Important Issues
Rainfall conditions for the Western Cape in the last few weeks are more favourable than expected.
La Nina development and Negative Indian Ocean Dipole (IOD) in coming months are positive for summer rainfall.
1. Current conditions
Better than expected rainfall occurred over most parts of the Western Cape and south western parts of the Northern Cape since the end of May 2020. There were also significant falls of rain in the Eastern Cape, large parts of KZN and the central tot northern interior of the country. Very important is that the drought-stricken areas of the western parts of the Northern Cape and adjacent parts of the Western Cape received significant falls. Springbok in the Northern Cape received more than 40mm, Garies more than 30mm, Nieuwoudtville more than 60mm and Calvinia more than 25mm. Other parts of the Western Cape that received significant falls were Ceres with more than 100mm, Paarl and Wellington with well over 60mm with parts of the Swartland (Malmesbury) also more than 50mm. Rain was also recorded in the drought stricken Eastern Cape with significant falls that were received in districts like Joubertinia and Port Elizabeth. In the period of 12 to 15 June, rainfall amounts of more than 20mm were measured in the Northwest Province, Mpumalanga and KZN.
Extreme low temperatures were recorded during the night of 13 June with minimum temperatures below of close to -10°C in many areas like Standerton, Warden, Douglas, Koppies and Bloemfontein. Compared to the minimum temperatures in the recent past two decades is it from the lowest temperatures measured in the month of June. Extreme cold events are usually more frequent in July.
The better than expected rainfall conditions for the Western and Eastern Cape, the extremely cold June and the significant rainfall event in the northern interior may be an indication of a step change in the climate and outlooks for the next 3 to 5 years.
Surface water is in general in a favourable state except for dams in the Eastern Cape like the Kougadam supplying water to the Nelson Madela Metro with only 7.8% of full capacity compared to the 44.3% last year the same time. Another source of concern is the Vaaldam with only 46.5% currently compared to the 68.5% last year the same time. Positive is that the Clanwilliam dam in the Western Cape started to recover with good falls of rain in the past weeks and is now at just below 20% compared to the about 6% a few weeks ago. The Theewaterskloofdam that is the largest storage dam in the Western Cape is currently at about 55% compared to 41.1% last year the same time.
The two most important dams in Lesotho, Katse and Mohale dams are currently about 35.3% and 11.4% of full capacity compared to the 30.1% and 33.4% last year the same time, respectively.
2. ENSO and Indian Ocean
2.1 ENSO (El Nino Southern Oscillation)
There was a total of five El Nino and four La Nina events since the 2009/10 season. El Nino events were 2009/10, 2014/15, 2015/16, 2018/19 and 2019/20 with 2009/10 and 2015/16 very strong events. The 2019/20 El Nino was a very weak event while 2014/15 and 2018/19 that were moderate. Only one of the four La Nina events (2010/11) can be classified as a strong La Nina event with the other three events weak to moderate.
The current status of ENSO is neutral with sea surface temperatures of all four Nino-areas in the neutral range or borderline La Nina. There was a cooling of between 0.5°C and 1°C since the middle of May 2020 in most of the Nino-areas. The Southern Oscillation Index or SOI that is a measure of the coupling of sea surface conditions with over lying climate systems, is however also neutral but still more towards El Nino (negative SOI).
Forecasts: Most forecasts are opting for a weak to moderate La Nina event from about July 2020 to at least March 2021. Cooling of waters in deeper layers of the tropical Pacific is also supporting a La Nina development to occur.
2.2 Indian Ocean
The Indian Ocean Dipole (IOD) is currently also in a neutral state. Most international models are forecasting a negative phase of the IOD to develop in coming months to become a moderate negative IOD event. A negative event is indicative of cooler waters towards the western Indian Ocean (closer to the African coastline) and warmer towards the eastern Indian Ocean (northern Australia). This decreases the risk of tropical cyclones and strong cyclonic depressions in summer and prevents the disturbance of normal rainfall patterns.
3. Rainfall and Climate expectations
3.1 Summer Rainfall Area
Short to medium term forecasts are not favourable for significant rainfall for the rest of the winter and spring. If the IOD or Indian Ocean Dipole develops rapidly towards a negative phase can it increase the probability for rain in spring and early summer. The rainfall event in the second week of June over the northern and north eastern interior can however be indicative of an earlier start of the summer rainfall season in the eastern parts of the Summer Rainfall Region.
Of significance for the mid to late summer is the forecast of La Nina conditions combined with a negative IOD phase. This combination in history was responsible for average to above average rainfall conditions in the first part of summer (November and December) over the eastern areas (KZN, Mpumalanga, Gauteng, Eastern Free State and south eastern parts of Limpopo) tapering off towards February and March with drier conditions. The band of rainfall then shifts towards the west becoming wet to very wet from about the December/January to March.
Although it is still very early in the season for accurate forecasts, are there for the first time in the near past recent years strong indicators for this time of the year.
Temperatures, especially minimum temperatures are expected to remain cold until about the first part of August before warming up.
3.2 Winter Rainfall Area
It is interesting that actual rainfall recorded since the last week of May 2020 was better than forecasts initially indicated. It is also significant that rain also occurred in the drought stricken West Coast and adjacent interior of the Western and Northern Cape.
Above average rainfall occurred in districts like Ceres, Paarl, Nieuwoudtville and Springbok. Less rain occurred over the Southern Cape compared to the Swartland and north along the Westcoast.
Short to medium term outlooks for the Swartland and surroundings are still positive for rain until at least the middle of August with regular at least light falls. It seems however that the probabilities are more favourable for the Rûens and South coast areas later in the winter and spring compared to the south western areas like the Swartland. This is the “normal” trend of the South African rainfall patterns starting in winter towards the south western areas and West Coast, moving gradually eastwards and northwards later in the winter and spring to start the summer rainfall in about September/October over the north eastern parts of the country and then move inland in mid and late summer.
3.3 Namibia
Very little rain is expected for most of the country until at least October. It is very likely that rainfall conditions will improve dramatically from about November to reach a peak in January to March. Above average rainfall is expected for most of the country for the 2020/21 summer season.
Lower temperatures with frost can still be expected until at least the last part of July.
4. Summary and conclusion
Production conditions improved over large parts of the Western Cape.
Both ENSO and Indian Ocean (IOD) are most probably heading for favourable rainfall conditions for the summer of 2020/21, especially the central to western parts of the Summer Rainfall Area.
