Séminaire

The semi-arid African Sahel has received unique attention in the climate science community and beyond since inception of persistent drought at the end of the 1960s. The hypothesis of a positive bio-geophysical feedback tied human-induced baring of the soils to a reduction of precipitation, which further exacerbated the loss in vegetation cover. In the more recent ~10 years the climate of the Sahel has again emerged as the focus of active research, this time as a possible "canary in the coal mine" for anthropogenic climate change. Advances in climate science have first conclusively tied persistent drought to subtle shifts in the surface temperature of the global oceans, "freeing farmers of blame" in the drought, then partially attributed these shifts to the influence of aerosols and greenhouse gases.

Here I present an interpretation for the role of the oceans in attributing precipitation change based on the most recent CMIP multi-model ensembles of simulations of historical and projected variations. In these I show that Sahel rainfall responds to the relative temperature of the North Atlantic, source of the moisture that converges in the region, with respect to the global tropical oceans. The temperature of the global tropical oceans, which is communicated first vertically through deep convection, then laterally by atmospheric waves, broadly determines the threshold for  convection.
The temperature of the North Atlantic relative to that of the global tropical oceans measures the potential for the moist, but cool air that is converged onto the African continent from the adjacent ocean to lead to deep convection and precipitation. This interpretation consistently explains past drought and sheds light on the uncertainty in future projections. This contribution aims to frame the physical context in which to discuss societal response to drought, and its applicability to adaptation to current variability and future change.

serge.janicot@locean.upmc.fr