The representation of dry-season low-level clouds over Western Equatorial Africa in reanalyses and historical CMIP6 simulations

Within the equatorial zone, Western Equatorial Africa (WEA) has a record low sunshine duration during the June-September dry season due to the persistence of low clouds. This study examines the ability of two reanalysis products (ERA5 and MERRA-2) and eight CMIP6 models (both coupled and atmosphere-only historical simulations) to reproduce the climatology of these low clouds, by comparing it with ground observations and a satellite product. All datasets show a reasonable representation of the regional distribution of low clouds over the Tropical Atlantic and the neighbouring African continent. However, CMIP6 models tend to underestimate the low cloud fraction, especially over WEA in the coupled simulations. This underestimation is partly due to an insufficient seasonal sea-surface temperature (SST) cooling over the Eastern Equatorial Atlantic from April to July in most models, which reduces the lower-tropospheric stability (LTS). However, the inability to reproduce the JJAS low cloud fraction does not necessarily scale with the SST biases of the CMIP6 models. Observed interannual variations of WEA low-cloud fraction are strongly controlled by LTS, itself mostly related to Atlantic SST. The strong dependence of low clouds on interannual SST variations is captured by most, but not all the CMIP6 models. Additional drivers of interannual variations identified in this study, such as mid-tropospheric temperatures over WEA and Bight of Bonny surface winds, emerge inconsistently in CMIP6. Further analyses are needed to disentangle the roles played by SST and independent atmospheric forcings on WEA low cloud formation.