The first significant summer rainfall was recorded in the first days of October. Vrede in the Free State received more than 40mm, Warden more than 30mm and Bothaville 20mm. Rain was also recorded in Marble Hall (17mm), Witbank (10mm) and Bethal (15mm), as well as Mokopane (30mm) in Limpopo. Grahamstown in the Eastern Cape recorded approximately 30mm, Port Elizabeth 25mm, and Graaff-Reinet in the Karoo 22mm. Up to 50mm was recorded in parts of KwaZulu-Natal.
Significant rainfall still occurred in the winter rainfall area in September, resulting in very favourable conditions for winter grain production. The relatively low temperatures that were experienced also made a positive contribution to these conditions.
The intense drought continued in parts of the Northern Cape although good rainfall occurred in the southwestern parts, including Springbok, Garies and Kamieskroon, among others. This offered a welcome release from the drought. Areas that still experience disaster drought conditions are Pofadder, Kenhardt, Prieska, Van Wyksvlei, Carnavon and other districts in the central to southern parts of the Northern Cape.
Most of the Kalahari (to the north of Upington) is also very dry and has been recording below-average rainfall for nearly a decade. The northern interior of the Western Cape and parts of the Eastern Cape are also still in the grip of drought.
The levels of surface water are critical in some larger storage dams. The Vaal Dam is only approximately 33% full compared to around 55% at the same time last year. The Tzaneen Dam supplying water to the Letaba irrigation valley is less than 10% full and the level is dropping rapidly.
In the Eastern Cape, the Kouga Dam is at approximately 8% compared to nearly 40% at the same time last year. In addition, Nelson Mandela Bay hit Day Zero in late September. This severe water shortage also threatens the Gamtoos irrigation area, which is an important citrus exporting area.
The water level of storage dams in the Western Cape is favourable. The Theewaterskloof Dam is overflowing for the first time in many years. The Clanwilliam Dam, which supplies water to the Citrusdal and surrounding irrigation areas, is 100% full after being below 10% in May 2020.
The water level in Lake Kariba in Zambia is at 30,5%, the Katze Dam in Lesotho at 24,3% and Hardap Dam in Namibia at 35%.
El Niño-Southern Oscillation
Sea surface temperatures in the Niño areas all indicated solid La Niña values at the end of September. The Australian Bureau of Meteorology favours a medium to strong La Niña event that may last for most of the southern hemisphere’s 2020/21 summer. The La Niña is now well established with a more than 80% probability that it will remain in place until approximately autumn of 2021. Sea surface temperatures in the important Niño 3.4 area are approximately 1°C lower than average with forecasts indicating that it will probably cool down further to approximately 1,5°C below average.
The Southern Oscillation Index (SOI), which is the measure of interaction between surface conditions, is in the consistently positive phase of the SOI system. The development of La Niña and the SOI’s reaction to this phenomenon this early in summer is highly significant for rainfall.
Further cooling of the southwestern Indian Ocean took place in September 2020 with warming in the central to eastern parts of the ocean. These conditions are favourable for the continued development of a negative phase of the Indian Ocean Dipole (IOD).
Rainfall and climate
The presence of a La Niña phenomenon that is associated with a negative phase of the IOD is positive for summer rainfall. Although a La Niña event is usually responsible for a normal to later-than-normal start of the summer rainfall season, the middle to second part of the summer is wet to very wet.
Similar seasons with both La Niña and a negative IOD include:
The previous strong La Niña from 2010 to 2012 also resulted in above-average rainfall, although the IOD was in a weak negative to neutral phase.
Similar years, as mentioned above, also resulted in above-average rainfall in the extreme western interior of the country. These parts of the country have been in the grip of a drought since around 2012. The drought will likely be broken in these areas in the coming months.
Minimum temperatures are likely to remain below average until around the end of October. Average to above-average temperatures can be expected over the central to western, as well as the extreme northern interior, for the last part of October and November until the start of the rainy season.
There is a high probability for a below-average number of heat units. Consequently, producers are advised not to plant summer crops too late. Expected excessive rainfall in January to March can also favour early plantings.
Winter rainfall area
It is expected that the rainfall pattern will further shift from the southwestern Cape towards the southern, eastern, and northeastern parts of the country in October. Rain is still possible over the winter rainfall area with cold fronts visiting the southern parts of the country. Light rain is possible on 7, 8, 15 and 16 October and during the last week of the month.
Outlook for Namibia
The status of the La Niña and Indian Ocean is also incredibly positive for above-average rainfall during mid- to late summer with a high risk for flooding in lower-lying areas. The short- to medium-term outlook for rainfall indicates potential light rainfall during the second week of October. However, the start of the rainy season is likely to only occur during the second part of November.
It is expected that the very dry southern to southwestern parts of Namibia will also experience sufficient rainfall to break the drought, but only towards December or in the second part of the summer.
A last thought
Below-average temperatures with late frost still occurred and caused damage to crops in the central to southern interior. Below-average temperatures are likely to remain dominant until the end of October, while further frost is not expected.
Both the El Niño-Southern Oscillation and IOD are likely to produce favourable rainfall conditions for the summer of 2020/21, especially the central to western parts of the summer rainfall area. There is a higher-than-average risk for flooding in the second part of summer.
Surface water conditions in the Western Cape are once again favourable for the first time in nearly five to six years. However, the low levels of the Vaal Dam, dams in the Eastern Cape, as well as large storage dams in neighbouring countries are still a cause for concern. – Johan van den Berg, Independent Agricultural Meteorologist