Robust Drying Influence of Mean Ocean Surface Warming on The Sahel and Implications for Constraining Future Rainfall Change

The Sahel is the semi-arid region spanning the Sahara Desert's southern border, and its hydrological state reflects a competition between the drying influences of the adjacent hyper-arid desert and the summer moistening influences of the West African Monsoon circulation. In at least one climate model, mean sea surface temperature (SST) warming with magnitude likely to occur in this century generates severe drying of the Sahel. In order to assess the plausibility of this drying response, we analyze uniform SST warming simulations in this and 16 other atmospheric models, using the moist static energy (MSE) budget to diagnose the underlying physical mechanisms and comparing model fields linked to the Sahel precipitation response with observational values.

In all models, the MSE difference between the Sahel and Sahara increases with warming, and the resulting increased dry advection into the Sahel drives anomalous subsidence. This yields reduced Sahelian rainfall in 14 of 17 atmospheric GCMs analyzed. The three models in which it does not dry are closely related model variants that share a weakly entraining convective parameterization that is largely insensitive to the increased dry air advection. The enhancement of the Sahel-Sahara MSE difference, and with it the Sahel drying response, depends on the Sahel convective depth in the control simulations, and most of the models that dry the Sahel most severely with warming exhibit ascent profiles that are anomalously top-heavy relative to multiple reanalysis products. We discuss the implications of these results and important caveats that complicate efforts to identify quantitative emergent observational constraints on future Sahel rainfall.