Climate and agricultural conditions - October 2020
- The winter of 2020 continued and frost damage occurred in the last week of September. Below average temperatures are still possible until the end of October but frost is not expected any more for this season.
- La Nina and favourable sea surface temperatures in the Indian Ocean are still on track for favourable summer rainfall conditions.
Frost and low temperatures continued to occur over much of South Africa. Mild to severe frost in the last week of September caused damage to grapes, fruit, nuts, potatoes and early planted winter wheat in parts of the Free State, Northern Cape and Northwest. These occurrences are on the later side of the expected spectrum for frost occurrences.
The first significant summer rainfall was recorded in the first days of October with Vrede in the Free State with more than 40mm, Warden with more than 30mm, Bothaville 20mm. Rain was also recorded in Marble Hall 17mm, Witbank 10mm and Bethal 15mm as well as Mokopane in Limpopo with 30mm. Grahamstown in dteh Eastern Cape recorded about 30mm Port Elizabeth 25mm and Graaff-Reinet in the Karoo 22mm. Up to 50mm was recorded in parts of KZN.
Significant rainfall still occurred in the Winter Rainfall Area in September, resulting in very favourable production conditions for winter grain production. The relative low temperatures that were experienced were also positive for favourable production conditions for winter grains.
The intense drought conditions continued in parts of the Northern Cape although very good falls of rain in the south western parts in districts like Springbok, Garies and Kamieskroon occurred and released these areas from the grip of the drought. Areas that are still experiencing 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) are also very dry with below average rainfall for nearly a decade. The northern interior of the Western Cape as well as parts of the Eastern Cape are also still in the grip of the drought.
The levels of surface water are critical in some larger storage dams like the Vaal dam with only about 33% of full capacity compared to about 55% last year the same time. The Tzaneen dam supplying water to the Letaba irrigation valley is at less than 10% of full capacity and the level is dropping rapidly. In the Eastern Cape is the Kouga dam at about 8% compared to nearly 40% last year the same time and the Nelson Mandela Metropole is rapidly closing in on Day Zero if replenishment is not taking place within weeks. It also threatens the water for the Gamtoos irrigation area that is a very important citrus exporting area. The level of storage dams in the Western Cape is favourable with the Theewaterskloof dam overflowing for the first time in many years. The important Clanwilliam dam supplying water to the Citrusdal and surrounding irrigation areas is also at 100% of full capacity 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%.
2. ENSO and Indian Ocean
2.1 ENSO (El Nino Southern Oscillation)
Sea surface temperatures in the Nino-areas were all indicating solid La Nina values at the end of September. The Australian Bureau of Meteorology favours a medium to strong La Nina event to last for most of the Southern Hemisphere summer of 2020/21. The la Nina is now well established and more than 80% sure that it will remain in place until about the fall of 2021. Sea surface temperatures in the important Nino3.4-area are about 1°C lower than average with forecasts indicating that it will probably further cool down to about 1.5°C below average.
The Southern Oscillation Index (SOI) that is the measure of the interaction between surface conditions is in the consistently positive phase of the SOI system. The La Nina-development and the SOI reacting to the development this early in the summer season, is highly significant for rainfall.
2.2 Indian Ocean
Further cooling of the south western Indian Ocean took place in September 2020 with warming in the central to eastern parts of the Ocean. This is favourable for the continued development of a negative phase of the Indian Ocean Dipole (IOD).
3. Rainfall and Climate
3.1 Summer Rainfall Area
Rainfall: The presence of a La Nina phenomenon associated with a negative phase of the Indian Ocean Dipole is positive for summer rainfall. Although a La Nina event is usually responsible for a normal to later than normal start of the summer rainfall season, is the mid- to second part of the summer wet to very wet.
Similar seasons in history with both La Nina and negative IOD were:
The previous strong La Nina from 2010 to 2012 resulted also in above average rainfall although the Indian Ocean Dipole was in a weak negative to neutral phase.
The similar years as mentioned above resulted also in above average rainfall in the extreme western interior of the country. These parts of the country are currently still in the grip of a drought since about 2012 and it is likely that the drought will be broken in these areas in the months to come.
Minimum temperatures are likely to remain below average until about the end of October. Average to above average temperatures can be expected over the central to western as well as 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 and farmers are advised not to plant summer crops too late. Expected excessive rainfall in January to March can also favour early plantings.
3.2 Winter Rainfall Area
It is expected that the rainfall pattern will further shift from the Southwestern Cape towards the southern, eastern and north eastern parts of the country in October. Rain is still possible over the Winter Rainfall Area with colds fronts visiting the southern parts of the country. Light rain is possible on 7-8, 15-16 October and in the last week of October.
The current status of the La Nina and Indian Ocean is also very positive for above average rainfall for the mid– to late summer with a high risk for flooding in lower lying areas. Short to medium term outlooks for rainfall indicates light falls of rain possible in the second week of October but the start of the rainy season is likely only to occur in the second part of November.
It is expected that the very dry southern to south western parts of Namibia will also experience sufficient rainfall to break the drought but only towards December or in the second part of the summer.
4. Summary and conclusion
- Below average temperatures with late frost still occurred and caused damage to crops in the central to southern interior. Below average temperatures likely to remain dominant until the end of October but further frost is not expected.
- Both ENSO and Indian Ocean (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 are for the first time in about 5 to 6 years again in a favourable state for the Western Cape but the Vaal dam, dams in the Eastern Cape as well as large storage dams in neighbouring countries are still reason for concern with very low levels.
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