EQUILIBRIUM AND FULLY COUPLED GCM SIMULATIONS OF FUTURE SOUTHERN AFRICAN CLIMATES

Changes in seasonally averaged surface air temperature, mean sea-level pressure and rainfall predicted by a representative range of current equilibrium climate models linked to mixed-layer oceans and fully coup led ocean-atmosphere models are compared. All fully coupled and mixed- layer models predict increased temperatures under enhanced greenhouse conditions throughout southern Africa. By comparison with the mixed-la yer models, the magnitude of warming predicted by the fully coupled mo dels is much diminished, particularly over the southern oceans. Howeve r it is important to compare predicted warming normalized to a global average of 1 degrees C in order to account for differences in experime ntal design between fully coupled and mixed-layer models. The pattern of normalized warming indicates that fully coupled models predict grea ter warming than the mixed-layer models over the subcontinental land m ass, whereas the mixed-layer models predict greater normalized warming in the high latitudes of the southern oceans. Predicted decreases in pressure simulated by both fully coupled and mixed-layer models over m uch of the tropics and subtropics are indicative of an intensification of the tropical easterly circulation. However, caution must be exerci sed in using mean sea-level pressures to estimate circulation changes over the interior plateau region. None the less, the magnitude of pred icted changes in pressure are similar to observed pressure anomalies a ssociated with extended periods of above- and below-normal rainfall ov er southern Africa. Little confidence may be expressed in simulated ci rculation changes over the oceans south of the subcontinent. Neither t he fully coupled nor mixed-layer models predict any significant change in rainfall seasonality over southern Africa under enhanced greenhous e conditions. Both types of model simulate 10-20% increases in rainfal l during summer in the tropics. These predicted changes are physically consistent with predicted increases in temperature and an intensified tropical easterly circulation. Over much of the rest of the subcontin ent, fully coupled models and to a lesser extent mixed-layer models si mulate rainfall decreases in summer which are not physically consisten t with similar changes in temperature and pressure. However both types of model display almost no agreement in the predicted sign of rainfal l change over the subcontinent during the second half of summer. As su ch, the uncertainties associated with the simulation of future rainfal l changes remain to be resolved and are a major obstacle to the confid ent development of future climate scenarios. Recent evidence suggests that significant errors may exist in current fully coupled model simul ations of the southern ocean. Currently, mired-layer models may be mor e reliable for developing regional climate change scenarios.