Towards constraining future rainfall in the Sahel using the moist static energy budget

Abstract:

Wet-season rainfall in the Sahel varies dramatically among climate model future projections. Its behavior is usefully characterized by the vertically integrated moist static energy (MSE) budget, which stipulates a balance between the net top of atmosphere radiative flux (R_toa) and the divergence of MSE by convection and by horizontal advection. We analyze this budget in multiple general circulation models in present-day and idealized warming experiments.

All models simulate a climatological leading-order budget wherein R_toa is balanced by horizontal advection of low MSE air from the Sahara, rather than by convection as in classical theories. Convection transitions from net divergence to convergence of MSE moving northward as the convective depth steadily lowers, but its mean height over the Sahel varies appreciably among models. This model variation imprints on the hydrological and energetic responses to global warming, as the resultant stabilization of the tropical upper troposphere inhibits convection more in models with climatologically deeper convection. Meanwhile, the semi-arid nature of the region's land permits a simple approximation of the change in R_toa solely in terms of climatological radiative and hydrological properties and the change in precipitation. Taken together, these point towards meaningful constraints on future Sahelian rainfall purely in terms of present-day quantities, and we discuss the prospects of using this framework to ultimately falsify model projections.