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 travels from the drier middle troposphere at higher latitudes to the lower
... e 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. Being dry, this 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 negatives impact 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 tropical cyclone convection. In this study, we use the Global Forecast System (GFS) analyses from 2000 to 2008 to perform a back trajec-tory analysis of air within 191 AEW cases to determine 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 have weaker convection and a lower probability of tropical cyclogenesis. We then investigate the role of sea surface temperature along the northwest African coast north of where AEWs track in moistening the dry air from isentropic descent and thus counteracting its inhibiting impact on convection and tropical cyclo-genesis. Based on a series of numerical modeling case studies, we find that warming (cooling) SST north of 15 • N along the African coast increases (decreases) the probability that an AEW will become a tropical cyclone.