Interactions Between Vegetation and the Dynamics of the West African Monsoon: A Mechanism for Abrupt Climate Change in the Sahel and Sahara

Interactions between vegetation and the dynamics of the West African summer monsoon are studied using a regional climate model that produces an excellent representation of today's North African summer climate, and a mechanism that produces abrupt climate change is identified. Substantiation for the operation of this mechanism is found in the evidence of rapid climate change in this region during the Holocene. The atmosphere and vegetation distribution are strongly coupled through the dependence of the dynamics of the African easterly jet (AEJ) on soil moisture, and through the dynamics of the low-level westerly flow from the tropical Atlantic. A present day simulation and five integrations with prescribed static idealized zonal vegetation distributions, in which the southern Saharan desert border is imposed at 10.0°N, 15.5°N, 17.9°N, 19.4°N, and 20.9°N, are conducted. The regional model simulates small changes in the strength of the AEJ and low-level westerly jet and the precipitation field when the desert border is located at 10.0°N, 15.5°N and 17.9°N. However, when the desert border is located at 19.4°N, the AEJ weakens by 50% and the low-level westerly jet strengthens by 20%. The southerly winds from the Gulf of Guinea are relatively unchanged. Intense positive summer precipitation anomalies develop over the Sahel and Sahara and interrupt the zonal structure of the present day climate. A deep moist thermal low replaces the dry shallow thermal low observed in the present climate. Moist static energy analysis shows that when the desert border is located north of the threshold latitude, it is the positive soil moisture anomalies beneath the region of upward vertical velocity associated with the thermal low that support enhanced convection. Asynchronous coupling with a simple vegetation model produces a new equilibrium vegetation distribution in which the central Sahara is vegetated when the desert border is initially at 20.9°N, and suggests that atmospheric conditions would allow vegetation regrowth when the desert border is initially at 10.0°N.