Belinda Beresford
It came from the north-east, the killer storm, following the Limpopo River, to deliver its fury on South Africa. An immature tropical cyclone is the way climatologists describe it – the phenomenon behind the deluge of rain which has washed over the subcontinent this week.
The storm – which just missed being accorded full cyclone status and thus being dignified with a name – was born in the Mozambique channel. It moved westward over the subcontinent, crossing southern Zimbabwe last weekend, and following the valley of the Limpopo, before forming a “deep low pressure system” over eastern Botswana. This system has been fairly stationary, although it is slowly moving towards KwaZulu-Natal. Weather experts expect that upper-level north-westerly winds will eventually take the system out over the ocean again, where it will finally dissipate.
Tropical cyclones aren’t that rare an event in South Africa. Weather records indicate the country encounters a tropical cyclone roughly once every five years, although there hasn’t been one for the past 11 years.
“Tropical systems or depressions are part of the normal weather patterns in the region, particularly at this time of year,” says Mike White, deputy director of forecasting at the Weather Bureau. Usually there are about six or seven a year in the Pacific Ocean, although very few ever reach the African coastline.
“But it is unusual that it has moved further inland than normal, so the effects are over a far wider region. It has also lasted longer and the intensity of the rainfall in particular areas, for example Kruger Park, justifies calling it an ‘extreme event’.”
Geography plays a major part in the implications of the system. In Mpumalanga, for example, the strong easterly air flows encountered the mountains. Unable to move horizontally, the air was pushed upwards, so increasing the probability and intensity of the rainfall. In Mpumalanga, Graskop recorded 543mm of rain from February 1 to February 9 – the total annual average is 281mm.
The highest recorded rainfall in the country was at Phiphidi in the Northern Province which up till Wednesday had received 544mm of rain so far this month. A Weather Bureau employee said Phiphidi had probably recorded more than 600mm by Thursday – unfortunately the rains had since washed away the telephone lines. “It’s not cats and dogs, it’s cows and horses up there,” he said.
In Gauteng Sandton had received more than double its expected monthly rainfall in the first nine days of February, recording 233mm instead of the monthly average of 100mm. The annual average rainfall for Sandton is 720mm. Pretoria had had 118mm of rain, again more than the expected monthly 76mm.
However, the storm system is weakening as it loses energy. Rainfall is decreasing in intensity, although this will not relieve the pain it causes. The earth is becoming saturated, meaning that further rainfall will simply swill over the surface rather than becoming absorbed.
Natural extremes in weather are the result of many factors, from sun spot cycles to changes in features of the oceans. But human activity can also have a profound effect. Dr Debbie Hudson of the University of Cape Town environment and geographical sciences department says the earth is in an energy balance, and any factor affecting that will alter the climate. This can include changing land use – urbanisation, for example, tends to mean more energy is reflected than from vegetation – or gaseous emissions which affect the chemistry of the atmosphere.
While more time is needed to confirm global warming, Hudson points out that since the late 19th century surface temperatures have risen by between 0,3% and 0,6%, while the sea level has risen by up to 25cm. Four of the hottest years on record have occurred since 1990.