The probability of a La Niña event increased to approximately 80% and is said to last from October 2021 to March 2022. Rainfall outlooks are favourable for early summer over the eastern parts, and mid- to late summer over the western parts of the summer rainfall area.

Current conditions

Very hot conditions started to occur in the last part of September over the central to northern and northwestern parts of South Africa (SA). Below-average temperatures still occurred over most of the winter rainfall areas, with a decrease of more than 10 to 70% in heat or growing units, compared to the two previous seasons since the beginning of August.

In the Ceres area of the Western Cape, the number of heat units from 1 August to 28 September 2021 was approximately 10% less compared to 2020 for the same period, and 65% less than in 2019. In the Breede River Valley near Robertson, the number was 7% less than in 2020 and 35% less than in 2019. In the area near Grabouw, the number of heat units for the period from 1 August was approximately 78% less compared to 2019, and 60% less compared to 2020. This will influence the growth and development of crops in the Western Cape.

Summer grain farmers are in a unique position for this time of the season with nearly all fields already cultivated and pre-plant fertiliser applied. The rain in April as well as some untimely rain in August in some areas enabled producers to start preparing fields roughly one to three months earlier. This can have an impact on the planting date as well as very favourable soil moisture and soil fertility conditions.

Drought conditions persist in the southwestern parts of the Northern Cape as well as the central to southern parts of the Eastern Cape and parts of the Klein Karoo. Grazing conditions are very poor in these areas, with borehole and surface water being nearly non-existent.   

Dam levels

The state of storage dams in the summer rainfall area are generally still very favourable with dams in the Free State at almost 90% capacity. Large storage dams like the Gariep and Vaal dams are at above 80% capacity, and Vanderkloof Dam is at almost 98%. Bloemhof Dam is still overflowing at 102%.

Because the buffer capacity is very low, there is a high risk of flooding should large amounts of rainfall occur in the catchment areas. Some dams in the Eastern Cape remain at critically low levels, while others are nearly empty, like Kouga Dam, which is at only 5,6% capacity. Western Cape dam levels are favourable although the rainy season is close to an end.

Water levels of Lake Kariba in Zambia are at approximately 39% capacity, compared to 31% during the same time last year. Katze Dam in Lesotho is at nearly 64%, while Mohale Dam is at 30%. The Hardap Dam in southern Namibia was at 58% capacity at the end of September this year.

El Niño–Southern Oscillation

El Niño–Southern Oscillation (ENSO) is still borderline between neutral and La Niña. All Niño areas were between 0,2°C warmer than usual (Niño 1+2) and 0,5°C (Niño 4) cooler than normal, which is solid in the neutral range. Forecasts are show increasing probabilities for the development of a full-scale La Niña event.

Probabilities for La Niña increased from just shy of 50% in July to between 70 and 80% presently. Forecasts also show an increase in intensity from approximately 0,5 to 0,9°C cooler than normal, to a current expectation of between 1 and 1,3°C cooler than normal. The current outlook represents a moderate to strong La Niña. Outlooks are assisted by the development of a cooling trend in the deeper layers below the Niño-areas.

The current strengthening of La Niña is significant because the months of September and October are very important to set the trend for the rest of the summer season. It is therefore very likely that the La Niña event will occur and will continue to influence weather systems until at least March 2022.    

The Southern Oscillation Index (SOI) as indicator of the effect of sea surface interaction with overlying weather systems in the Niño areas, remains positive since June 2021.                   

Indian Ocean

Sea surface temperatures showed a cooling trend for nearly 80% of the Indian Ocean, except for some warming taking place in the northern Indian Ocean. Although the Indian Ocean Dipole (IOD) is now in a neutral phase, it is due to the lack of a warming dipole in the central to eastern Indian Ocean. The Indian Ocean in its current state will still play a positive role towards summer rainfall over SA.      

Summer rainfall area

The persistency of the positive phase of the SOI since June 2021 as La Niña indicator is very positive for summer rainfall. The positive phase of the SOI also occurred in September 2020, 2010, 2008, 2000, 1998, 1996, 1988 and 1981, resulting in above-average rainfall for most of the summer rainfall area. 

The neutral phase of the Indian Ocean Dipole (IOD) will not have a negative effect on rainfall and, considering the current state of the Indian Ocean where approximately 80% of the surface area is cooler than normal, it can rather have a positive effect on rainfall. High pressure systems will dominate the Indian Ocean and adjacent parts of eastern Africa up to Somalia with below-average rainfall. However, it will assist in the development of low-pressure systems with above-average rainfall over the western and southern African sub-continent.  

Average to above-average rainfall is expected over the central to eastern parts of SA from the middle of October, reaching a peak in November/December. The improved rainfall conditions will then spread westwards over the central to western parts from January to March.

It is still uncertain if the westward forcing of weather systems will be sufficient to reach the extremely dry south-western parts of the Northern Cape. Rainfall conditions are positive for the Eastern Cape and average to above-average rainfall is expected from the last part of October. Extreme wet conditions are expected in October and November over the northern parts of the Eastern Cape, most of KwaZulu-Natal (KZN), and adjacent parts of Mpumalanga.

Short-term outlooks are positive for the central to south-eastern and eastern parts of the country for the spells between 15 to 20 October and the first week of November. 

Temperature

Hot to very hot conditions will still be present over the northern to northwestern and western parts for most of October until the first rain is due. Cold snaps may still occur with a sharp drop in temperature and possible frost damage in the Southern Free State, eastern parts of the Northern Cape, central to north-eastern parts of the Eastern Cape, and southwestern parts of KZN. 

A mild season is expected over the central to eastern parts from about November, but hot to very hot in the west and northwest.  

Winter rainfall area

Rainfall conditions deteriorated during September over the Southwestern Cape with only light falls. It seems that the probability for further rain is very low over the Swartland and adjacent areas. Yet rain is still expected over the Southern Cape until at least the middle of October. With the development of the current La Niña, it is possible that summer rainfall may occur in mid- to late summer as a result of summer systems moving southwards over Namibia.        

Namibia

Not much rain is expected for October and the first part of November. Outlooks for rain improve from the second part of November 2021. Very warm to hot conditions are expected for most of October and the first part of November.  

In conclusion

The probability for a La Niña event increased to approximately 80% and a medium level intensity is expected, with temperatures in the La Niña areas to reach between 0,9 and 1,3°C cooler than normal. The La Niña will remain prominent until at least March 2022.

Most of the Indian Ocean except for the northern parts near India was at cooler-than-normal levels at the end of September. Average to above-average rainfall is expected over the central to eastern summer rainfall area in the first part of summer, extending to the west in the mid-to second part of summer.

Short-term outlooks are favourable for rain over the central to eastern parts in the first week of March 2022, from 15 to 20 March and first week of November.

There is a possibility of frost over the southern parts of SA in the first week of March 2022. –Johan van den Berg, Independent Agricultural Meteorologist