{{Short description|Jetstream over West Africa}} The '''African easterly jet''' is a region of the lower troposphere over [[West Africa]] where the seasonal mean wind speed is at a maximum and the wind is easterly. The temperature contrast between the [[Sahara Desert]] and the [[Gulf of Guinea]] causes the jet to form to the north of the [[monsoon trough]]. The jet's maximum wind speeds are at a height of {{convert|3|km}}. The jet moves northward from its south-most location in January, reaching its most northerly latitude in August. Its strongest winds are in September while it begins shifting back towards the equator. Within the easterly jet, [[tropical wave]]s form. Convective complexes associated with these waves can form [[tropical cyclone]]s. If the jet is south of its normal location during August and September, [[tropical cyclogenesis]] is suppressed. If [[desertification]] continues across [[Sub-Saharan Africa]], the strength of the jet could increase, although tropical wave generation probably would decrease, which would decrease the number of tropical cyclones in the Atlantic basin. A [[Jet Stream]] is the physical mechanism of a [[teleconnection]].
==Formation and characteristics== During January, the African easterly jet lies at {{convert|3000|m}} above sea level at five degrees north latitude. Winds within it increase in speed from {{convert|30|km/h|abbr=on}} in January to {{convert|40|km/h|abbr=on}} in March. Shifting northward in April to the seventh parallel, winds within the jet increase to {{convert|45|km/h|abbr=on}}. By June, it shifts northward into northwest Africa.<ref name="marcel">{{cite book | url=https://books.google.com/books?id=v32qnYWh5gQC&pg=PA139 | pages=138–139 | title=The Meteorology and Climate of Tropical Africa | author=Marcel Leroux | year=2001 | publisher=Springer | accessdate=5 February 2011 | ISBN=978-3-540-42636-3}}</ref>
From June into October, the [[thermal low]] over northern Africa leads to a low-level westerly jet stream to the south of the [[Intertropical Convergence Zone]].<ref>B. Pu and K. H. Cook (2008). [http://adsabs.harvard.edu/abs/2008AGUFM.A13A0229P Dynamics of the Low-Level Westerly Jet Over West Africa.] American Geophysical Union, Fall Meeting 2008, abstract #A13A-0229. Retrieved on 8 March 2009.</ref> The mid-level African easterly jet develops because heating of the West African land mass during this time of year creates a surface temperature and moisture gradient between the Gulf of Guinea and the Sahara Desert. The atmosphere responds by generating vertical [[wind shear]] to maintain [[thermal wind]] balance. The jet reaches its zenith in August, lying between the 16th and 17th parallels. In September, winds maximize near {{convert|50|km/h|abbr=on}} between the 12th and 13th parallels. The easterly jet weakens and drops southward during October and November.<ref name="marcel"/>
==Impact== {{See also|Tropical cyclones and climate change#Hurricanes}} The mid-level African easterly jet is considered to play a crucial role in the West African [[monsoon]],<ref>Kerry H. Cook. [https://archive.today/20130101205758/http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175/1520-0442(1999)012%3C1165:GOTAEJ%3E2.0.CO;2 Generation of the African Easterly Jet and Its Role in Determining West African Precipitation.] Retrieved on 8 May 2008.</ref> and helps form the tropical waves that move across the tropical Atlantic and eastern Pacific oceans during the warm season.<ref>[[Chris Landsea]]. AOML Frequently Asked Questions. [https://www.aoml.noaa.gov/hrd/tcfaq/A4.html Subject: A4) What is an easterly wave ?] Retrieved on 8 May 2008.</ref> The jet exhibits both [[barotropic]] and [[baroclinic instability]], which produces [[synoptic scale]], westward-propagating disturbances in the jet known as African easterly waves or tropical waves. These instabilities, particularly in the presence of moist-convection, cause intense lower-layer cyclonic vortices at the northern flank of the jet.<ref>Rostami, M., Zeitlin, V., 2021, [https://doi.org/10.1080/03091929.2021.1959574 Instabilities of low-latitude easterly jets in the presence of moist convection and topography and related cyclogenesis, in a simple atmospheric model.] Geophysical & Astrophysical Fluid Dynamics, DOI:10.1080/03091929.2021.1959574. </ref>
A small number of mesoscale storm systems embedded in these waves develop into tropical cyclones after they move from west Africa into the tropical Atlantic, mainly during August and September. When the jet is south of normal during the peak months of the [[Atlantic hurricane season]], tropical cyclone formation is suppressed.<ref>{{cite web | url=https://www.cpc.ncep.noaa.gov/products/special_summaries/97_4/fig7.gif | author=Climate Prediction Center | author-link=Climate Prediction Center | title=Figure 7 | accessdate=5 February 2011 | date=November 1997 | publisher=[[National Oceanic and Atmospheric Administration]]}}</ref>
==See also== *[[Saharan Air Layer]] *[[Tropical Easterly Jet]] *[[Somali Jet]]
==References== {{Reflist|2}}
==Further reading== *[[Kerry H. Cook]]: Generation of the African Easterly Jet and Its Role in Determining West African Precipitation [https://journals.ametsoc.org/doi/full/10.1175/1520-0442%281999%29012%3C1165%3AGOTAEJ%3E2.0.CO%3B2] *Holton, James R.: ''An Introduction to Dynamic Meteorology'', 2004. {{ISBN|0-12-354015-1}}
[[Category:Atmospheric dynamics]]