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- The weather system known as a cut-off low that occurred this weekend is not unusual for springtime.
- The cut-off low forms a closed low-pressure system that is stuck outside the jet stream.
- On average, one in 11 cut-off lows lead to flooding, as was seen this past weekend, according to a meteorologist.
- For climate change news and analysis, go to News24 Climate Future.
A weather phenomenon called a "cut-off low" contributed to severe storms in the Western Cape that led to flooding, mudslides, collapsing bridges and road infrastructure over the past few days.
A cut-off low is a low-pressure system that becomes "cut-off' from the jet stream – the strong winds that usually blow from west to east across the globe. The cut-off low forms a closed system that is stuck outside the jet stream. It is often composed of very cold polar air, and moves slowly – which is why it can affect an area for a long period.
Professor Liesl Dyson of the University of Pretoria's geography, geoinformatics and meteorology department says cut-off lows are usually expected in South Africa this time of year.
The South African Weather Service (SAWS) also confirmed that for the Western Cape, cut-off lows are normally associated with spring and autumn, and they may lead to severe impacts. "A similar event occured in May 2021, over the Overberg region," SAWS said. Many rain and wind disasters also occur in the Eastern Cape during this time of year, the SAWS noted.
On average, only one in every 11 cut-off lows causes floods, said Dyson.
In combination with the weather systems that caused cold fronts recently in Cape Town, the cut-off low caused heavy rainfall and cold temperatures.
The effects of El Niño – which are associated with higher temperatures and low rainfall – will only be felt in the summer months, from December to February, Dyson said.
The weather phenomenon, which sees warm sea surface temperatures in the Pacific Ocean, will only affect SA's weather two months down the line. "There is a lag effect," Dyson said.
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She said that while a weather system cannot necessarily be connected to climate change – the number of severe weather events can be blamed on the climate crisis.
Dyson points to the fact that the strong cut-off low follows a massive spring tide along the Western Cape and KwaZulu-Natal coasts. In some cases, waves as high as 9.5 metres were recorded.
Dr Andries Kruger of the SAWS said that the weather conditions in the Western and Eastern Cape provinces were unusual - based on anecdotal evidence that the rainfall amounts over the "relatively short period" were in some places "unprecedented" in human memory.
Further analysis of historical figures and comparisons with actual rainfall amounts will indicate "quantitatively" exactly how extreme the rainfall was.
He added that a dedicated research study will be required to attribute climate change to this specific event. "But it is generally acknowledged among climate experts, and also reflected in the most recent IPCC (Intergovernmental Panel on Climate Change) reports, that short-term rainfall episodes have become more extreme and will be more so in future," said Kruger.
Given the damage observed over the past two weekends, it is apparent that government and planning institutions need to consider that extreme weather events are becoming all the more extreme.
Western Cape MEC for Local Government, Environmental Affairs and Development Planning Anton Bredell, at a briefing on Tuesday, noted the reality of climate change – and indicated that the province has mitigation measures for climate change and has implemented early warning systems.
While he believes efforts must be "stepped up", there are "huge budget pressures".
It will take a long-term investment to prepare the province for such severe storms, he added.
Premier Alan Winde, at the same briefing, said that he is not sure "everybody on the planet" is prepared for climate change. Regions around the world have the same issue.
"I don't think you could actually invest enough to prepare for a flood like we are having at the moment," he said.
Going forward, planning and infrastructure build would have to take into account extreme weather, for example replacing existing drain pipes with those which are twice the size to account for higher volumes of water.
This would cost hundreds of billions of rands, he said.
This article was updated at 10:00 on Wednesday 27 September 2023 to include additional comment from the SAWS.
