Sponsored by Santam Agriculture- Johan van den Berg -Independent Agricultural Meteorologist (M.Sc Agric, Agricultural meteorology, UFS)
La Nina still is in a development phase but it already reached moderate to strong levels and will be present until at least March/April 2021.
Rainfall conditions up until now were in general less favourable than expected but outlooks are still very positive for the next months and the risk for high rainfall amounts remain very good for most of the Summer Rainfall Area.
1. Current conditions
November 2020 was characterised by average to above average rainfall over the central to eastern parts of the Summer Rainfall Area as well as regular lighter falls of rain over the southwestern and Southern Cape and Garden Route.
Most of the Summer Grain Area received significantly more rain from 1 July 2020 compared to the same period in 2019 except for some areas in Mpumalanga:
Free State: Bothaville 205mm in 2020 and 99mm in 2019; Hoopstad 135mm 2020 and 50mm in 2019; Vrede 417mm in 2020 and 160mm in 2019; Warden 319mm in 2020 and 106mm in 2019.
Mpumalanga: Amersfoort 197mm in 2020 and 203mm in 2019; Standerton 224mm in 2020 and 192mm in 2019; Delmas 124mm in 2020 and 283mm in 2019; Witbank 207mm in 2020 and only 74mm in 2019; Morgenzon 203mm in 2020 and 223mm in 2019.
Northwest: Coligny 153mm in 2020 and 120mm in 2019; Mareetsane 316mm in 2020 and 81mm in 2019; Schweizer Reneke 110mm in 2020 and 40mm in 2019.
Very little or no rain was recorded from 1 July 2020 over the western and north western parts of the Northern Cape, except for areas closer to the coastline like Springbok and Garies that received above average rainfall due to strong cold front activity in 2020.
Average to above average rainfall occurred over parts of the Western Cape: Ceres received about 540mm since 1 July with the long term average for the same time about 654mm; Paarl received about 380mm and the average is 440mm; Malmesbury about 250mm and the average of 190mm and Kirstenbosch 680mm and the average of 720mm since 1 July. The distribution over time was also excellent with only short dry periods. This resulted in very good production conditions.
Levels of surface water are in general favourable, especially in the Western Cape. Although there was some inflow of water in the Vaal dam is it still below 36% currently and critically low. Levels of the important dams in the Eastern Cape also remain very low. The Tzaneen and Middle Letaba dams in Limpopo is below 10% of full capacity and close to empty, respectively.
The water level in Lake Kariba in Zambia is at 18.9%, the Katze dam in Lesotho is at 26% and the Hardap dam in Namibia at 27.4%.
2. ENSO and Indian Ocean
2.1 ENSO (El Nino Southern Oscillation)
Surface temperatures in the Nino-areas are constantly at La Nina levels. Temperatures of the important Nino-3.4-area is ranging between 1.4°C and 1.7°C cooler than normal for at least the last two months. Forecasts indicate that it will reach its peak in December 2020 or January 2021 and will last until at least March 2021.
The Southern Oscillation Index (SOI) that is a measure of the interaction between surface conditions in the Nino-areas and over lying weather systems, is also consistently reflecting La Nina conditions since August 2020.
2.2 Indian Ocean
Large parts of the southern and south western Indian Ocean are still about 1°C to 2°C cooler than normal but the Indian Ocean Dipole (IOD) index is now already for a few weeks in a neutral phase. Some warming took place in the southern areas of the Ocean and cooling in the central Indian Ocean to result in more neutral or average conditions.
3. Rainfall and Climate
3.1 Summer Rainfall Area
Rainfall: Although there was some average to above average rainfall during November 2020, was the rain less than expected over the eastern areas. With the La Nina solidly settled are outlooks still very positive for further rain in the next months. There were also La Nina events in history that resulted in a late start to the rainy season like in 1973/74 but Upington in the Northern Cape received more than 400mm in January and February 1974.
Short term outlooks are still not positive over the western parts of South Africa and Namibia due to the overriding effect of the Atlantic Ocean High Pressure that is still dominant, resulting in unfavourable rainfall conditions. Heavy rain is expected over Zambia and Zimbabwe in the first part of December that can start to move west over Namibia, Botswana and South Africa from the second part of December.
Heavy falls of rain are still a very high probability for the January to March 2021 period or even later in the season.
Temperature
Average to above average daytime temperatures can be expected over the western interior for most of December but mild conditions over the central to eastern parts. Minimum temperatures remain in general below average.
3.2 Winter Rainfall Area
Frontal activities are still in progress with weak cold fronts still regularly moving over the southern parts of the country. Short term forecasts show some rain over the south western Cape on 8 and 9 December and again on 10 and 11 December. Heavier falls of more than 20mm are possible over the Southern Cape on 6 and 7 December as well as in the last two weeks of December as a result of summer rainfall systems.
La Nina not always favour rain in the winter period and is it possible that drier conditions may occur in the winter of 2021.
3.3 Namibia
Some rain occurred in November 2020 over the northern parts of Namibia but very dry conditions still prevail in the central to southern parts. With the La Nina still present until at least March 2021 and a relatively moderate to strong development, is it favourable for rainfall in the second part of summer.
The risk for heavy falls and flooding remains high in the January to March period, even for the current dry southern parts of Namibia.
4. Summary and conclusion
The lack of rain up until now over the western interior is still not representing a deviation from the La Nina effects on rainfall. If rainfall is still lacking towards the second part of January is there reason for concern.
Although less rain than expected occurred over the eastern parts of the country, was it still above average for large parts.
La Nina development is still in progress but the Indian Ocean Dipole (IOD) returned to neutral values, being in a negative phase in August and September.
Surface water conditions are favourable for the Western Cape but the Vaal dam, dams in the Eastern Cape and Limpopo as well as large storage dams in neighbouring countries are still at low levels although some improvement start taking place in some areas and dams.
