Forced modulations of Sahel rainfall at decadal timescales over the20th Century using CMIP6 models

During the 20th Century, Sahel rainfall has experienced strong variability at decadal timescales, partly attributed to the internal variability of the climate system, mediated by changes in oceanic sea surface temperature (SST). However, a stronger emphasis has been more recently given to the role of external forcing. Thus, the attribution of past decadal modulations of Sahel rainfall is still under debate.

In this study, we propose a diagnostic of the contribution of external forcing to observed Sahel rainfall decadal modulations based on a correlation analysis. We apply it to six models of the sixth coupled model intercomparison project (CMIP6) in the whole 20th Century. Our results show that external forcings induce a weak amplitude of Sahel rainfall modulations in the models and these modulations are in general insignificantly correlated with the observed modulations. There are two notable exceptions: IPSL-CM6A-LR and INM-CM5-0 models.

With the IPSL-CM6-LR model, our results show that this correlation primarily arises from the role of anthropogenic aerosols. This effect is partly explained via the ocean mediated mechanism in particular the North Atlantic ocean and Mediterranean sea. In CanESM5 and CNRM-CM6-1 models, the Sahel rainfall decadal forced modulations are also dominantly due to anthropogenic aerosols, although not significantly correlated with the observations. In addition, the greenhouse gases (GHG) contribute significantly to the forced response of these models, which could explain partly the insignificant correlation of these CMIP6 models.