Diabatic heating, divergent circulation and moisture transport in the African Monsoon System

Field properties of divergent circulation are utilized to identify the roles of various diabatic processes in forcing moisture transport in the African Monsoon System. In this study, the divergence field from global reanalyses is treated as a set of point sources and is partitioned into two sub-sets corresponding to latent heat release and surface sensible heat flux at each respective point. The divergent circulation associated with each set is then calculated from the Poisson's equation using Gauss-Seidel iteration. Moisture transport by each set of divergent circulation is subsequently estimated.

The surface sensible heating gradient between the Saharan heat low and Gulf of Guinea drives a shallow meridional overturning circulation (extending from the surface to 700 hPa) whose southerly lower branch transports moisture deep into the continent and thereby promotes the seasonal northward migration of monsoon precipitation during boreal spring. In contrast, the overturning circulation directly associated with latent heating is deep (surface to the tropopause) and the corresponding boundary-layer moisture convergence maximum is within the region of precipitation. Latent heating also induces dry air advection from the north. The seasonal northward migration of precipitation is encouraged by neither of the two effects, even though the latent-heating induced moisture convergence feeds back positively to local monsoon precipitation. This technique also enabled the isolation of effects divergent circulation forced by latent heating over the remote regions on local moisture distribution through advection. Specifically by bringing Saharan air from the north, and driving moisture to the adjacent oceans, global latent heating has an overall drying effect over the Sahel.