The onset of the West African monsoon: A numerical study

The West African monsoon onset is studied using a regional climate model. The nature of the observed abrupt latitudinal shift of the precipitation maximum from the coast of Guinea into the Sahel region, "the monsoon jump", is investigated by performing moisture, momentum and energy budget analysis on the model output. The results suggest that the onset process can be divided into three stages; (i) During the second half of April, warming on the Gulf of Guinea and the subsequent arrival of the ITCZ at the coast suppresses convection in the Sahel region by introducing meridional circulation and advection of dry air from the north. This cools down the mid-troposphere over the Sahel while the boundary layer over land warms up by sensible heating. This builds potential instability. (ii) Cooling over the Gulf of Guinea leads to a decrease in the latitudinal extent of the ITCZ and the associated northerly dry air advection over the Sahel at around mid-May, resulting in the release of the potential instability there. (iii) During the first week of June, condensational heating associated with the release of this instability warms the continental mid-troposphere above a threshold level. Deep convection moves from the coast into the continental interior as a result of inertial instability.