A model investigation of interannual winter rainfall variability over southwestern South Africa and associated ocean-atmosphere interaction

We have investigated the variability of inter-annual winter rainfall over the southwestern Cape region of South Africa and associated large-scale atmosphere-ocean interaction upstream over the South Atlantic using the HadAM3 atmospheric general circulation model. This model was run for the period from 1990 to 1999 using mean monthly global sea-surface temperature (SST) as surface boundary condition over the global ocean. Diagnostics of winter (May to September) model output averaged over 1990-99 suggest that the HadAM3 model represents the general circulation in the South Atlantic/African sector reasonably well for this season at least. In addition, model years with wet and dry winters over the study area tended also to be those that were observed to be anomalously wet or dry. Wet minus dry season composite fields were used to investigate the model's inter-annual variability. The composite difference fields for low- and mid-level winds, sea-level pressure, and moisture flux all indicated wet winters being associated with increased inflow from tropical South America (originating in the equatorial western Atlantic at low levels) contributing relatively moist air to the westerly flow heading towards the southwestern Cape. A stronger jet over the South Atlantic promoted the passage of storms towards the Cape. Large areas of cyclonic vorticity anomalies, enhanced eddy activity, increased thickness in the lower atmosphere and low-level convergence near and upstream of the southwestern Cape in the model composite differences all favoured increased storm systems as well as their local intensification, implying enhanced rainfall. The results presented here suggest that the model can represent the inter-annual variability of winter rainfall over the study region and shed light on the mechanisms potentially associated with anomalously wet winters there.