Favorable Monsoon Environment over Eastern Africa for Subsequent Tropical Cyclogenesis of African Easterly Waves

Abstract Eastern Africa is a common region of African easterly wave (AEW) onset and AEW early life. How the large-scale environment over East Africa relates to the likelihood of an AEW subsequently undergoing tropical cyclogenesis in a climatology has not been documented. This study addresses the following hypothesis: AEWs that undergo tropical cyclogenesis (i.e., developing AEWs) initiate and propagate under a more favorable monsoon large-scale environment over eastern Africa when compared with nondeveloping AEWs. Using a 21-yr August–September (1990–2010) climatology of AEWs, differences in the large-scale environment between developers and nondevelopers are identified and are proposed to be used as key predictors of subsequent tropical cyclone (TC) formation and could inform tropical cyclogenesis prediction. TC precursors when compared with nondeveloping AEWs experience an anomalously active West African monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet. These large-scale conditions are linked to near-trough attributes of developing AEWs that favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, a deeper wave pouch, and arguably more AEW and mesoscale convective systems interactions. AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or West Africa. Developing AEWs are more likely than nondeveloping AEWs to be southern-track AEWs.