Horn of Africa Rainfall Sensitivity to Regional Sea Surface Temperature Forcing in the Indian and Atlantic Oceans

Major features of the tropical atmospheric circulation, averaged over time-scales longer than a month or two, are largely determined by sea surface temperature (SST) variations. This study employs the Abdus Salam International Center for Theoretical Physics (ICTP) version 4.4 Regional Climate Model (RegCM4) to investigate the rainfall response over the Horn of Africa to SST forcing in the Indian and Atlantic Oceans. To isolate the effects of SST forcing and smooth out chaotic internal model variability, 10 individual simulations obtained from random perturbations of the initial and time-varying boundary conditions are analyzed. In these simulations, a constant or linearly zonally varying SST anomaly (SSTA) forcing is prescribed in individual basins while specifying the 1971–2000 monthly varying climatological SSTs across the remaining model domain. The nonlinear rainfall response to SST anomaly strength is also investigated by separately specifying +1K, +2K, and +4K SST anomaly forcing in the Atlantic and Indian Oceans.

The model sensitivity experiments showed that the vertically integrated (1000–600 hPa) meridional moisture flux across the Ethiopian southern boundary generally is more than twice as large in magnitude as the zonal flux, and thus the southerlies from the Indian Ocean are the primary moisture driver for Ethiopian June–September rainfall season. The simulation results also show that warm SSTs over the entire Indian Ocean produce drier conditions across the larger Blue Nile catchment, whereas warming exceeding +2K generates large positive rainfall anomalies exceeding 10 mm·day-1 over drought prone regions of northeastern Ethiopia. However, the June–September rainy season tends to be wetter (drier) when the SST warming (cooling) is limited to either the Northern or Southern Indian Ocean. Wetter rainy seasons generally are characterized by deepening of the monsoon trough east of 40°E, intensification of the Mascarene high, strengthening of the Somali low level jet and the tropical easterly jet, enhanced zonal and meridional vertically integrated moisture fluxes, and steeply vertically decreasing moist static energy. The opposite conditions hold for drier monsoon seasons.