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|Title: ||Isentropic Descent beneath the Saharan Air Layer and its Impact on Tropical Cyclogensis|
|Authors: ||Diaz, Michael|
|Advisors: ||Dr. Lian Xie, Committee Member|
Dr. Anantha Aiyyer, Committee Member
Dr. Fredrick Semazzi, Committee Chair
|Keywords: ||Saharan Air Layer|
African Easterly Wave
|Issue Date: ||24-Nov-2009|
|Discipline: ||Marine, Earth and Atmospheric Sciences|
|Abstract: ||We investigate the driving mechanism behind strong climatological isentropic descent in the eastern Atlantic and how it aÃ¯Â¬â‚¬ects 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 Ã¯Â¬â€šow on the eastern Ã¯Â¬â€šank of the Azores high. Since this northerly Ã¯Â¬â€šow 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 oÃ¯Â¬â‚¬ 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 diÃ¯Â¬â‚¬erent; 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 Ã¯Â¬Â nd 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 Ã¯Â¬Â nd that warming (cooling) SST north of 15Ã¢â€”Â¦ N along the African coast increases (decreases) the probability that an AEW will become a tropical cyclone.|
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