It is known that drought conditions over the Sahel increases the intensity and frequency of dust outbreaks (Prospero and Nees 1986; Swap et al. 1996). Intense and widespread dust within the Saharan Air Layer (SAL), in turn, is found to inhibit convection (on the equatorside of the middle level jet) in conjunction with less perturbed zonal flow (Randall et al., 1984), which may impede the growth of African waves (Karyampudi and Carlson 1988).

The contrasting influence of the SAL on the genesis of tropical cyclones in both the normal and dry Sahel rainfall years is examined in this study by utilizing the European Center for Medium Range Weather Forecasting (ECMWF) gridded data analysis. Two tropical cyclogenesis cases, Hurricanes Andrew and Luis, which formed over the Eastern Atlantic, respectively, during the dry Sahel rainfall year of 1992 and the normal Sahel rainfall year of 1995, are analyzed.

The analysis of H. Luis (1995) indicated that it formed from the merger of the low-level (925 hPa) and mid-level (700 hPa) vortices over the E. Atlantic within the monsoon trough enhanced by surges in the trades. The mid-level vortex evolved from wave troughs enhanced by cyclonic shear vorticity of the mid-level easterly jet (MLEJ) located on the southern edge of the SAL. Vorticity budget calculations suggested that vortex stretching dominated the enhancement of low-level vortex, whereas positive vorticity advection (PVA) on the south and leading edge of the MLEJ but ahead of the trough axis contributed to the development of mid-level vortex. No upper-level anticyclonic circulation appears to have aided in the genesis of Luis. On the other hand, H. Andrew (1992) analysis suggested that it formed from a deep African wave vortex. Vortex stretching contributed to the development of low-level vortex. Although cyclonic shear vorticity to the south of the MLEJ is present, the mid-level contribution from PVA on the southside of the jet to the maintenance of the mid-level vortex is found to be insignificant due to less perturbed easterly flow to the south and ahead of the wave trough. However, the pre-Andrew growth appears to have been dominated by PVA at upper-levels associated with easterly wave perturbations to the south of an anticyclonic circulation center but to the north of an upper-level easterly jet.

It is found that the sign reversal of the meridional gradient of potential vorticity (between 850 and 700 hPa levels) in Luis case satisfied the Charney and Stern (1962) criterion for barotropic and baroclinc instability across the MLEJ, whereas in Andrew's case the instability criterion was not satisfied. Furthermore, it was shown that the baroclinic mechanism, proposed by Karyampudi and Carlson (1988), was able to explain part of the mechanisms involved in the genesis of Luis but not in Andrew. By contrasting the results between these two cases, it is concluded that that SAL had a positive influence on Luis, which occurred in the normal year, in contrast to negative influence on Andrew, which occurred in an extremely dry year.