Impact of Enhanced Low-level Stratus on Simulated SSTs, Precipitation and the Circulation in the Tropical Atlantic Sector

Most coupled atmosphere-ocean general circulation models (AOGCMs) show a substantial warm bias in sea-surface temperatures (SSTs) in the eastern tropical Atlantic. The impact of enhanced low-level clouds on SST, precipitation and the circulation in the tropical Atlantic sector is tested. Therefore, we have conducted sensitivity experiments with the atmospheric model ECHAM6 and the coupled version of it (MPI-ESM1) in which we enhance the formation of low-level stratus at the inversion layer in the low troposphere. The impact of enhanced low-level clouds is compared to the standard version of the models. There is a direct cloud impact by reducing the incoming solar radiation at the surface. The reduced incoming solar radiation leads to a cooling of SSTs in the eastern tropical Atlantic in the coupled atmosphere-ocean model. This in turn causes not only locally rainfall reductions in oceanic precipitation but also a remote precipitation enhancement over north east Brazil. These precipitation changes are associated with changes in the equatorial wind-stress forcing. The impact of the wind stress changes on the equatorial zonal SST-gradient and the seasonal cycle is also analysed.