Atmospheric Moisture Transport Associated with the West African Sudan-Sahel Rainfall Variability

Understanding the mechanisms and associated predictability of intraseasonal to seasonal scale rainfall prediction requires accurate estimates of the hydrological cycle and its linkages to regional atmospheric circulation futures. In this study, the origins and transport of water vapor into the West African Sudan-Sahel zone are examined for the summer monsoon rainfall seasons. Vertically integrated fluxes and flux divergences of water vapor are computed for 30 summers from 1979-2008 using NCEP/NCAR reanalysis data. Also investigated are the importance of atmospheric moisture transport for West African rainfall variability and prediction on intraseasonal, annual, and interannual time scales. Our results support the established notion that rainfall in the coastal region of the Gulf of Guinea is associated with the northward migration of the Intertropical Front (ITF) and the accompanying meridional moisture transport from the Gulf of Guinea. Both the synoptic- and climate-scale circulations transport moisture meridionally from the tropical South Atlantic Ocean toward the Gulf of Guinea during the spring (April-May) season, which coincides with the West African monsoon (WAM) onset. The study also demonstrates that the low-frequency climate-scale circulation drives much of the mean zonal moisture transport across the west coast of Africa into the Sahelian zone. However, whereas the mean zonal moisture transport originating from the eastern Atlantic is in phase with the annual cycle of rainfall of the Sahelian region, it is found to be out of phase with rainfall variability in the Gulf of Guinea. This suggests that the two rainfall regimes of the WAM are connected with large scale moisture transport from two different regions in the Atlantic Ocean – the south and east Atlantic Ocean.