The annual cycle of the zonal wind over equatorial and West Africa is described. Monthly NCEP reanalysis and radiosonde data are utilized. The region features three jets, the mid-tropospheric African Easterly Jets of the northern and southern Hemisphere (NAEJ and SAEJ) and the 200mb Tropical Easterly Jet (TEJ). These are described in terms of their varying strength, location and their horizontal and vertical shears. In addition the differences between the annual cycles of wet and dry years in the region are examined.
The jets are caused by the local meridional temperature gradients. Because the source of the gradients are different for the AEJs and the TEJ, their temporal evolution is also distinct. The NAEJ is discernable throughout the year. During the October to April dry season it is located around 0-5N and is relatively weak. It strengthens during the rainy season and also moves some 10-15 degrees to the north. The SAEJ is most clearly defined from September through November, when it is located at around 7S, but is is not evident during March and April. The TEJ has a strong semi-annual cycle. It has maxima at around 8S in February and around 8N in August. However it appears not to be present at all during some transition months.
Annual cycles of horizontal and vertical shear reveal peaks around the time of maximum jet strength. The annual cycles display systematic differences between wet and dry years. In particular, in the wet years the NAEJ is weaker and more poleward between March and October. However, the low level westerly flow tends to be stronger and deeper. This is important in increasing horizontal shear at 600mb, particularly during the late rainy season