Understanding CMIP6 biases in the representation of the Greater Horn of Africa long and short rains

Abstract The societies of the Greater Horn Africa (GHA) are vulnerable to variability in two distinct rainy seasons, March–May ‘long’ rains and October–December ‘short’ rains. Recent trends both possibly related patterns low-frequency variability, have increased interest future climate projections from General Circulation Models (GCMs). However, previous generations GCMs historically poorly simulated regional hydroclimate. This study conducts a process-based evaluation simulations long short CMIP6, latest generation GCMs. Key biases CMIP5 remain or worsened, including that too weak strong. Model driven by complex set oceanic atmospheric factors, Walker Circulation. Biased wet models connected with Indian Ocean zonal sea surface temperature (SST) gradients warm west convection is deep. connect equatorial African winds strength rains, though observations robust connection primarily found mean state timing western SST seasonal cycle associated certain rainfall biases, may be due common source. Simulations historical SSTs (AMIP runs) often larger than fully coupled runs. A path towards using better understand uncertainty GHA suggested.