Posted by & filed under Climate Change, CSAG Blog, CSAG student blog.

Purpose of this piece

In this short companion blog, a link is provided to an article in The Conversation, in which Sabina and I give a short, broad overview of our current understanding of storms affecting Cape Town and surrounding areas and how they may be affected by climate change. In this blog, we provide a somewhat more in-depth perspective. Interested readers can also read the last sections of the blogs I wrote about March and September last year, covering similar topics.

The April 2024 storm

A severe storm hit South Africa’s Western Cape Province between 6 and 9 April 2024. Extreme winds on Sunday 7 April, gusting up to 135km/h, left a trail of destruction across Cape Town and the Boland. Recorded wind speeds were generally greater even than during the famous “Cape Storm” of June 2017. Various informal settlement and wildland urban interface fires were fanned by these violent wind gusts. 

Early on Tuesday the 9th, the same weather system brought heavy rain which led to flooding along the Garden Route and southern Karoo. The Gamkaskloof Dam level surged from under 50% to over 150% in a few days.

Cape Town storm types

South Africa’s western coastal belt from Cape Town to far south-western Namibia is unique in southern Africa in that it receives its rainfall mostly in winter. This winter rainfall zone (WRZ) covers only the western half of the Western Cape province; the rest of the province falls into the all-year and (late) summer rainfall regions.

The two primary weather systems that bring heavy rain here are cold fronts and cut-off lows (COLs). 

Cold fronts occur mostly in winter. They bring cold, wet and windy weather from westerly winds. They are most likely to deliver flooding rains when they are accompanied by an atmospheric river. This happened with the historic flooding of early June 2023, the heavy rains of February 2023 and the record single-day rainfall at Cape Town airport in June 2022. Atmospheric rivers can bring particularly heavy rains in the wettest dam catchment areas of the WRZ, even the rare summer events.

COLs can form at any time of year, but occur preferentially in autumn and spring.  Near Cape Town they usually occur with southerly or easterly winds. They are the most common cause of the “Black South-Easter” phenomenon – when the south-easterly wind, usually associated with fair weather, brings in thick and dark clouds. 

Cut-off lows and South African flooding events

COLs are the most common flood-producing weather system across South Africa, as discussed in Dr. Mary-Jane Bopape’s keynote presentation at the recent Extreme Climate Events conference in Stellenbosch. The best known examples are the catastrophic KwaZulu-Natal floods of April 2022 and September 1987 and the Laingsburg flood of January 1981. Overnight on Sunday 14 April, another COL brought well over 200mm to Margate in KwaZulu-Natal, resulting in at least 5 fatalities. The extreme weather currently gripping much of South Africa is also related to an extraordinarily large and wet winter cut-off low.

Notably, most of the severe storms to affect the winter rainfall zone over the last 2 years have been COLs. These include intense thunderstorms in December 2022 and March 2023, the floods of late June 2023 and the deadly Heritage Day 2023 storm. The Boland and Overberg regions have been hardest hit.

Most of these COLs formed in a similar area off the South African West Coast. Why this was the case is a topic of active research.  

Many other severe flooding events across the world, including June 2024 flooding in southern Germany, and April 2024 flooding in Dubai, have also been associated with cut-off low type weather systems.

Can these events be attributed to climate change?

Climate change attribution studies have detected an increase in the risk of observed flooding rains across many parts of the world. This includes the 2022 COL-related KwaZulu-Natal floods.

As the Earth warms, more water evaporates from warmer surface oceans into a warmer atmosphere, capable of holding more moisture. Hence, more rain can fall, more quickly.

Around Cape Town, attribution studies have identified a significantly increased risk of droughts like the 2015–2017 “Day Zero” event due to human-driven climate warming. A climate change fingerprint was also detected in the extreme fire weather seen with the Devil’s Peak fire of April 2021.  

This may seem to imply that climate change can’t also increase the risk of flooding rains. An attribution study following the catastrophic Storm Daniel in coastal Libya concludes that climate change is driving “a long-term decline in average rainfall, and at the same time, [an] increase in extreme rainfall.” The number of storms there are decreasing, while the most severe storms become ever wetter. 

Could this be happening around Cape Town, too? Studies looking at climate model projections of future extreme rainfall risk suggest not. Unlike all the other African cities investigated, these models suggest that the frequency of heavy rains over Cape Town will decrease as the planet warms. This is mostly due to the expected weakening and reduced occurrence of cold fronts. But what about COLs? And beyond Cape Town, does this also apply in the Overberg?

The observed record since 1979 does not indicate a clear decrease or increase in COLs or associated total rainfall in the winter rainfall zone. There has been pronounced multi-year variability recently. Heavy COL rainfall in 2013–2015 was followed by record low COL counts between 2016 and 2020. Since then their frequency has spiked again. 

Given the generally short period of people’s “weather memory” this may explain why it seems like there has been an increase in storm occurrence. 

There is considerable uncertainty about the future risk of COL-induced floods over the Western Cape. This holds, even as much of the province is expected to become drier due to climate change. Researchers have also suggested that changes in atmospheric rivers stretching across the Atlantic from Brazil could modify future heavy rainfall risk along the western mountains. More work is needed to clarify the picture.

For now, we know that COLs are a major severe weather risk, including around Cape Town. They are, unfortunately, particularly difficult to forecast—a recent media release from the South African Weather Service (SAWS), labelled them “fickle”. 

Accordingly, future COLs could impact regions outside of where they are expected, or they could have different impacts than anticipated. They may sometimes be less severe than suggested by warnings. However, underestimating the next predicted intense COL storm is dangerous, as there will also be times when impacts will be worse than envisaged. 

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