The potential for an abrupt failure of the West African monsoon as climate warms

Paleo-proxy evidence suggests that the West African monsoon is susceptible to abrupt climate change. The change may be associated with the changes in external forcing factors, such as Atlantic SSTs, or nonlinear processes within the monsoon system in the presence of smooth external forcing. Here we investigate the potential for abrupt change associated with the internal dynamics of the West African monsoon system using a series of idealized regional model (WRF v. 3.1.1) simulations in which uniform warm SSTAs of 1K, 1.5K, 2K, 2.5K, 3K and 4K are imposed over the Atlantic Ocean in the regional model domain. The control model climate validates well, including a good simulation of the West African monsoon jump, a shift in rainfall from the Guinean Coast to the Sahel that occurs over a few days in July and marks the onset of the monsoon season in the Sahel. With 1K and 1.5K SSTAs prescribed in the Atlantic rainfall increases by about 25% over West Africa, including the Sahel. But there is a change in the sign of the Sahel precipitation anomaly as SSTs warm. With 2K warming Sahel rainfall decreases by about 30%, and with 2.5K Atlantic warming rainfall is reduced by 75%. The physical processes that cause the nonlinear response of Sahel precipitation to Atlantic Ocean warming, and the reasons that a threshold SSTA occurs between 1.5K and 2K, are investigated. The low level southwesterly flow over the Gulf of Guinea that brings moisture into the southern Sahel is diverted farther west when SSTs are warmed above 2K. The moist westerly flow present in the control and with Atlantic warming below 1.5K over the Sahel turns into easterly flow when the Atlantic is warmed to 2K and above. Associated with the increase in Atlantic SSTAs, geopotential heights decrease in the lower atmosphere. The geopotential height gradient over the Sahel at 1K and 1.5K warming is similar to that in the control simulation, but it increases abruptly at when Atlantic warming reaches 2K and higher.