On the detection of externally-forced decadal modulations of the Sahel rainfall over the whole 20th century in the CMIP6 ensemble.

International audience ; Abstract The Sahel semiarid region was marked during the 20th century by significant modulations of its rainfall regime at the decadal timescale. 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 20th Century using a set of twelve (12) models from the sixth Coupled Model Intercomparison Exercise (CMIP6). We begin by proposing a physically-based definition of Sahel rainfall that takes into account the southward hias 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 show that anthropogenic aerosol dominate the decadal modulations of Sahel rainfall simulated by this model, and suggest that at least a part of the impact is ocean-mediated and operated through shifts in the ITCZ and the Saharan Heat Low.