Rainfall Recovery in the Sahel: the Roles of Storm Frequency, Intensity, and Efficiency.

During the recovery from the depths of the 1980s drought, extreme rainfall in the Sahel has increased faster than the mean seasonal rainfall, so that currently more than a third of the summer rainfall falls in the form of deluges.

Tracking storms in a combination of satellite-based rainfall and emission temperature data shows that, from the early 1980s to the late 2000s, the increase in extreme rainfall fraction was the consequence of the strongest storms becoming more frequent, likely because the enhanced warming of the Sahara produced enhanced thermal wind and wind shear, which better supported the development of well-organized Mesoscale Convective Systems with very intense convective towers.

However, since the late 2000s, the number of strong storms has plateaued:  warming continued to intensify in the Sahara, but the anomalous temperature gradient moved north, leading to reduced shear in the key storm development region. The intensity of the strongest convection, as measured by the coldness of the cloud tops, also stopped increasing.

Yet, extreme rainfall has kept increasing apace. We interpret this as a thermodynamic effect: influenced by warming ocean waters in the tropical Atlantic and the Mediterranean, moisture levels have been rising throughout the region and throughout the depth of the atmosphere, leading to heavier rainfall being produced by the same convective intensity.