Oceanic origin of the precipitation jump in the Sahel

The West African Monsoon region is of great concern due to ecological, economical, and societal impact of the persistent drought. New satellite data and techniques let us combine the effect of the onshore flow at the surface and offshore transport aloft, and show that the depth-integrated moisture advection across the Atlantic coast into the Sahel region is in-phase with the annual and interannual variations of Sahel rainfall, but off-phase with the transport across the northern coast of the Gulf of Guinea and the rainfall just inland from this coastline. The onset of Sahel rainfall is associated with moisture from the Atlantic Ocean more than local instability embedded in the easterly wave or land-use changes. The onset of summer rain in the Sahel occurs in August when the temperature in Atlantic Ocean offshore is lowest and the easterly waves is weakened. The result will provide a new perspective and advance our understanding of the onset of West African monsoonal rainfall and the ability to predict it, which is important to survival in the marginal agriculture of the Sahel.