Large-Scale Stability and the Greater Horn of Africa Long and Short Rains

The societies of the coastal regions of the Greater Horn of Africa (GHA) experience two distinct rainy seasons: the generally wetter "long" rains in the boreal spring and the generally drier "short" rains in the boreal fall. The GHA rainfall climatology is unique for its latitude in both its aridity and for the dynamical differences between its two rainy seasons. This study explains the drivers of the rainy seasons through the climatology of moist static stability, estimated as the difference between surface moist static energy hs and midtropospheric saturation moist static energy h*. In areas and at times when this difference, hs -h*, is higher, rainfall is more frequent and more intense. However, even dur -ing the rainy seasons, hs -h* , 0 on average and the atmosphere remains largely stable, in line with the GHA's aridity. The seasonal cycle of hs -h*, to which the unique seasonal cycles of surface humidity, surface temperature, and midtropospheric temperature all contribute, helps explain the double-peaked nature of the regional hydroclimate. Despite tropospheric temperature being relatively uniform in the tropics, even small changes in h* can have substantial impacts on instability; for example, during the short rains, the annual minimum in GHA h* lowers the threshold for convection and allows for instability despite surface humidity anomalies being relatively weak. This hs -h* framework can help identify the drivers of interannual variability in GHA mean rainfall or diagnose the origin of biases in climate model simulations of the regional climate.