Understanding Inter-Model Variability in Future Projections of a Sahelian Storm Proxy and South Saharan Warming

Abstract

Projected changes in the intensity of severe rain events over the North African Sahel – falling from large mesoscale convective systems – cannot be directly assessed from global climate models due their inadequate resolution and parameterization of convection. Instead, the large-scale atmospheric drivers of these storms must be analyzed. Here we study changes in meridional lower tropospheric temperature gradient across the Sahel (ΔTGrad), which affect storm development via zonal vertical wind shear and Saharan air layer characteristics. Projected changes in ΔTGrad vary substantially amongst models, adversely affecting planning decisions that need to be resilient to adverse risks, such as increased flooding. This study seeks to understand the causes of these projection uncertainties and finds three key drivers. The first is inter-model variability in remote warming, which has strongest impact on the East Sahel, decaying towards the West. Second – and most important – a warming-advection-circulation feedback in a narrow band along the South Sahara varies in strength between models. Third, variations in South Saharan evaporative anomalies weakly affect ΔTGrad, although for an outlier model these are sufficiently substantive to reduce warming here to below that of the global mean. Together these uncertain mechanisms lead to uncertain South Saharan / North Sahelian warming, causing the bulk of large inter-model variations in ΔTGrad. In the South Sahel, a local negative feedback limits the contribution to uncertainties in ΔTGrad. This new knowledge of ΔTGrad projection uncertainties provides understanding that can be used, in combination with further research, to constrain projections of severe Sahelian storm activity.