[[Visakhapatnam]]}}</ref><ref name="Weakening">{{cite journal |last1=Abish |first1=B. |last2=Joseph |first2=P.V. |last3=Johannessen |first3=Ola M. |title=Weakening Trend of the Tropical Easterly Jet Stream of the Boreal Summer Monsoon Season 1950–2009 |journal=Journal of Climate |date=1 December 2013 |volume=26 |issue=23 |pages=9409–9414|doi=10.1175/JCLI-D-13-00440.1 |bibcode=2013JCli...26.9408A |doi-access=free }}</ref> This trend has been attributed to a combination of low-level [[global warming]] with cooling near the [[tropopause]] over continental Asia.<ref name="Weakening"/> Other studies show that the greatest weakening has occurred in the highest-altitude parts of the jet, and in the westerly section over the [[Arabian Sea]].<ref>{{cite journal|title=Effect of changing tropical easterly jet, low level jet and quasi-biennial oscillation phases on Indian summer monsoon|first1=P.|last1=Rai|first2=A. P.|last2=Dimri|journal=Atmospheric Science Letters|volume=18|pages=52–59|year=2017}}</ref> The declining strength of the Tropical Easterly Jet has been tightly linked with declining rainfall over [[Arunachal Pradesh]], [[Jammu and Kashmir (union territory)|Jammu and Kashmir]] and certain parts of central India and the [[Malabar Coast]].<ref>{{cite conference|title=A study on the decreasing trend in tropical easterly jet stream (TEJ) and its impact on Indian summer monsoon rainfall|first1=P. P.|last1=Sreekala|first2=S. V.|last2=Bhaskara Rao|first3=M. S.|last3=Arunachalam|first4=C.|last4=Harikiran|conference=Theoretical and Applied Climatology|date=November 2013}}</ref> Further weakenings of the jet are expected under enhanced greenhouse gases – by at least 11 percent at the end of the twenty-first century.<ref name="Dramatic">{{cite journal|journal=Journal of Climate|volume=33|issue=19|date=October 2020|pages=8439–8455|title=Dramatic Weakening of the Tropical Easterly Jet Projected by CMIP6 Models|author1=Sihua Huang|author2=Bin Wang|author3=Zhiping Wen|doi=10.1175/JCLI-D-19-1002.1 |bibcode=2020JCli...33.8439H }}</ref>
The northeastern Pacific easterly jet, however, is expected to weaken by as much as 77 percent under likely global warming scenarios,<ref name="Dramatic"/> which could reduce annual rainfall over [[Central America]] and [[Administrative divisions of Mexico#Regions and districts|southern Mexico]] by as much as {{convert|1,000|mm|in|-1|disp=or}}.
==See also== * [[African easterly jet]] * [[Somali Jet]]
==Notes== {{reflist|group=note}}
==References== {{reflist}}
[[Category:Atmospheric dynamics]] [[Category:Winds]]