Observation of oceanic origin of Sahel precipitation from space

The West African Monsoon region is of great concern due to the negative ecological, economical, and societal impacts of the persistent drought. Our results support the established postulation that the onset of summer rain in the southern region is associated with the migration of the inter-tropical convergence zone and in phase with the meridional moisture transport from the Gulf of Guinea. We present recent satellite measurements that illustrate the phase shift of the precipitation in the northern region, the Sahel, with respect to the region to the south. These satellite data and associated analysis techniques let us combine the effect of the onshore flow at the surface and offshore transport aloft to demonstrate with sufficient clarity that the depth-integrated moisture advection across the west coast, from the open Atlantic Ocean into the Sahel region, is in-phase with the annual and inter-annual variations of Sahel rainfall, but out-of-phase with rainfall in the southern region around the Gulf of Guinea. The peak Sahel rainfall is associated with the short-lived net moisture influx from the Atlantic, which may interact with local instability related to the African Easterly Waves. The data show two precipitation regimes in Western Africa Monsoon region associated with moisture input from two different regions of the ocean.