The probability for La Nina to occur increased to about 80% and 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 north-western parts of the country. Below average temperatures still occurred over most of the Winter Rainfall Areas with a more than 10%-70% reduction 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 were about 10% less compared to 2020 for the same period and 65% less than in 2019. In the Breede River Valley near Robertson the number were 7% lower than 2020 and 35% lower than in 2019 and in the area near Grabouw, the number of heat units for the period from 1 August were about 78% lower compared to 2019 and 60% lower in comparison to 2020. This will impact on the growth and development rate 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 fertilizer applied. The rain in April as well as some untimely rain in August in some areas enabled farmers to start preparing fields about 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 south-western parts of the Northern Cape as well as the central to southern parts of the Eastern Cape and parts of the Kleinkaroo. Grazing conditions are very poor in these areas with bore hole and surface water nearly non-existent.
State of storage dams in the Summer Rainfall Rea are in general still very favourable with dams in the Free State at about 90% of full capacity with large storage dams like Gariep and Vaal dam above 80% and Vanderkloof dam about 98% full. The Bloemhof dam is still over-flowing at 102%. There is a high risk for flooding if high rainfall amounts occur in the catchment areas because the buffer capacity is very low. Some dams in the Eastern Cape remain at critical low levels or is nearly empty like Kouga with only 5.6%. Western Cape dam levels are very favourable although the rainy season is very close to an end.
The water level in Lake Kariba in Zambia is at about 39% compared to about 31% last year the same time while the Katze dam in Lesotho is at about 64% and the Mohale dam 30%. The Hardap dam in southern Namibia was at the end of September 2021 at 58% of full volume.
ENSO and Indian Ocean
ENSO (El Nino Southern Oscillation)
ENSO is still borderline between neutral and La Nina. All Nino areas were between 0.2°C
warmer than normal (Nino1+2) and 0.5°C (Nino 4) cooler than normal that is solid in the neutral range. Forecasts are showing increasing probabilities for the development of a full scale La Nina event. Probabilities for La Nina increased from just shy of 50% in July to between 70% and 80% currently. Forecasts also are showing an increase in intensity from about 0.5°C to 0.9°C cooler than normal to a current expectation of between 1°C and 1.3°C cooler than normal. The current outlook represents a moderate to strong La Nina. Outlooks are assisted by the development of a cooling trend in the deeper layers below the Nino-areas.
The current strengthening of La Nina 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 therefor very likely that the La Nina will occur and will remain 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 Nino 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 that is taking place in the northern Indian Ocean. Although the Indian Ocean Dipole (IOD) is now in a neutral phase, is it 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 Southern Africa.
Summer Rainfall Area
Rainfall
The persistency of the positive phase of the Southern Oscillation Index (SOI) since June 2021 as La Nina indicator, is very positive for summer rainfall. The positive phase of the SOI also occurred in September months of 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 about 80% of the surface area is cooler than normal, 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 but will assist in the development of low-pressure systems with above average rainfall over the western and southern African subcontinent.
Average to above average rainfall is expected over the central to eastern parts of South Africa from about the middle of October and 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 about the last part of October. Extreme wet conditions are expected in October and November over the northern parts of the Eastern Cape, most of 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 30 September and 7 October, 15 to 20 October and the first week of November.
Temperature
Hot to very hot conditions will still be present over the northern to north-western and western parts for most of October until the first rain is due. Cold snaps may still occur with a sharp drop in temperature from for example 2 to 4 October with 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 to 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 but rain is still expected over the Southern Cape until at least the middle of October.
With the development of the current La Nina is it 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 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 first part of November.
Summary and conclusion
The probability for a La Nina event increased to about 80% and a medium level intensity is expected with temperatures in the Nino-areas to reach between 0.9°C and 1.3°C cooler than normal. The La Nina will remain prominent until at least March 2022.
Most of the Indian Ocean with the exception of 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 and 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, from 15 to 20 March and first week of November.
There is a possibility of frost over the southern parts of the country in the first week of March.
Johan van den Berg
Independent Agricultural Meteorologist
(M.Sc Agric, Agricultural Meteorology, UFS)
