AGCM Precipitation Biases in the Tropical Atlantic

Many general circulation models (GCMs) share similar biases in the representation of the intertropical convergence zone (ITCZ) in the Atlantic, even when they are forced with the time series of the observed sea surface temperature (SST). Specifically, they overestimate precipitation in the Southern Hemisphere in boreal spring and in the Caribbean region in boreal summer. The majority of the models considered here place the rainfall maximum over the SST maximum, although the true precipitation maximum does not occur there. This is the case even though these GCMs accurately place the maximum in surface wind convergence away from the SST maximum, at the location where the observed precipitation maximum lies. Models that overrespond to SST in this way tend to (i) have fewer heavy-rain events, (ii) rain more for a smaller amount of water vapor in the atmospheric column, and (iii) couple rainfall and surface humidity too strongly and rainfall and humidity above the surface too weakly.