Forced Sahel rainfall trends in the CMIP5 archive

The simulations of the fifth Coupled Models Intercomparison Project (CMIP5) strengthen previous assessments of a substantial role of anthropogenic emissions in driving precipitation changes in the Sahel, the semi-arid region at the southern edge of the Sahara. Historical simulations can capture the magnitude of the centennial Sahel drying over the span of the 20th century and confirm that anthropogenic forcings have contributed substantially to it. Yet, the models do not reproduce the amplitude of observed oscillations at multi-decadal time scales, suggesting that either oscillations in the forcing or the strength of natural variability are underestimated. Projections for Sahel rainfall are less robust than the 20th century hindcast and outlier projections persist, but overall the CMIP5 models confirm the CMIP3 results in many details and reaffirm the prediction of a rainy season that is more feeble at its start, especially in West Africa, and more abundant at its core across the entire Sahel. Out of 20 models, 4 buck this consensus. Idealized simulations from a sub-set of the CMIP5 ensemble—simulations designed to separate the fast land-atmosphere response to increased greenhouse gases from the slow response mediated through changes in sea surface temperature (SST)—confirm that the direct effect of CO2 is to enhance the monsoon, while warmer SST induce drying over the Sahel. At the same time, these simulations suggest that the seasonal evolution of the rainfall trends in the scenario simulations, spring drying and fall wetting, is an inherently coupled response, not captured by the linear superposition of the fast and slow response to CO2.