Homepage: https://www.albany.edu/atmos/christopher-thorncroft.php

This presentation will provide a review or the nature and causes of African easterly waves (AEWs) including presentation of some recent results concerned with the variability of AEW structures. The structural variability of African easterly waves (AEWs) is explored using an empirical orthogonal function (EOF) approach to extract the dominant modes. The structure of AEWs is obtained by projecting the wind fields from reanalysis data and satellite-derived brightness temperature (T_b) onto the principle 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 confined south of the African easterly jet. It shares many characteristics with most AEWs previously analysed and discussed in the literature. In contrast, the v700 EOF exhibited less familiar characteristics. The convective patterns are characterized by a “checkerboard” pattern 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 former has a modified MRG structure in the southern portion of the wave and the inseparable nature of the AEW-MRG hybrid confirms the prevalence of such a structure. The latter interaction brings dry air from the subtropical region in tandem with the northern vortex. The two contrasting circulations of AEWs captured by the two kinds of EOFs are also associated with different precipitation patterns. This study highlights the AEW interaction with the large-scale environment and with other synoptic waves. Discussions and implications of such AEW behaviors are provided. Some brief comments will also be included that discuss the relationship between AEWs and downstream tropical cyclogenesis including some recent results.