Heavy rainfall occurred over parts of the Cape Winelands district and the Cape metropole until yesterday (Tuesday, 13 August), with lighter rainfall over the rest of the winter rainfall region.
“Although a few more frontal systems are forecast for the rest of August – particularly in the last 10 days of the month – these rains could be much lighter,” says Johan van den Berg, independent agricultural meteorologist.
“Light rain is also possible in September with further cold fronts, but they appear to be less intense than the very strong systems of the past six weeks.”
The lowest and the highest
Van den Berg says the winter rainfall region is experiencing one of the wettest recorded winters, with some of the lowest minimum temperatures recorded elsewhere in the country.
In areas of the Cape Winelands district, the Cape metropole and western parts of the Overberg, more than 500 mm of rain was recorded in July. “In many cases, more than three to four times the long-term average rainfall for July fell.”
Extremely low minimum temperatures also occurred during July over parts of the interior, as far north as Limpopo. Other parts of the world are experiencing some of the highest recorded temperatures, but the south and east of Australia also experienced very cold conditions with extreme weather.
According to an article in The Conversation by Martin Jucker, senior lecturer in atmospheric science at the University of New South Wales in Sydney, these extreme conditions are linked to upper atmospheric conditions over Antarctica.
Temperatures as low as -80 °C are present in the stratosphere – the layer of air that lies 10-50 km above the Earth’s surface. Very strong winds of up to 300 km/h, known as the stratospheric polar vortex, are raging around the South Pole. (These types of conditions can also occur in the northern hemisphere.)
“Factors such as a strong high-pressure system in the lower parts of the atmosphere are sometimes present. This can cause large-scale waves in the stratosphere around the poles, which can cause wind speeds to decrease. In extreme cases, the wind speeds decrease to such an extent that very rapid warming closer to the surface occurs.”
Weather systems move closer to the equator
Van den Berg explains that once the polar vortex is disturbed, it causes weather systems from the Southern Ocean to move closer to the equator. Weakening stratospheric winds put the Southern Annular Mode (SAM) in a negative phase, which means frontal systems with rain and cold are pushed further north during winter. However, during spring and early summer, the negative phase is associated with dry and warm conditions.
Since mid-July, temperatures in the stratospheric polar vortex have been about 20 °C warmer than the long-term average. Wind speeds have also decreased by 70 km/h to about 230 km/h. These changes do not yet represent a typical so-called rapidly warming stratospheric event, but there are signs that it may already be responsible for the exceptional weather conditions.
Van den Berg says SAM is in a very negative phase. Two possible outcomes could occur in the coming months:
- The stratospheric winds and temperatures may recover and the impact on surface weather systems will disappear.
- Temperatures in the stratosphere may continue to warm and winds may weaken further. This could cause warmer conditions with below-average rainfall to occur much more quickly in spring and early summer.
“Currently, there is no certainty about which outcome will occur. The direct impact on Southern Africa is not clear, but there is usually a very strong correlation between especially the southeastern parts of Australia and the summer rainfall region of South Africa.
“With above-average global temperatures, very cold conditions in parts of the southern hemisphere, as well as other phenomena such as the developing La Niña system and conditions in the Indian Ocean, there is a possibility that very stormy conditions could occur in the coming months.”
