Role of cross-equatorial SST gradients and orography in African easterly wave genesis

African easterly waves (AEW) constitute an important component of the African and tropical Atlantic Ocean climate during the boreal summer. An understanding of this important component of the tropical climate is essential since AEW are closely related with tropical Atlantic storm activity. We adopt an idealized modeling approach using the WRF initialized with ERA-40 to study the mechanisms that trigger the formation and maintenance of AEW. The model domain includes the African continent, central and eastern Atlantic Ocean and the Western Indian Ocean Experiments are designed to test the relative importance of the thermal effect of the east African topography and the influence of cross-equatorial pressure gradient, induced by the sea surface temperature (SST) on the origins and maintenance of AEW. Topography and SST variation are selected added and removed. The control experiment shows that the model reproduces many of the mean features observed during the summer Westward propagating disturbances of 3-8 day period that originate between 30 and 40 E at the surface levels and the African easterly jet (AEJ) is well depicted. When all topographic features are removed, there is a weakening of the AEJ over land and ocean, however, longitude - time sections of meridional velocity still exhibit westward propagating disturbances that reach the western African coast at the surface and at the jet level with the same 3-8 day period. Maximum amplitude of the disturbances occurs over the ocean.

When the zonal SST gradient is removed, a weaker AEJ and a weaker monsoon flow ensues. Spectral analysis of meridional velocity displays a variance reduction in the 3 - 8 day band at the 850hP a level in western and eastern Africa. At the 650 hPa level significant changes are not observed at the latitude of the AEJ (15N), however, a decrease in the variance associated with AEW occurs at the southern flank of the jet. In summary, the present study suggests that orography plays an important role in determining the variability of meridional wind associated with AEW over Eastern Africa at the lower levels. Further, zonal SST gradients over the Atlantic favor intensification of waves when they reach the coast and the maintenance of disturbances across the Ocean. Also, results could suggest that SST gradients support genesis of AEW just off the coast of Africa.