Significance The Cape Town “Day Zero” drought was caused by an exceptional 3-y rainfall deficit. Through the use of a higher-resolution climate model, our analysis further constrains previous work showing that anthropogenic climate change made this event five to six times more likely relative to the early 20th century. Furthermore, we provide a clear and well-supported mechanism for the increase in drought risk in SSA through a dedicated analysis of the circulation response, which highlights how—as in 2015–2017—a reduction in precipitation during the shoulder seasons is likely to be the cause of drought risk in southwestern South Africa in the 21st century. Overall, this study greatly increases our confidence in the projections of a drying SSA.