Climate and agricultural conditions - November 2021
- La Nina in full swing.
- Above average rainfall expected for December and first part of January for the Summer Rainfall Area and further rain for the Winter Rainfall Area in the first part of December.
November 2021 was characterised by spatially highly variable rainfall amounts. Most rain occurred over the northern, central and south-eastern parts. This is due to the fact that the flow of tropical moisture was mainly from Angola over Botswana. Intense or cut-off low pressure systems developed over parts of the country like the south eastern parts resulting in heavy rainfall and flood conditions over parts of the Garden Route with George that received between 100mm and 200mm. Heavy falls also occurred in Gauteng in the last week of November receiving between 60mm and 100mm and Giyani and Marken in Limpopo with more than 60mm between 22 and 23 November. Heavy rain and hail also caused severe damage in Klerksdorp in Northwest. Untimely falls of more than 20mm also occurred from 27 November over parts of the Western Cape that can have serious negative consequences on grains not yet harvested.
There was an area over the eastern parts of Northwest Province and adjacent parts of the Free State and Mpumalanga that received very little rain until the last week of November and farmers in these areas only started to plant following rain in the last week of November.
Parts of the Eastern Cape are still very dry and districts like Steytlerville, Willowmore and Jansenville are still very dry with only light falls that were received. Although the drought-stricken areas of the Northern Cape received some good falls in October was there a lack of rain since then with very high temperatures. The result is that the drought conditions continue and recovery is not taking place. Locusts are again starting to become serious problem.
State of storage dams in the Summer Rainfall Area is in general still very favourable with dams in the Free State at about 88.3% of full capacity with large storage dams like the Vaaldam at 78.5%, Gariep 90% and Vanderkloof at about 86.4% full. There is a high risk for flooding if high rainfall amounts occur in the catchment areas because the buffer capacity is very low. Important dams in the Eastern Cape showed some improvement but remain at critical low levels or is nearly empty like Kouga with only 10.6%. Western Cape dam levels are very favourable at about 80.7`% 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 28% compared to about 23% last year the same time while the Katze dam in Lesotho is at about 69% and the Mohale dam 35% that is an improvement of about 3% in the last part of November. The Hardap dam in southern Namibia was at the end of September 2021 at 52% of full volume compared to 28% last year the same time.
2. ENSO and Indian Ocean
2.1 ENSO (El Nino Southern Oscillation)
The current La Nina intensity is still classified as a weak event with all Nino-areas between 0.5°C and 1°C cooler than normal. It is expected that temperatures will further cool down to reach levels of about 1.3°C cooler than normal towards the end of December that will represent a moderate La Nina and about the same intensity as in 2020/21. Forecasts indicate that the current La Nina will last until about March/April 2022.
Although very early are there some indications that the 2022/23 season will be either neutral or El Nino-like. This is despite a so called “wet cycle” that will probably last until about 2025. El Nino can disrupt this wetter trend.
The Southern Oscillation Index (SOI) as indicator of the effect of sea surface temperature interaction with overlying weather systems in the Nino areas remains positive for the fifth consecutive month. On a scale from -30 (Strong El Nino) to +30 (strong La Nina) is the current 30-day average SOI about +12,1 indicating a weak to moderate La Nina. Of significance is the consistent La Nina like pattern from about July 2021.
2.2 Indian Ocean
The Indian Ocean Dipole (IOD) is now in a neutral phase and will remain neutral for coming months.3. Rainfall and Climate
3.1 Summer Rainfall Area
With the La Nina in place and lesser influence of the Indian Ocean due to the current neutral state, are outlooks positive for average to above average rainfall for most of the Summer Rainfall Area. With less rain recorded over the eastern areas than forecasts indicated during October and November 2021, are there strong indications of heavy falls to follow. It seems that globally was there a delay in the La Nina impact but that it will eventually result in the expected impact.
Short to medium term outlooks are favouring above average rainfall over the central to eastern parts of the country for the first and second week of December and again in the last week to ten days of December and first week of January. Rainfall outlooks are less favourable for the extreme western parts and only start to improve from about the last week of December.
Rainfall outlooks are very positive in the second part of the season over the central to western parts but drier conditions expected in and around February over the eastern parts.
Temperatures will be mostly average to below average due to the expected rainfall and high cloud load with the exception of the western parts of the country where warm to hot conditions is expected.
3.2 Winter Rainfall Area
Further rain is expected in the first week of December until about 8 December with more than 10mm to 20mm expected. Cold fronts still reaching the Winter Rainfall Region and is associated with the development of relative strong low-pressure systems over the south eastern Atlantic Ocean. Cloudy conditions are expected for most of the first two weeks of December that can have serious negative impacts on nearly all crops.
There is also a possibility of further rain towards Christmas.
With the development of the current La Nina is it possible that summer rainfall may also occur in mid to late summer as a result of summer systems moving southwards over Namibia and western parts of the Northern Cape.
Rainfall conditions are improving over the northern parts and above average rainfall is expected for December over the areas to the north of roughly Otjiwarongo. The most likely period for rain is from about 10 to 24 December. The central parts can expect good falls from before Christmas.4. Summary and conclusion
- La Nina in full swing although currently still in a weak phase but it is likely to develop towards a moderate event.
- Rainfall outlooks are very positive for most of the Summer Rainfall Area with the exception of the extreme western parts for December and first part of January.
- Winter Rainfall Area can still expect rain in the first part of December.
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Source: Johan van den Berg, Independent Agricultural Meteorologist (M.Sc Agric, Agricultural Meteorology, UFS)
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