Synoptic-Scale Influence of the Saharan Air Layer on Tropical Cyclogenesis over the Eastern Atlantic

Mohan Karyampudi, V.; Pierce, Harold F.
December 2002
Monthly Weather Review;Dec2002, Vol. 130 Issue 12, p3100
Academic Journal
The formations of Hurricane Andrew, Tropical Storm Ernesto, and Hurricane Luis, which occurred, respectively, during the 1992, 1994, and 1995 hurricane seasons over the eastern Atlantic, have been investigated by utilizing the European Centre for Medium-Range Weather Forecasts (ECMWF) gridded data analyses. These cases were selected to illustrate the contrasting influences of the Saharan air layer (SAL) on tropical cyclogenesis. Analyses results show that Tropical Storm Ernesto (1994) and Hurricane Luis (1995) formed from the merger of the low-level (925 hPa) and midlevel (700 hPa) vortices over the eastern Atlantic within the monsoon trough enhanced by surges in the trades. Midlevel vortices associated with each case appear to evolve from African wave troughs enhanced by cyclonic shear vorticity of the midtropospheric jet, which existed to the south of an SAL anticyclonic eddy as an elongated wind maximum. Vorticity budget calculations suggest that vortex stretching dominated the enhancement of low-level vortices, whereas positive vorticity advection (PVA) on the south and leading edge of the midlevel easterly jet (MLEJ) but ahead of the trough axis contributed to the enhancement of midlevel vortices for both cases. Persistent upper-level divergence associated with an anticyclonic circulation appears to have aided in the formation of Ernesto, whereas for Luis, no such prior forcing is evident. Hurricane Andrew (1992), on the other hand, appears to form from a deep African wave vortex. Vortex stretching contributed to the development of low-level vortices. Although cyclonic shear vorticity to the south of the MLEJ is present in association with a deeper and wider SAL devoid of its characteristic anticyclonic eddy (unlike in Ernesto and Luis), the midlevel contribution from PVA on the south side of the jet to the maintenance of the midlevel vortex is found to be insignificant in Andrew due to negligible cross-(vorticity) contour flow to the south and ahead of...


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