Simulations by the atmospheric model intercomparison project of atmospheric circulation over southern Africa

Abstract

Seven simulations by the Atmospheric Model Intercomparison Project (AMIP) of mean climate and interannual variability over southern Africa during the 1979-1988 decade are evaluated. The focus of the study is the representation in the models of both regional and hemispheric circulation features which influence the climate of the subcontinent. The AMIP integrations provide an improved simulation of mean circulation around southern Africa when compared with earlier atmospheric general circulation models. Improvements are noted in the representation of the position and intensity of the subtropical anticyclones, the circumpolar trough and the strength of the mid-latitude westerlies. The representation of tropospheric circulation features throughout the Southem Hemisphere, including zonal asymmetries and planetary wave 1 at 500 hPa, appear to be dependent on the spatial resolution of the models. No single model simulates all features of southern African circulation accurately, and model performance varies with the seasons. Several of the AMIP models are capable of simulating the observed features of interannual variability in circulation over southern Africa. Simulated interannual variability in both summer rainfall and circulation is weaker than observed. Simulated circulation adjustments in the subtropics and mid-latitudes are in good agreement with observed adjustments associated with periods of above and below average summer rainfall over the subcontinent. It is recognized that considerable uncertainty is associated with the single integrations using observed sea-surface temperature forcing considered in the present analysis. Ensemble approaches are required to provide a more comprehensive assessment of the simulated response to imposed forcing.