SST VARIABILITY IN THE SOUTH INDIAN-OCEAN AND ASSOCIATED CIRCULATION AND RAINFALL PATTERNS OVER SOUTHERN AFRICA

A general circulation model is used to study the response of the atmos phere to an idealised sea surface temperature (SST) anomaly pattern (w arm throughout the southern midlatitudes, cool in the tropics) in the South Indian Ocean region. The anomaly imposed on monthly SST climatol ogy captures the essence of patterns observed in the South Indian Ocea n during both ENSO events and multidecadal epochs, and facilitates dia gnosis of the model response. A previous study with this anomaly impos ed in the model examined differences in the response between that on t he seasonal scale (favours enhancement of the original SST anomaly) an d that on the decadal scale (favours damping of the anomaly). The curr ent study extends that work firstly by comparing the response on the i ntraseasonal, seasonal and interannual scales, and secondly, by assess ing the changes in the circulation and rainfall over the adjoining Afr ican landmass. It is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and incl uding, annual. However, as the scale becomes interannual (i.e., 15-21 months after imposition of the anomaly), the model response suggests t hat damping of the original SST anomaly becomes likely. Compared to th e shorter scale response, the perturbation pressure and wind distribut ion on the interannual scale is shifted poleward, and is more reminisc ent of the decadal response. Winds are now stronger over the warm anom aly in the southern midlatitudes suggesting enhanced surface fluxes, u pper ocean mixing, and consequently, a damping of the anomaly. Examina tion of the circulation and rainfall patterns indicates that there are significant anomalies over large parts of southern Africa during the spring, summer and autumn seasons for both short (intraseasonal to int erannual) and decadal scales. It appears that rainfall anomalies are a ssociated with changes in the advection of moist tropical air from the Indian Ocean and its related convergence over southern Africa. Over e astern equatorial Africa, the austral autumn season (the main wet seas on) showed rainfall increases on all time scales, while parts of centr al to eastern subtropical southern Africa were dry. The signals during summer were more varied. Spring showed generally dry conditions over the eastern half of southern Africa on both short and decadal time sca les, with wet areas confined to the west. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90-180 mm, and therefore represent a si,significant fraction of the annual total of many areas. It appears that relatively modest SST anomalies in the South Indian Ocean can lead to sizeable rainfall anomalies in the model. Although precipitation in general circulation models tends to be less accurately simulated than many other variables , the model results, together with previous observational work, emphas ize the need for ongoing monitoring of SST in this region.