Two Contrasting African Easterly Wave Behaviors

The dominant structural variability of African easterly waves (AEWs) is explored using an empirical orthogonal function (EOF) approach. The structure of AEWs is obtained by projecting the wind fields from reanalysis data and satellite-derived brightness temperature T-b onto the principal components associated with EOF patterns of filtered T-b (T-b EOF) and 700-hPa meridional wind (v700 EOF). The wave structure depicted by the T-b EOF has confined convection and circulation mostly south of the African easterly jet. It shares many characteristics with AEWs analyzed and discussed in the literature. In contrast, the v700 EOF exhibits less familiar characteristics and includes interactions with the equatorial and subtropical regions. The convective patterns are characterized by a "checkerboard'' pattern of convection that has not been emphasized before. The most striking feature is the broad meridional extent, which depicts interactions with a mixed Rossby-gravity wave (MRG) in the equatorial region and interactions with the basic-state potential vorticity in the subtropics. The southern portion of the wave has a modified MRG structure, and this AEW-MRG hybrid cannot be separated using the EOF technique, indicating the prevalence of such structures. The subtropical interaction at mid-to lower levels establishes a vortex off the coast of Morocco that results in dry-air advection into the tropics in tandem with the northern vortex. At upper levels, a subtropical wave train is induced by the AEW-associated convective inflow and outflow. The contrasting AEW circulations are associated with differences in the precipitation rates and patterns over Africa. These results highlight the variability of AEW structures and their interactions with equatorial and subtropical waves.