Isentropic descent beneath the Saharan Air Layer and its impact on tropical cyclogenesis

We investigate the driving mechanism behind strong climatological isentropic descent in the eastern Atlantic and how it affects tropical cyclogenesis from African Easterly Waves (AEW). Our results suggest that this isentropic descent is forced by the warm thermal structure associated with the Saharan Air Layer (SAL) combined with northerly flow on the eastern flank of the Azores high. Since this northerly flow descends from the drier middle troposphere at higher latitudes to the lower troposphere at lower latitudes, it provides a nearly continuous source of dry air off the West African coast. Thus, AEWs traveling south of the SAL often ingest dry air from the middle latitudes into their circulation. Once ingested by an AEW, this dry air mass may suppress the moist convection required for tropical cyclogenesis. Although this process is intimately linked with the SAL, the air mass involved is distinctly different; it originates from the middle latitudes and travels beneath the SAL. In contrast, previous research emphasizes the direct negative impacts of the SAL itself on tropical cyclogenesis and concentrates primarily on how strong vertical wind shear, dry mid-level air, and high static stability suppress AEW convection. In this study, we use the Global Forecast System (GFS) analyses from 2000 to 2008 to perform a back trajectory analysis of air within 191 AEW cases to determine the dominant air mass source regions. We find that AEWs contain a large fraction of low level air mass which has undergone isentropic descent along the African coast. Our results suggest that AEWs containing larger amounts of this air mass tend to be associated with weaker convection and a lower probability of tropical cyclogenesis. However, we also note that, since this isentropically descending air mass occurs beneath the SAL, its negative impacts often occur in conjunction with those investigated in earlier studies of the SAL's influence on AEWs and tropical cyclones. Nevertheless, our results suggest that climatological isentropic descent in the eastern Atlantic may have important consequences for tropical cyclogenesis and that further isolating its specific role may be an important topic for future research.