by: Johan van den Berg, Independent Agricultural Meteorologist (M.Sc Agric, Agricultural Meteorology, UFS)
- Favourable winter rainfall conditions are expected to continue for the Winter Rainfall Area for most of August but drier conditions in September.
- A La Nina event from about September/October is more likely to develop than initially anticipated. Favourable Indian Ocean conditions combined with La Nina is very positive for summer rainfall.
Above-average rainfall occurred over many parts of the Winter Rainfall Region in June and the first part of July 2021. The south-western Cape in districts like Ceres and Malmesbury received on average the best falls. Franschhoek and Kirstenbosch more towards the Winelands and Cape Town Metropole also received very good falls. Rain also occurred over parts of the Westcoast and adjacent interior as far north as Springbok and Garies but it was less than 40mm and much less compared to 2020 at the same time.
There was some rain over the coastal areas of the Eastern Cape but it was insufficient to replenish surface freshwater sources. Very little or no rain occurred over most of the Summer Rainfall Region although light to moderate snowfalls occurred over parts of the southern interior as far north as the southern Drakensberg range in areas like Barkly East and the southern Free State in Trompsburg.
Record low minimum temperatures were recorded in the first part of July in the central interior and eastern Highveld. Standerton recorded a low of -11.3°C on 14 July, Koppies -10.9°C, Bothaville -10.3°C and Prieska in the Northern Cape -8.1°C. Extreme cold conditions are also likely to occur from 22 to 25 July.
Production conditions for winter crops in the Western Cape are very favourable with regular rainfall events and significant falls.
Grazing conditions are in general favourable for most of the country for this time of the year with sufficient amounts of dry matter but with lower quality, except for the dry southern parts of the Northern Cape. Veld fires are becoming already a serious problem early in the fire season. Very windy conditions as part of cold fronts sweeping over the country occurred regularly in June and July. With the fire season to last until at least September/October is it expected that the fire risk will increase.
State of storage dams is in general favourable with the average level of Free State dams on about 94.8%. The dams in the Free State are the most important suppliers of water to the major irrigation areas in the country and will ensure that there is sufficient water for the next summer season. There is however a risk of flooding if above-average rainfall is to occur in 2021/22. Dams in the Eastern Cape remain at low levels with the Kouga Dam at 4.0%, Impofu dam and 14.3% and Bridle Drift dam at 23.6%.
The Clanwilliam dam in the Western Cape which was at very low levels in the first part of June reached full capacity in the middle of July 2021 following the above average rainfall. Levels of storage dams in the Western Cape improved by about 20% since the middle of June 2021 and are currently on average about 75% of full capacity. Of importance is that all the larger and important storage dams like Theewaterskloof, Clanwilliam and Bergrivier dams are all at 100% capacity or overflowing.
The water level in Lake Kariba in Zambia is at 50.6% compared to about 40.8% last year at the same time while the Katze dam in Lesotho is at about 69.9% and the Mohale dam 33%. The Hardap dam in southern Namibia was in the middle of July 2021 at 63.6% of full volume.
ENSO and the Indian Ocean
ENSO (El Nino Southern Oscillation)
ENSO is currently in a neutral state since about April 2021. There was an upgrade in the probability for an occurrence to just under 70% probability for the development of a La Nina event to reign from about October 2021 to March 2022. The probability for an El Nino event is about zero with an about 30% to 40% probability of neutral conditions to continue. This increased probability from about 50% to 70% for La Nina conditions is very significant, especially for this time of the year.
The expected intensity of the developing La Nina is still uncertain but it will most probably be moderate.
The Southern Oscillation Index as an indicator of the effect of surface conditions in the Nino areas also showed a rapid change from solid neutral values since about April 2021 to positive values (La Nina) in July but it may be due to localised short-term disturbances. It is however significant that the SOI is already on the positive side of neutral with some other indicators like trade winds also stronger than normal, assisting in the forecast for La Nina conditions to develop.
Indian and the Atlantic Ocean
A stretch of between 2,000km and 3,000 km of cooler than normal waters in the western Indian Ocean (adjacent to the Africa coastline) from southeast of Cape Town up to the Horn of Africa (Somalia), was still present in the second part of July 2021 with warmer than normal waters east towards Australia.
This type of situation is consistent with a negative phase of the Indian Ocean Dipole (IOD) index. The IOD was already in a negative phase for 8 of the last 9 weeks and forecasts also favour the continuation of the negative IOD in months to come until at least December 2021. A strong negative IOD is positive for increased rainfall probabilities for Southern Africa for the spring and summer.
Rainfall and Climate
Summer Rainfall Area
Short term outlooks for the rest of the winter and early spring are negative for most of the Summer Rainfall Area. It is however possible that rainfall may start to occur over the very dry Eastern Cape coastal areas and adjacent interior as well as the KZN coast in August as part of the migration of rainfall patterns from the winter rainfall areas to the summer rainfall areas.
It is most likely that conditions in the Indian Ocean will be the main driving force of climate and weather patterns over Southern Africa in the next summer season but if the expected La Nina event will intensity, the combined effect can be responsible for increased probabilities of above-average rainfall. Rainfall outlooks for the summer of 2021/22 are positive for the central to eastern parts of Southern Africa from about October that can expand from about December to the more western parts. Drier conditions are possible towards February 2022 for the eastern summer grain production areas with the migration of rainfall more towards the west later in summer.
Very cold conditions in the middle to second part of July signals the peak of winter with warmer conditions to start developing. There are however still indications of below average temperatures in the first part of August as well as in the last week of August. The risk for late frost damage is again very high, especially when temperatures start to increase and plant development starts. Cold snaps or short duration drops in temperature during these critical and sensitive periods can cause severe damage.
Winter Rainfall Area
Favourable rainfall conditions are likely to continue until at least the end of August but drier conditions possible from the beginning of September. Temperatures are likely to remain below average for the rest of July and most of August.
Little or no rainfall is expected for the winter and spring. Outlooks are positive for summer rainfall starting from about November 2021.
Temperatures will still be below average until at least the second part of August, especially minimum temperatures and frost may occur until the end of August.
Summary and conclusion
- The probability for La Nina development increased and there is an about 60-70% probability that La Nina will develop from September/October 2021. This is positive for summer rainfall.
- Favourable Indian Ocean temperatures in combination with La Nina can further enhance the rainfall probabilities for the Summer Rainfall Area and can even assist in an earlier onset of rainfall for the summer.
- Rainfall conditions are positive for most of the Winter Rainfall Region until at least the end of August but drier conditions are expected in the second part of September.
- Extreme cold conditions reach a peak in the last 10 days of July but frost still possible until at least the end of August and even later in the central to southern parts of the country.
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