On the detection of externally forced decadal modulations of the Sahel rainfall over the whole twentieth century in the CMIP6 ensemble

Abstract

The Sahel semiarid region was marked during the twentieth century by significant modulations of its rainfall regime at the decadal time scale. Part of these modulations have been associated with the internal variability of the climate system, linked to changes in oceanic sea surface temperature. More recently, several studies have highlighted the influence of external forcings during the dry period in the 1980s and the recovery around the 2000s. In this work we evaluate the internally and externally driven decadal modulations of Sahel rainfall during the entire twentieth century using a set of 12 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). We begin by proposing a physically based definition of Sahel rainfall that takes into account the southward bias in the location of the Sahelian ITCZ simulated by all the models. Our results show that the amplitude of the decadal variability, which is underestimated by most models, is mainly produced by the internally driven component. Conversely, the external forcing tends to enhance the synchrony of the simulated and observed decadal modulations in most models, providing statistically significant correlations of the historical ensemble mean with observations in 1/3 of the models, namely IPSL-CM6A-LR, INM-CM5-0, MRI-ESM2-0, and GISS-E2-1-G. Further analysis of the detection and attribution runs of the IPSL-CM6A-LR shows that anthropogenic aerosol dominate the decadal modulations of Sahel rainfall simulated by this model, suggesting that at least a part of the impact is ocean-mediated and operated through shifts in the ITCZ and the Saharan heat low.