Above-average rain expected from November to January

The development of the La Niña system so far suggests that above-average rainfall may occur from November to January, but not necessarily for the rest of the season.

By Alani Janeke, Senior Journalist at African Farming and Landbouweekblad

Rain has fallen over large parts of the country over the past 10 days, except in the Northern Cape and the western parts of North West. It is estimated that up to 70% of the summer rainfall region received above-average rain during September, and precipitation over the winter rainfall region was below average.

La Niña’s Impact on the Rest of the Summer Rainy Season

Two main factors influence seasonal forecasts. The first is the El Niño–Southern Oscillation (ENSO), which occurs in the Pacific Ocean and has two primary phases: La Niña and El Niño. The second is the state of sea surface temperatures in the Indian Ocean, which can also influence summer rainfall.

“Sea surface temperatures are very important because they determine the overlying weather and climate systems, which in turn have a chain reaction that influences conditions far from the oceans,” explains independent agricultural meteorologist Johan van den Berg.

Sea surface temperatures in the Niño regions are currently slightly cooler than normal, but still very close to normal, Van den Berg says. However, temperatures in the key Niño 3.4 region of the equatorial Pacific have now been below the threshold of 0,5°C cooler than average for a second consecutive week. “Technically, we cannot yet speak of a full-scale La Niña system, because temperatures must remain at least 0,5°C cooler than normal for five consecutive months, but current developments do indicate a La Niña trend.”

Van den Berg says forecasts suggest that this trend could continue and develop into a full-scale La Niña event. The system is expected to peak around December, after which it may weaken fairly quickly during the second half of summer.

If La Niña continues to strengthen from October to December and then weakens, it increases the likelihood of good rainfall during November and December, particularly over the central to eastern parts of the country, but not necessarily further west, he says. From January – and especially in February – weaker rainfall conditions may develop over large parts of the country.

Because the La Niña system may start to weaken earlier than usual, the western parts of the summer rainfall region might not benefit as much from it.

Also read: Summer rainfall: Wetter days ahead for the Eastern Cape

State of Sea Surface Temperatures in the Indian Ocean

The other important factor is what is happening in the Indian Ocean. The position of warmer and cooler sea surface temperatures in the Indian Ocean, a climate phenomenon known as the Indian Ocean Dipole (IOD), determines how weather and climate systems respond, Van den Berg says.

He notes that warmer water closer to the African coastline is often associated with tropical storms and cyclones, leading to wetter conditions over East Africa but drier conditions over Southern Africa, and vice versa.

A strong signal now indicates that the IOD has become more negative, meaning sea surface temperatures are cooler closer to Africa and warmer near Australia. This is favourable for rainfall over Southern Africa. Van den Berg adds that the IOD was last in such a strong negative phase in October 2022, which is likely a major factor behind the current good summer rainfall conditions.

Also read: Chance of frost in the third week of October

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Outlook for Summer Rainfall

The IOD’s influence is usually stronger during the early part of summer, whereas ENSO plays a greater role in rainfall variations from mid- to late summer.

Given the strong negative phase of the IOD, conditions are favourable for rainfall from October to December, Van den Berg says. The weaker La Niña system that developed relatively early will have more of an effect around midsummer, but rainfall may start to taper off from February as the system weakens. Overall, indications point to above-average rainfall from November to January over the central to eastern parts of the country.