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

The paleoclimate record indicates that northern Africa is a region where abrupt climate change has occurred in the past, so it is important to evaluate the potential for such abrupt change in the future under forcing from increasing levels of greenhouse gases. Abrupt change in this region may be associated with abrupt changes in the external forcing factors that the region's rainfall is sensitive to, such as SSTs, or it can arise from 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 simulations in which uniform warm SSTAs of 1K, 2K, 2.5K, 3K and 4K are imposed over the entire 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 within a few days at the end of June and marks the onset of the monsoon. With the gradual increase of prescribed SSTAs from 1K to 2K, rainfall increases over much of West Africa in a pattern similar to today's. But there is a change in the structure as SSTs warm, especially for SSTAs greater than 2.5 K. In particular, the simulated Sahelian rainfall increases for 1 K warming, and then decreases abruptly between the 2.5K and 3K warming experiments as the monsoon onset fails.

The physical processes that cause the failure of the monsoon onset, and the resulting reduction (increase) in Sahel (Guinean coast) rainfall 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 by 2.5K and above. The African easterly jet in the mid troposphere intensifies and moves further south with the warming SSTs. Sinking motion that is typically observed over the Gulf of Guinea is replaced by rising motion when SSTAs are above the threshold level, so convection develops over the Atlantic Ocean and along the coast instead of over the continental interior.