Water vapour transport over Southern Africa during wet and dry early and late summer months



Southern Africa is semi-arid to arid, and the moisture that contributes to rainfall over the summer rainfall region is largely imported from other areas. Interseasonal and interannual variations in rainfall must result from changes in the circulation and in vapour fluxes over the subcontinent. It is shown that important changes in vapour fluxes occur between October (early summer) and January (late summer), with zonal fluxes being more important in October and meridional fluxes in January. Wet and dry months of October and January are characterized by enhanced zonal (meridional) flow in wet Octobers (Januaries), and reduced importance of these flows in the dry months. Adjustments in the areas of vapour flux convergence and divergence are as important as changes in the fluxes. The convergence and divergence changes between wet and dry months are confirmed by decreases in outgoing longwave radiation over central southern Africa in wet months and increases in dry months. Tropical zonal circulations, and tropical—subtropical meridional circulation cells, conform to the patterns of enhanced (reduced) convection over the central subcontinent in wet (dry) months.