General circulation model simulations of Southern African regional climate.

[摘要] Six general circulation model simulations of present-day southern Africanclimate are assessed, Each of these models are early-generation equilibriumclimate models linked to simple mixed-slab oceans. Simulations of surfaceair temperature over the subcontinent are sensitive to the grid-scaleparameterisation of convection in summer. At high latitudes, large simulationerrors are caused by errors in the specification of sea-ice albedo feedbacks.Increased spatial resolution and the inclusion of a gravity wave drag term inthe momentum equations results in a markedly-improved simulated mean sealevel pressure distribution. Tho models successfully simulate the pattern ofrainfall seasonality over the Subcontinent, although grid-point simulation ofprecipitation is unreliable. Treatment of convection, cloud radiative feedbacksand the oceans by this generation of models is simplistic, and consequentlythere is a large degree of uncertainty associated with predictions of futureclimate under doubled-carbon dioxide conditions. For this reason, morereliable estimates of future conditions will be achieved using only thosemodels which reproduce present climate most accurately. Early-generationgeneral circulation models suggest a warming of 4°C to 5°C for the southernAfrican region as a whole throughout the year. Over the subcontinent,warming is expected to be least in the tropics, and greatest in the drysubtropical regions in winter. Estimated changes in mean sea level pressureindicate a southward shlft of all pressure systems, with a weakening of thesubtropical high pressure belt and mid-latitude westerlies. Little agreementexists between the models concerning predictions of regional precipitationchange. However, broad scale changes in precipitation patterns are inaccordance with predicted circulation changes over the subcontinent.Generally wetter conditions may be expected in the tropics throughout theyear and over the summer rainfall region during summer. Decreased winterrainfall may be expected over the winter rainfall region of the south-westernCape. However, estimated precipitation changes are grid-point specific andtherefore must riot be over-interpreted. The present climate validation hasresulted in more reliable estimates of future conditions for the southernAfrican region. This approach should be extended to recent slrnulations whichinclude more comprehensive treatment of important physical processes.

[发布机构] University of the Witwatersrand