Climate and agricultural conditions - October 2021
- La Nina now more than 90% sure to occur and to last until at least March 2022 with a moderate intensity similar to 2020/21 or even more intense.
- Rainfall outlooks are favourable for most of the Summer Rainfall Area but may be drier in January/February over the eastern areas. Isolated rainy systems can however already start to occur over the western areas during November.
October is characterised by the start of the summer rainfall season although rain also occurred in the Winter Rainfall Area. For the first time in many years was significant rainfall also recorded in the extremely dry south western parts of the Northern Cape and adjacent areas of the Western Cape and Eastern Cape in the last 10 days of October. More than 20mm also occurred in parts of the Kalahari like Askham and Van Zylsrus, Upington 50mm and Augrabies 25mm. Further south in the Boesmanland and Karoo rain also occurred in places like Pofadder 48mm, Williston 45mm, Brandvlei 33mm and Calvinia 19mm. Some good falls were also recorded between 18 and 24 October in parts of the Western Cape like Malmesbury 30mm, Villiersdorp 57mm and Paarl 24mm but more important is that the Kleinkaroo received also significant rain with Ladismith 57mm, Oudtshoorn 41mm and Laingsburg 21mm. Aberdeen, Willowmore and Joubertinia in the Eastern Cape also received about 20mm that is also the first significant rain in a long time.
Frost damage occurred in the middle parts of October in the southwestern Free State as well as parts of the Northern Cape that caused frost damage to grapes, winter wheat and pecan nuts.
The fire risk remains high in the central to western grassland areas with high temperatures and strong winds to persist. Although some rain occurred, was it insufficient to lower the fire risk.
State of storage dams in the Summer Rainfall Area is in general still very favourable with dams in the Free State at about 90% of full capacity with large storage dams like Gariep and Vaal dam above 80% and Vanderkloof dam about 94% full. There is a high risk for flooding if high rainfall amounts occur in the catchment areas because the buffer capacity is very low. Some dams in the Eastern Cape remain at critical low levels or is nearly empty like Kouga with only 6.5%. Western Cape dam levels are very favourable at about 81% of full storage capacity although the rainy season is very close to an end.
The water level in Lake Kariba in Zambia is at about 35% compared to about 28% last year the same time while the Katze dam in Lesotho is at about 66% and the Mohale dam 32%. The Hardap dam in southern Namibia was at the end of September 2021 at 55% of full volume.
2. ENSO and Indian Ocean
2.1 ENSO (El Nino Southern Oscillation)
All Nino-areas except Nino-1+2 (closest to South America) were La Nina-like and below 0.5°C cooler than normal. The probability for a full scale La Nina from now towards March/April 2022 is more than 90%. The intensity seems to be in the same order as in the previous season or even stronger to be moderate to even moderate/strong. The turning point of the La Nina of the 2020/21 season occurred towards the end of November that was earlier than expected. The forecast for the 2021/22 seems to be later in December 2021 to January 2022. This can be significant because it can ensure that favourable rainfall conditions in summer can last longer until March and April 2022.
The Southern Oscillation Index (SOI) as indicator of the effect of sea surface interaction with overlying weather systems in the Nino areas, remains positive for the fourth consecutive month.
2.2 Indian Ocean
The Indian Ocean Dipole (IOD) is still borderline negative, mainly due to warming of the eastern Indian Ocean (towards Australia) and cooling towards the African coastline. This sea surface temperature pattern can cause important shifts in the weather and rainfall patterns with the Indian Ocean high pressure system over the western Indian Ocean closer to the Southern African sub-continent. Low pressure systems are now more likely to develop over the central to western parts of the subcontinent.
3. Rainfall and Climate
3.1 Summer Rainfall Area
With the strengthening of La Nina and with the peak that will most probably be reached later in the summer than the past season, are the probabilities for summer rainfall very favourable for the season. The return of the Indian Ocean Dipole (IOD) to a negative phase in the recent past weeks will also improve rainfall probabilities.
The spatial and time distribution is however more difficult to interpret because there will be drier and wetter parts of the season in specific areas of southern Africa and more specific South Africa. A La Nina phenomenon is usually responsible for above average rainfall in the November to first part of January over the eastern production areas with drier conditions in February/March while the rainy season starts from about the end of December to last until March over the central to western parts with a dry spell at the start of the summer. With the current setting of the Indian Ocean with cooler waters near the African coastline, is the moisture and rain more concentrated over the central to western parts of southern Africa, resulting is a higher probability for earlier rain over the western parts.
Average to above average rainfall is expected over the central to eastern parts of South Africa to reach a peak in December. The improved rainfall conditions will then spread westwards over the central to western parts from December to March.
With the exception some very hot conditions in the second week of October, are temperatures average to below average. This trend is expected to last over the central to eastern parts due to cold air still in circulation but also with more rain and cloudy conditions that are expected. Minimum temperatures between 10°C and 15°C are expected until at least the middle of November and it can result in unfavourable soil temperatures for germination of summer crops.
3.2 Winter Rainfall Area
Stronger than expected frontal systems returned and are expected to cause light falls of rain in the last week of October and first week of November over most of the Winter Rainfall Region. This is not good news for winter wheat farmers that are in the peak harvesting season and the rain and high humidity can cause damage to quality.
With the development of the current La Nina is it possible that summer rainfall may occur in mid to late summer as a result of summer systems moving southwards over Namibia and western parts of the Northern Cape.
Light falls of between 10mm and 20mm are expected in the last week of October over the north-eastern parts. Very little of no rain is expected for the rest of the country until at least the last week of November. Outlooks are favourable from about December to March.
Very warm to hot conditions are expected for most of November.
4. Summary and conclusion
- The probability for a La Nina event increased to more than 90% and moderate to moderate/strong event is now expected. It is about the same intensity as in 2020/21 or even a bit stronger. It also is likely that the peak will be reached later compared to 2020/21 that is positive for rainfall towards the western parts.
- The Indian Ocean Dipole (IOD) index returned to a weak negative phase that is also positive for summer rainfall.
- Average to above average rainfall is expected over the central to eastern Summer Rainfall Area in the first part of summer and extending to the west in the mid to second part of summer.
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Source: Johan van den Berg (Independent Agricultural Meteorologist (M.Sc Agric, Agricultural Meteorology, UFS)