For communities across the Western Cape, cut-off low weather systems are a familiar threat. They can dump torrents of rain in a matter of hours, flooding roads, damaging homes and infrastructure, and in worst cases, causing loss of lives. What scientists have long wanted to understand is why some of these storms turn so destructive, and researchers at the University of Cape Town (UCT) found that the answer lies far offshore, in the warm waters of the Agulhas Current.
In a study published in Atmospheric Research , the team found that the Agulhas Current acts as a powerful source of heat and moisture that intensifies rainfall during cut-off low systems over the Western Cape. The neighbouring Benguela Upwelling System, by contrast, appears to play only a minor role.
Cut-off lows are among the country’s most dangerous weather systems. They account for only a small portion of the Western Cape’s annual rainfall, but they are responsible for many of its most severe floods. The cut-off low in September 2023 caused widespread destruction across the province, damaging roads, homes and public infrastructure; and claiming lives. Similar events in 2007 and 2008 caused hundreds of millions of rand in damage and displaced thousands of people.
The team recreated the September 2023 flood-producing event using the Model for Prediction Across Scales–Atmosphere (MPAS-A), a high-resolution weather model. That allowed researchers to isolate the influence of South Africa’s two dominant ocean current systems by running experiments that removed the warming effect of the Agulhas Current and, separately, weakened the cooling influence of the Benguela Upwelling System.
When the warming influence of the Agulhas Current was removed, rainfall across the Western Cape dropped by more than 18%. The simulations showed that cooler sea temperatures reduced the transfer of heat and moisture from the ocean into the atmosphere, weakening instability, limiting upward motion and sharply reducing rainfall.
“The Agulhas Current is far more than a warm ribbon of water flowing along South Africa’s coastline,” said lead author Chelsey Jansen of UCT’s Department of Environmental and Geographical Science and the Nansen-Tutu Centre for Marine Environmental Research .
“Our simulations show that it actively supplies the heat and moisture that strengthen these weather systems. Without that energy source, the storms become noticeably weaker and produce substantially less rainfall over the Western Cape.”
The Benguela Upwelling System told a different story. Although warmer Benguela waters increased the amount of heat and moisture entering the atmosphere, rainfall changed by less than 2%. The extra moisture remained trapped on the western side of the storm and never reached the area where the heaviest rainfall develops.
“This tells us that moisture alone is not enough,” Jansen said. “The atmosphere has to be organised in a way that allows that moisture to rise into the storm. In the case of the Benguela Upwelling System, the circulation doesn’t allow that to happen.”
The study also found that the MPAS-A model closely reproduced the September 2023 storm, accurately simulating its movement, internal structure and rainfall distribution. Compared with widely used reanalysis datasets, the model produced the most realistic representation of rainfall across the province, strengthening confidence in its use for studying extreme weather events.
Jansen noted that the study deepens understanding of the processes behind extreme rainfall in southern Africa.
“Understanding the interaction between the ocean and the atmosphere is essential if we want to improve predictions of extreme weather,” she said. “Our findings demonstrate that the Agulhas Current plays a dominant role in determining how intense these storms become, and that knowledge can help improve weather forecasting and disaster risk management.”
Jansen said that future work will examine multiple cut-off low events across different years to determine whether the Agulhas Current exerts the same dominant influence under a wider range of weather conditions, helping improve confidence in future climate projections and flood prediction systems.
Atmospheric Research
10.1016/j.atmosres.2026.109220
Computational simulation/modeling
Not applicable
Cut-off low–induced extreme rainfall in the Western Cape, South Africa: The roles of the Agulhas and Benguela current systems
14-Jul-2026
No conflicts of interest were declared by the researchers in the media release.