Large-scale mechanisms linked to anomalously wet summers over the southwestern Cape, South Africa

Although on average, the southwestern Cape (SWC) of South Africa is a winter rainfall dominated region, almost 30% of the total rainfall occurs during the extended summer (October-March). A previous study showed that anomalously wet summers may help mitigate the effects of severe winter drought. Apart from that study, very little work has been done on summer rainfall variability over the SWC or the mechanisms associated with it. Here, station data and ERA5 reanalyses are used to investigated summer rainfall day variability and associated mechanisms. Interannual variability in summer rainfall day frequencies appears related to that in the South Atlantic High Pressure (SAHP) and westerly moisture fluxes across the midlatitude South Atlantic. Increased rainfall days are associated with cyclonic anomalies over the region and enhanced westerly moisture fluxes. Some of these circulation changes are related to the Southern Annular Mode, and in late summer, also to ENSO and changes in the zonal wavenumber 3 pattern. Significant decreasing trends in rainfall days were found in the mid- and late summer for the southern part of the region where most of the population lives and the main water supply dams are located. These trends seem associated with significant trends found in the southern boundary of the SAHP and in decreasing (increasing) South Atlantic storm counts in the 35°–45° S (50°–60° S) latitude bands.