A numerical study of moisture build-up and rainfall over West Africa

Empirical studies have shown that a necessary condition for abundant rainfall in any year at Kano in Nigeria is a pre-rainy season moisture build-up for 6-8 weeks at the surface or below 800 hPa. The International Centre for Theoretical Physics (ICTP) regional climate model (RegCM3) is used to study the mechanism and variability of the inflow of this moisture over West Africa over a 21-year period (1980-2000). The study focused on three dry (1983, 1989 and 1990) and three wet years (1980, 1988 and 1996) and uses rainfall and other surface data such as specific humidity, over Nigeria for the same years for model validation. Comparison of RegCM3 simulation of rainfall and moisture patterns over West Africa with both CRU and Nigerian stations observations shows that the model performed very well with respect to the observations over the region, particularly over the Sahel and Savana zones. The composites of moisture inflow and wind anomalies for the wet years are found to be associated with areas of boundary layer moisture build-up, overlain by relatively drier (but still moist) air driven by strong west to southwest wind vector anomalies at 700 hPa (African Easterly Jet, AEJ, level). These strong wind anomalies, which occur south of the AEJ between May and September and transport moisture towards it, seem unconnected with the boundary layer winds and are not found in the dry composites. In sharp contrast, the dry composites are characterized by negative moisture anomalies in the boundary layer overlain by drier than normal air associated with northerly to easterly wind anomalies at the AEJ level. Secondly, while the ITD reached the same northernmost limit of about 20 degrees N in August during both dry and wet periods, the AEJ advanced faster across West Africa, reaching a mean position of 16.5 degrees N in August during wet years but 15 degrees N in dry years. Also, on a monthly and inter-annual basis, moisture adequacy and variability analysis show that, for dry years, moisture build-up is delayed but starts early and strongly during wet years. Furthermore, rainfall does not begin properly until about 2.5 months after the start of the moisture build-up. Finally, RegCM3 predicted rainfall correlates very well (r >= 0.6) with observed rainfall for stations within Nigeria. Copyright (C) 2007 Royal Meteorological Society.