SA has a unique rainfall dilemma
SOUTH Africa is among a handful of countries that experience winter rainfall in some areas and summer rainfall in others. The south-western tip of the country has a Mediterranean climate, with hot, dry summers and cool, wet winters.
The interior is dry and cold in winter, with subsiding air from strong high-pressure systems. In summer this high pressure moves south, causing the dry conditions in the Western Cape and allowing convective storms in the interior.
All of this is well documented. What is unclear is where exactly the boundary between the winter and summer rainfall zones exists. There is consensus that Cape Town is in the winter rainfall zone and Joburg in the summer rainfall zone, but these cities are separated by 1 400km. What about the zone in between?
Not even climate scientists are sure. This was established in research aimed at pinning down where the line might be drawn.
Geo-referencing all the existing rainfall seasonality maps, using Geographic Information System software to determine the areas of overlap between the maps, it was found that there was no agreement on the position of this line.
This is a problem for farmers, tourists and even high school geography pupils who need to accurately classify the rainfall seasonality of a particular city.
A large area of the country has an agreement in rainfall zone classification of 75% or more. However, the region between the winter and summer rainfall zones is highly contested.
A large band is classified by half of the scientists as a winter rainfall zone and half as a summer rainfall zone.
A smaller area, including the town of Sutherland, is completely disputed, with researchers classifying it as summer, winter and year-round rainfall zones.
This problem is largely unique to South Africa. Much of the Mediterranean, which experiences a similar climatic transition, comprises small countries with relatively coherent rainfall regimes.
Other countries that have changes in rainfall zone have a density of rainfall stations, which means that they don’t face the same problem.
To complicate matters, boundaries appear to be shifting under climate change. In South Africa, the winter rainfall zone extended as far north as Lesotho and the Free State during the Last Glacial Maximum, 24 000 years ago. It has progressively migrated south over the past 20 000 years. Fitchett is a senior lecturer in physical geography, Wits University; Curtis is a professor of geography, Wits; Roffe is a PhD student of climatology, Wits.