The seasonal cycle of the African easterly jet (AEJ) is very important, since it determines the nature of the African easterly waves (AEWs) and tropical squall lines. Despite its importance, the seasonal cycle of the AEJ has been understudied.
We focus on the idea that the AEJ is diabatically forced. Deep moist convection and upper level diabatic heating take place in the inter-tropical convergence zone (ITCZ), situated to the south of the AEJ. Shallow dry convection and intense low level diabatic heating take place in the Saharan heat low (SHL), situated to the north. As earlier studies have pointed, both of the above convective areas are sources for thermally direct circulations and potential vorticity (PV) anomalies. A PV maximum on the southern flank of the AEJ is the response to the ITCZ convection, whereas a PV minimum on the northern flank of the AEJ is the response to the dry convection in the SHL. The resulting negative meridional PV gradient in the mid-troposphere is consistent with the AEJ.
We use ECMWF re-analysed data and TAMSAT satellite data to study the seasonal cycle of the AEJ from May to October. The results from this study show that the AEJ is indeed a combined effect of the ITCZ and the SHL. Intense low level diabatic heating and dry convection in the SHL are more important early in the season, as the height of the sun and the surface dryness are maximum then. However, upper level diabatic heating and moist convection in the ITCZ are more important during the rainy Sahelian season (August - September), when the West African monsoon maximizes and the ITCZ moves north