Easterly waves over Africa. II: observed and modeled contrasts between wet and dry years

Differences in the basic state over West Africa between wet and dry years are well documented. This study investigates whether there are also observable differences in the easterly waves between wet and dry years and if these differences might be attributed to the changes in the basic state. Contrasting basic states from the rainy seasons of the four wet years (1958–61) and four dry years (1982–85) were derived from the NCEP reanalysis. The basic states served as input for the linear instability model. The model results indicated faster growth rates and greater phase speeds in the wet years. These results were consistent with a wavelet analysis of the 600-mb meridional wind. This analysis showed that waves were stronger and tended to have a greater contribution from the longer periods during the wet years. The differences in the waves appear to be due to the greater horizontal and vertical shear in wet years. The relative importance of these two were assessed using the Charney–Stern necessary condition for instability. It appears that the horizontal shear is more important in causing the differences. Although the baroclinic and barotropic terms were of similar magnitude in dry years, in wet years the barotropic term increased significantly, whereas the baroclinic term did not. Implications of the results for the understanding of interannual and interdecadal rainfall variability over West Africa are discussed.