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North Carolina State University (2009)

Isentropic Descent beneath the Saharan Air Layer and its Impact on Tropical Cyclogensis

Diaz, Michael

Titre : Isentropic Descent beneath the Saharan Air Layer and its Impact on Tropical Cyclogensis

Auteur : Diaz, Michael

Université de soutenance : North Carolina State University

Grade : Master of Science 2009

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 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. 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 trajectory analysis of air within 191 AEW cases to determine dominant air mass source regions. We ﬠnd 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 cyclogenesis. Based on a series of numerical modeling case studies, we ﬠnd that warming (cooling) SST north of 15â—¦ N along the African coast increases (decreases) the probability that an AEW will become a tropical cyclone.

Mots-clés : Saharan Air Layer ; African Easterly Wave ; tropical cyclone


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Page publiée le 2 mai 2011, mise à jour le 9 octobre 2018