A Study of the Dynamic Factors Influencing the Rainfall Variability in the West African Sahel

Abstract

This study examines selected dynamical factors associated with wet and dry years in the West African Sahel. The approach is to evaluate the temperature, wind, and moisture fields and the dynamic instabilities for a 4-yr?wet? composite (1958?61) and a 4-yr ?dry? composite (1982?85). The analysis, limited to the months of June through September, is carried out using the National Centers for Environmental Prediction?National Center for Atmospheric Research (NCEP?NCAR) 40-Year Reanalysis Project dataset. Two upper-air datasets are also evaluated to confirm trends apparent in the NCEP?NCAR data. This study confirms some of the results of earlier studies, such as the weaker African easterly jet (AEJ) and stronger tropical easterly jet (TEJ) during wet years, but suggests a different interpretation of the wet?dry contrasts. In the Sahel, the most important characteristic of the AEJ appears to be its latitudinal location rather than its intensity. This governs the instability mechanisms. The AEJ is displaced northward during the wet years, thereby enhancing both the horizontal and vertical wind shear over the Sahel. Baroclinic instability is probably the dominant mechanism here. South of the Sahel, both the location and intensity of the AEJ appear to be important, and the dominant instability mechanism appears to be barotropic. Results presented here further suggest that equatorial westerlies significantly modulate interannual variability. These serve to displace the jet and associated disturbances poleward and to enhance convective activity. The TEJ may also play an important role by promoting ascent in the lower troposphere on the cyclonic side of the AEJ, where the disturbances develop. Contrasts in the moist layer between wet and dry years may be consequences of these other changes. On an interannual basis, the rainfall maximum and the surface position of the ITCZ are effectively decoupled, with the ITCZ keeping a relatively stable location from year to year despite large latitudinal shifts in the rain belt. As a consequence, the length of the rainy season does not change markedly. Instead, wet years in the Sahel are characterized by more intense rainfall (mm month?1). This is linked to a northward shift in the belt of maximum rainfall, which is coincident with the northward shift in the AEJ.