Australia, Brazil, Thailand, the Philippines, South Africa … is there a pattern to the floods that seem to be drowning half the world?
Yes, says senior weather service forecaster Jan Vermeulen, but it is to some extent predictable. January is a mid-summer month in the southern hemisphere, Vermeulen points out, meaning plenty of rain in summer rainfall areas.
The additional factor, however, is the meteorological phenomenon of La Niña (Spanish for “the girl”), which causes lower than normal temperatures in the eastern equatorial area of the Pacific Ocean.
This, in turn, causes “good or above-normal rainfall” in areas such as Southern Africa, northeastern Australia and northeastern Brazil. The current La Niña is expected to last until April.
“At this time of year the ITCZ, the inter-tropical convergence zone, moves southwards and lies over South Africa. There is currently a low-pressure system over Namibia and that is why the rainfall is continuing,” Vermeulen says. “This trough [extended area of low pressure] brings in warm, very moist air which enhances rainfall in the area.”
Douw Steyn, professor of atmospheric science at the University of British Columbia, who is on sabbatical at the African Institute of Mathematical Sciences in Cape Town, explains that the temperature of water on the sea surface affects the global atmospheric temperature.
It alters precipitation patterns in sub-tropical regions because of global weather patterns such as trade winds. Stronger trade winds bring increased warm, moist air into Southern Africa, leading to more rain. La Niña is the obverse of El Niño, which causes dry weather in the southern hemisphere.
Vermuelen says it is very likely that La Niña is also causing the heavy downpours in eastern Asia. “In some places it’s just been raining non-stop. And remember, they have monsoons during our winter, so those rains are still coming. They are probably a spillover effect from La Niña,” he says.
Steyn agrees, saying that La Niña has caused tropical cyclones to move westward, bringing “enormous precipitation” to southern and eastern Asia. More than a million people have been affected in Sri Lanka, while four provinces in Thailand have been inundated and 1,4-million people in the Philippines have been displaced by more than three weeks of torrential rain.
While the rainfall in South Africa has been “above normal”, Vermeulen says he would not yet class it as “extraordinary”. “These floods are equivalent to those in 1988. I would say it is once-in-25-years flooding.”
Since heavy rains and flooding started in December, eight of South Africa’s provinces have been hit.
Vuyelwa Qinga Vika, spokesperson for the department of cooperative governance and traditional affairs, provided the following details of the toll in lives and infrastructure damage: