A general view of containers that fell over at a container storage facility following heavy rains and winds in Durban, on April 12, 2022. (Photo by PHILL MAGAKOE / AFP)
Liesl Dyson, an associate professor in meteorology at the University of Pretoria, said the extreme rainfall that unleashed the widespread flooding in eThekwini metro and nearby coastal areas of KwaZulu-Natal was an “anomaly” and an “extreme event”.
According to the South African Weather Service, a cut-off low system was responsible for the inclement weather that brought heavy rainfall to the province. These systems, it said, are associated with widespread instability in the atmosphere, which can promote periods of prolonged rainfall, as witnessed over many of the interior provinces of the country last weekend.
“For KwaZulu-Natal however, the effect of the cut-off low system has been markedly enhanced by the presence of sustained low-level maritime air, which has been fed in from the southern Indian ocean, thus driving the system to produce more rainfall,” said weather service.
“Moreover, the original source of the maritime air was from warmer, subtropical parts of the ocean, with a greater capacity to transport moisture, an essential ingredient of any rain-producing system.”
On Monday night, some 24 hour falls exceeded 200mm in KwaZulu-Natal, it said. “More noteworthy, is that a few stations even reported 300mm or more. A selection of the highest overnight rainfall measured in KwaZulu-Natal includes King Shaka International Airport (225mm), Margate (311mm), Mount Edgecombe (307mm), Port Edward (188mm), as well as Virginia airport (Durban North) with 304mm.”
Such rainfall, it said, is of the order of values normally associated with tropical cyclones. “However, [the weather service] must strongly emphasise that this system is not tropical in nature, nor is it a tropical cyclone.”
Dyson explained that, “cut off lows develop predominantly in the transition season so in spring and in autumn”.
“So you get a combination of tropical air that has been around since January, February and March, we’ve had the tropical cyclones that came in, so the circulation and the air over South Africa was very tropical. Now, this weather system is more associated with the extra topics or the mid-latitudes.
“If it was dry, if it wasn’t such a wet summer season, then there wouldn’t have been moisture to rise. But because there was already such a lot of rainfall, there was a lot of moisture, the dams are full, there’s a lot of moisture in the ground, so there was a lot of moisture to rise.”
Dyson said that early on Monday morning, the little low pressure system associated with a cut-off low developed on the North Coast of KwaZulu-Natal. The circulation is clockwise and so air that flows from the ocean is pushed from the coast.
“The topography in KwaZulu-Natal rises very quickly … so the air that comes in is forced to rise against those hills and escarpments and the rainfall … continues for hours and days,” she said.
“So it was kind of a perfect storm situation with the wet season that came before and this little low pressure system that developed — a small thing but it’s kind of an offspring of the cut-off low.”.
[/membership]