Important Issues:
Climate and agricultural conditions – April 2022 Johan van den Berg
1. Current conditions
The month of April 2022 will be remembered as one of the wettest April months on record. The worst hit was the South Coast of KZN where Margate for example received more than 600mm in April with more than 300mm on 12 April alone. This is about six to seven times the long-term average for rainfall for April. Heavy rain also occurred in April over the central to eastern Summer Rainfall area with more than 150mm in areas like Koppies, Middelburg (Mpumalanga), Witbank, Coligny, Bloemfontein and Leeudoringstad. Between 100mm and 150mm occurred also in districts like Frankfort, Vrede, Hoopstad, Warden, Marble Hall and Schweizer Reneke. These amounts of rainfall are between four and five times the long-term average rainfall for April.
The very wet conditions are becoming an increasingly serious problem to harvest soyabeans, sunflower and early planted maize. Many fields are still waterlogged with free water on the soil surface, preventing the movement of harvesting implements. The cool and humid conditions also prevent drying of soils.
Of importance is that some of the dry areas in the Eastern Cape also received some good falls of rain and it was sufficient to break the drought in areas like Cradock and Jansenville.
The heavy rains in April in the catchment areas of the larger storage dams resulted in a flood risk for the lower Vaal and Orange rivers but it is not expected to reach critical levels.
Nearly all dams in the Summer Rainfall Area are at very high levels due to the heavy rains and are in the process of overflowing. The average level of Free State dams was at 107% in the third week of April with the Vaal dam at 115%. Although the average level of dams in the Eastern Cape stabilised at about 66%, concerning is the level of the important Kouga dam at 13% and still dropping. Western Cape dam levels are on average at about 57% compared to 50% the same time in 2021. Evaporation and consumption are responsible for levels dropping by between 1% and 1.5% weekly.
The water level in Lake Kariba in Zambia is at about 26% compared to 22% a few weeks ago but is still way below the 48% last year the same time while the Katze dam in Lesotho is at 98.4% and the Mohale
dam at 91.4%. The Hardap dam in southern Namibia was at 19 April 2022 at 71.0% of full volume while the largest storage dam, the Neckartal dam, is at 99%.
2. ENSO and Indian Ocean2.1 ENSO (El Nino Southern Oscillation)
The current La Nina phenomenon maintains strong levels, especially for this time of the year. La Nina/El Nino usually weakens in the autumn to winter months but currently it keeps strong levels. Of importance is the high index values of the Southern Oscillation Index (SOI) that is a measure of coupling of weather and surface conditions. The 30-day running average of the SOI wat on 25 April 2022 on a scale from +30 (La Nina) to -30 (El Nino) at a value of +18. This is the highest value since the start of the current La Nina in about July 2021.
The probability for a full scale La Nina to continue increased to above 50%, that is significantly better than the normal occurrence of about 20-30% of years. Neutral conditions have an about 35% to occur and El Nino less than 10% in the next months until mid-summer of 2022/23.
There will be more certainty from about August about the progress of La Nina conditions but at this early stage it is leaning heavily towards a next consecutive La Nina season. The pool of cooler subsurface water that is expanding, providing more impetus to maintain La Nina levels for the next months.
2.2 Indian Ocean`
The Indian Ocean Dipole Index (IOD) is currently in a neutral phase, but the latest forecasts indicate a sharp drop towards the negative phase from about May to at least September. (A negative IOD is positive for summer rainfall, indicating cooler surface water towards the western Indian Ocean (African coast) and warmer water towards Australia, as was the case in 2021).
3. Rainfall and Climate
3.1 Summer Rainfall Area
Rainfall
With the current relative strong levels of La Nina and its indicators is it still positive for rain before winter. What can have a significant impact on climate conditions is the expected rapid change of the Indian Ocean Dipole (IOD) index towards the negative phase. This is a very positive signal for rainfall in the post-winter period and in conjunction with the expected continuation of La Nina, can again be responsible for above average rainfall in the 2022/23 summer.
Rain is still possible until at least the end of May over the Summer Rainfall Area although short term outlooks are currently less positive.
A dry spell is expected from about June to November that is in line with La Nina conditions.
Average to above average rainfall is again expected from about November 2022 over the eastern production areas and shifting towards the central and western parts in the second part of summer.
Temperature
The first moderate frost over the central to southern parts of the country is likely to occur before the end of April.
The probability for severe frost is low until the middle to end of May.
Day time temperatures will be cool to mild over the central to southern parts but warm towards the west and north.
3.2 Winter Rainfall Area
Weaker cold fronts started to move more frequently reaching the southern parts of the country, bringing colder conditions with light falls of rain. This is expected to continue for most of May before frontal systems will increase in intensity with more rain.
Longer term outlooks are inconclusive, but the most probable scenario is a late start to the rainfall season (June) to reach a peak in July and August. A very dry season is not very likely.
Temperature
Temperatures will be much cooler from now on and it seems that the extremely hot conditions are unlikely to return again before winter.
3.3 Namibia
Not much further rain is expected for the rest of April. It is possible that the southern parts can expect light falls of rain in the second week of May from a frontal system bringing cloudy and cooler conditions.
4. Summary and conclusion
La Nina conditions continue that is very unique for this time of the year.
Short term outlooks are not very positive for further rain but long-term outlooks are still positive until about the end of May for the Summer Rainfall Area.
Light falls of rain are possible for the rest of April and first part of May for the Winter Rainfall Area. The start of the real winter rainfall season is expected from about the end of May. It may be associated with stormy conditions and heavy snow over the higher lying areas.
