Analysis of Low-Level Atmospheric Moisture Transport Associated with the West African Monsoon

The major objective of this study is to re-evaluate the ocean-land transport of moisture for rainfall in West Africa using 1979-2008 NCEP-NCAR reanalysis data. The vertically integrated atmospheric water vapor flux for the surface-850 hPa is calculated to account for total low-level moisture flux contribution to rainfall over West Africa. Analysis of mean monthly total vapor fluxes shows a progressive penetration of the flux into West Africa from the south and west. During spring (April-June), the northward flux forms a "moisture river" transporting moisture current into the Gulf of Guinea coast. In the peak monsoon season (July-September), the southerly transport weakens, but westerly transport is enhanced and extends to 20 degrees N owing to the strengthening West African jet off the west coast. Mean seasonal values of total water vapor flux components across boundaries indicate that the zonal component is the largest contributor to mean moisture transport into the Sahel, while the meridional transport contributes the most over the Guinea coast. For the wet years of the Sahel rainy season (July-September), active anomalies are displaced farther north compared to the long-term average. This includes the latitude of the intertropical front (ITF), the extent of moisture flux, and the zone of strong moisture flux convergence, with an enhanced westerly flow. For the dry Sahel years, the opposite patterns are observed. Statistically significant positive correlations between the zonal moisture fluxes and Sudan-Sahel rainfall totals are most pronounced when the zonal fluxes lead by 1-4 pentads. However, although weak, they still are statistically significant at lags 3 and 4 for meridional moisture fluxes.