Large Summer Rainfall Events and Their Importance in Mitigating Droughts over the South Western Cape, South Africa

Although the south Western Cape receives most of its rainfall between May and September, there are substantial rainfall events in some summers. These events are of interest in themselves as well as for their possible role in mitigating the frequent winter droughts that the region suffers from. Most recently, greater Cape Town suffered a devastating drought during 2015-18 known as the Day Zero drought due to the high risk of urban areas running out of piped water supply. Estimated data from the city show that major dam levels in the south Western Cape increased more than 1% in some cases after large rainfall events (LREs) in the summer of 2018/19. This increase is significant as dam levels often decrease by several percent per month during the hot summer. In this study, LREs over the south Western Cape during the summer (October-March) are investigated together with dam level data. Most summer LREs result from atmospheric rivers (ARs) or cutoff lows (COLs). ARs have not been previously studied in the South African region except for one study for winter that showed they are responsible for almost all the heavy rainfall events in the Western Cape. Although COLs are most common in the transition months, they can also occur in midwinter and summer. COLs tend to last longer and cover larger areas than ARs, which typically yield relatively short bursts of intense rainfall mostly concentrated around greater Cape Town. After each summer LRE, average dam levels increase by up to 5%, suggesting they are very important for drought recovery. In particular, summer LREs following the anomalously dry winters of 1980, 1984, 2003, 2004, and 2015-18 played an important role in mitigating those droughts.