Historical Multidecadal Variability of Intraseasonal Features of Sahel Rainfall in CMIP6 Models and Observations

We compare observations with seven large ensembles of CMIP6 historical simulations to understand the role that external forcings and internal variability may have played in modulating the frequency and intensity of daily rainfall, and the duration of the rainy season in the Sahel. We compare three periods: [1950–1970], [1970–1990] and [1994–2014]. The results show that the anomalously wet [1950–1970] and [1994–2014] periods are characterized by more extreme rainfall days and longer monsoon seasons than the drier [1970–1990] period. In exploring the contributions of anthropogenic aerosols (AAs) and greenhouse gases (GHGs) in both transitions, we find results to be consistent with partial cancellation of the effects of AA and GHG in the [1950–1970] to [1970–1990] transition to drought, which allows internal variability to contribute significantly, and with the emergence of the external forcing effect in the [1970–1990] to [1994–2014] recovery. In particular, the shift of AA emissions from North America and Europe to Asia between [1970–1990] and [1994–2014] increases the number of extreme rainy days and lengthens the wet season. The increase in GHG concentrations between these two periods also acts to increase the number of extreme rainy days. However, this effect is not statistically significant in all models. Finally, in all the models analyzed, we cannot discard the fact that the individual effects of AAs and GHGs add up linearly in setting the total precipitation change.