ASSESSMENT OF TROPOSPHERIC CIRCULATION FEATURES DURING WET AND DRY YEARS OVER WEST AFRICA

Abstract

The most important climate variable over West Africa is rainfall, where many sectors of the economy depend on water resources. In this study we assess the tropospheric circulation features during wet and dry years over West Africa. Annual rainfall data from 1950 to 2012 was used in this study to ascertain the wet and dry years, using Standardized Precipitation Index (SPI) plots. Ten wet years (1958 – 1967) and dry years (1979 – 1988) were singled out and used in analysis. Zonal and meridional components of the monthly mean wind are computed directly with European Centre for Medium-range Weather Forecasts (ECMWF) data for the main rainy season of June, July, August, and September. The moisture variables and equivalent potential temperature are also examined for the periods. Results confirm that the African Easterly Jet (AEJ) is stronger during the dry years than in the wet years while the Tropical Easterly Jet (TEJ is stronger during the wet years than in the dry years. Significantly, however, it is shown that during the wet years, there is stronger and deeper early season (April–June) build-up of moisture below the AEJ. There is also more moisture advection in the wet years than dry years. The value of equivalent potential temperature is high in the wet years compared to the dry years at the surface depicting that more moisture and instabilities is experienced in the wet years compared to the dry years resulting to more precipitation in the wet years. Finally, it is evident that the convective process over the West Africa is dynamically influenced by activities at the surface and that of the 700 – 600mb level. The results from this study serve as bases for further work on the tropospheric circulation features over the region of West Africa for an enhance understanding of the regional climate and future forecast of extreme events.