The winter of 2020 experienced one of the most intense spells of low temperatures in at least the past two decades.
The development of a La Nina event and favourable Indian Ocean surface temperatures can be responsible for above average rainfall conditions in the mid to late summer for South Africa and neighbouring countries to the north.
1. Current conditions
Extreme low temperatures remained prevalent since the last week of May 2020. The winter of 2020 is turning out to be one of the coldest in at least the past two decades. For example, Bothaville in the Free State where the number of days with minimum temperatures below freezing point since the beginning of the winter, tallied 75 days. This are nine days more than the previous “record” since 2001 with 66 days that was set in 2012. The average number of days where temperatures were below freezing point since 2001 is 49 days with the lowest number of days of 31 recorded in 2001 and 2002.
Rainfall conditions were also more favourable than initial forecasts indicated for most of the Winter Rainfall Area. Highly significant is the rainfall that were recorded in the drought stricken western parts of the Northern Cape and adjacent Western Cape for districts like Springbok, Garies, Kamieskroon, Nieuwoudtville and further south Clanwilliam and even inland. The highest rainfall amounts since about 2012 were measured in these areas.
Drought conditions still remain very serious in parts of the Northern Cape like the Kalahari (north of Upington), southern half of the Northern and adjacent Western Cape interior with the exception of the Westcoast and adjacent areas, large areas of the Eastern Cape and parts of Limpopo.
Agricultural conditions improved for most of the country with the exception of some smaller areas since the beginning of 2020. The harvesting of the summer crop harvesting is almost completed with average to above average yields in most areas. Smaller patches in the maize area suffered drought or water-logged conditions and there were also sporadic incidences where grain quality was very low but it was in general a bumper year for most summer crop farmers in the central to western parts.
The condition of surface water is favourable for this time of the year with the exception of a few critical areas like the Vaal dam with only 38% of full capacity and dropping weekly by more than 1%, the Letaba irrigation valley where the Tzaneen dam is at only 13% of full capacity and in the Eastern Cape where the Kouga dam is at 7.5%. The level of storage dams in the Western Cape is favourable with most of the larger storage dams above 80% of capacity.
Water levels in Lake Kariba in Zambia is at 36%, the Katze dam in Lesotho at 27.7% and Hardap dam in Namibia at 38% and the low levels is evidence of the effect of the drought conditions since about 2012. Average water levels of Namibia dams are about 49% compared to about 25% last year the same time.
2. ENSO and Indian Ocean
2.1 ENSO (El Nino Southern Oscillation)
All four Nino-areas are now consistently cooler than average since the middle of May 2020: Nino1+2 = minus 0.6°C (closest to South America), Nino3 = minus 0.7°C, Nino3.4 = minus 0.7°C and Nino4 = minus 0.1°C (closest to Australia). (Remember that the range between -0.5°C and +0.5°C is still in the neutral range.) This is in line with the development towards La Nina type of conditions for the summer season.
Forecasts: The probability for a full scale La Nina event increased to more than 60% with a less than 40% probability for Neutral conditions and less than 5% for return of El Nino conditions.
The Southern Oscillation Index (SOI) that is the measure of the coupling between surface conditions in the Nino-areas and the overlying weather systems, is now in the so called consistently positive phase of the SOI system. This is significant for this time of the year because it is a strong indication of La Nina conditions that reacts with overlying weather systems.
2.2 Indian Ocean
The Indian Ocean Dipole (IOD) is for three consecutive weeks in a negative phase, meaning that the surface water of the western Indian Ocean (towards the African coastline) is cooler than the eastern Indian Ocean and most forecasts indicate that this trend will continue for the next month of two.
This is an important development seen in combination with the La Nina strengthening that is taking place at the same time. A negative IOD is favourable for improved rainfall conditions over the South African sub-continent as well as Australia.
3. Rainfall and Climate
3.1 Summer Rainfall Area
The presence of a La Nina phenomenon at the end of winter is a positive indication of favourable summer rainfall conditions for the summer to follow. La Nina usually indicates a normal to somewhat later than normal start of the rainy season. With the negative phase of the IOD (Indian Ocean) present, is it possible for an earlier start to the rainy season. The current scenario is that rainfall conditions will start to improve over the eastern parts of the country from about the end of September to reach a peak in November and December over these parts. La Nina usually is responsible for above average rainfall to commence over the central to western parts of the country from about November to reach a peak in late summer.
There is a more than average risk of flooding over parts of the Summer Rainfall Area, especially the central to western parts in the second part of the season.
Minimum temperatures are likely to remain below average for at least the next weeks but the probability for frost is lower from the middle of September. With a positive rainfall outlook and associated higher cloud load and the cooling effect of rain, is it expected that maximum temperatures will not reach extreme high values for prolonged periods and that heat unit accumulation will be lower than average that can impact on summer crop production.
3.2 Winter Rainfall Area
Most of the Winter Rainfall Area received above average rainfall since the beginning of the winter rainfall season. Short to medium term outlooks remain positive for further rain until about the end of September with at least four to five possible rainfall events still to come. Rainfall amounts will become less and rainfall events after September will become less frequent.
With a strong La Nina event possible for 2020/21 is there also a higher probability for rain in summer, especially in the second part of summer. La Nina is usually responsible for rain over the western parts of South Africa that could also reach as far as the Western Cape during strong developed systems.
3.3 Namibia
Short to medium term outlooks are not positive for much rain to occur before the end of October. It is likely that rainfall conditions will improve dramatically from about November to reach a peak in January to March 2021 due to the positive effect of the La Nina and Indian Ocean setup that is currently in progress.
Temperatures will recover after the cold spell at the end of August and the probability for further frost and low minimum temperatures declined rapidly. There is however a possibility of minimum temperatures to drop to about 5°C in the second part of September over the southern parts but frost is not expected.
4. Summary and conclusion
The 2020 winter was one of the coldest in terms of minimum temperatures in at least the past two decades. This is probably an indication of a step change in the climate for the next few seasons.
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 in general favourable, especially in the Western Cape but dams in the Eastern Cape and the Vaal dam as well as dams the northern parts of Limpopo (Tzaneen) are critically low.
