African easterly jet

Formation and characteristics

During January, the African easterly jet lies at 3000 m above sea level at five degrees north latitude. Winds within it increase in speed from 30 km/h in January to 40 km/h in March. Shifting northward in April to the seventh parallel, winds within the jet increase to 45 km/h. By June, it shifts northward into northwest Africa.

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. 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 50 km/h between the 12th and 13th parallels. The easterly jet weakens and drops southward during October and November.

Impact

The mid-level African easterly jet is considered to play a crucial role in the West African monsoon, and helps form the tropical waves that move across the tropical Atlantic and eastern Pacific oceans during the warm season. 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.

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.

References

References

1. Marcel Leroux. (2001). "The Meteorology and Climate of Tropical Africa". Springer.
2. 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.
3. 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.
4. [[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.
5. 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.
6. Climate Prediction Center. (November 1997). "Figure 7". [[National Oceanic and Atmospheric Administration]].