The Representation of Dry-Season Low-Level Clouds Over Western Equatorial Africa in Reanalyses and Historical CMIP6 Simulations

Abstract 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 8 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 contributes to reducing the lower-tropospheric stability (LTS). ERA5 data, whose dry season WEA low-cloud fraction is close to observations, indicate that LTS strongly controls the interannual variability of WEA low clouds. Interestingly, the ability to reproduce the mean low cloud fraction does not necessarily scale with the SST biases of the CMIP6 models. The strong dependence of low clouds on interannual SST variations in ERA5 is captured by less than half of the CMIP6 models. Additional key drivers of interannual variations identified in this study, such as Bight of Bonny surface winds and mid-tropospheric temperatures, actually show up inconsistently in CMIP6. Further analyses are needed to disentangle the roles played by SST and independent atmospheric forcings on WEA low cloud formation.