27 September 2023
By: Alani Janeke
The well-established rainfall patterns observed over the past three summers may be one of the reasons for the stormy weather and heavy rain that occurred over the weekend.
Over the past 10 months, abnormally strong cut-off low-pressure systems have regularly developed over the western and southwestern parts of South Africa and the adjacent Atlantic Ocean. Such a system led to heavy rains over the southwestern parts of the Western Cape and the Southern Cape at the weekend.
Since last November, there has been a lot of rain over parts of the Western Cape, and a cut-off low-pressure system brought heavy rain to parts of the West Coast and the adjacent interior in December 2022.
“It was a similar system that caused the floods in the Laingsburg area in January 1981, as well as the floods in the Free State and Northern Cape in March 1988,” says Johan van den Berg, an independent agricultural meteorologist.
Van den Berg says the intensity of the systems that brought heavy rain followed below-average rainfall in the winter of 2022 over the Western Cape. Some places had less than 70% of long-term average rainfall.
Southward shift
During the summer seasons of 2021 and 2022, the same type of cut-off low-pressure systems occurred over the central to eastern parts of the summer rainfall area, and floods in interior and coastal areas of KwaZulu-Natal are still fresh in South Africans’ memories.
“It therefore appears that there has been a southward shift during the winter and spring of 2023,” says Van den Berg.
The big question is why it happened and how long it will continue. “It is speculated that the three consecutive La Niña events from 2020 until the end of the 2022-2023 summer season began to follow a fairly strong semi-permanent pattern, with below-average conditions favourable for the development of strong low-pressure systems.
“The shift in rainfall patterns further westward over Southern Africa during La Niña episodes also means that conditions were favourable for the development of stronger-than-normal low-pressure systems and more rain. Although La Niña conditions transitioned to El Niño conditions from April and May of this year, the well-established patterns are still present.”
The history
On the other hand, Van den Berg says history shows that above-average rainfall is very possible from September to November over large parts of South Africa when El Niño development occurs strongly and rapidly during winter.
“This was also the case in August 2023, when the Southern Oscillation Index (SOI), which is an indicator of sea surface temperature response in the central Pacific Ocean to overhead climate systems, was in a rapidly declining phase or represented a rapidly strengthening El Niño.”
The current outlook, as shown in Figure 1, indicates that nearly all of Southern Africa, including Namibia, Botswana, Zimbabwe and the southern parts of Mozambique, has a 60% to 80% chance of above-average rainfall for September to November 2023.
“It is also interesting that the strongest tendency for above-average rainfall is concentrated over the southern and southeastern parts of South Africa, where most of the rainfall is currently occurring,” says Van den Berg. “What is happening now is typical when an El Niño system rapidly develops in late winter and spring.”
Outlook for October
Based on the long-term forecasts in Figure 1, it is likely that further rain will occur, especially in October. “This may again be more over the southeastern parts of South Africa, but it should also shift northward later,” says Van den Berg.
“Cold fronts will continue to affect the southern parts of the country with cold air from polar regions. Along with this, tropical moisture moving southwest over Namibia and Botswana caused the very stormy conditions from September 23 to 25 when the cold air and warm, moist air from the north came together.
“It appears that the higher layers of the atmosphere are still favourable for the development of cut-off low-pressure systems, leading to very strong winds, hail and rain. This situation is expected to change from November or December when South Africa is expected to be more influenced by strong high-pressure systems that can suppress rainfall development.”
