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Klimaat- en reënvaltoestande vir Oktober 2020- Suid Afrika

Die eerste betekenisvolle somerreënval het in die eerste week van Oktober voorgekom, met Vrede in die Vrystaat wat bykans 40mm gemeet het, Warden meer as 30mm en Bothaville ongeveer 20mm.

Reën het ook in Marble Hall (17mm), Witbank (10mm) en Bethal (15mm) in Mpumalanga, sowel as in Mokopane, Limpopo, waar 30mm gemeet is, voorgekom. In die Oos-Kaap het Grahamstad byna 30mm reën gehad, Port Elizabeth ongeveer 25mm en Graaff-Reinet in die Karoo 22mm. Op plekke in KwaZulu-Natal is tot 50mm gemeet.

Betekenisvolle reënval het in September in die winterreënvalgebied voortgeduur en gunstige produksietoestande vir wintergrane tot gevolg gehad. Die relatiewe lae temperature was ook gunstig vir wintergraanproduksie.

Die hewige droogte duur steeds voort in dele van die Noord-Kaap, alhoewel goeie reënneerslae in die suidwestelike dele in distrikte soos Springbok, Garies en Kamieskroon gemeet is, wat sal help om die droogte in hierdie gebiede op te hef.

Distrikte waar die droogte nog baie intens is, sluit Pofadder, Kenhardt, Prieska, Van Wyksvlei, Carnavon en ander distrikte in die sentrale tot suidelike dele van die Noord-Kaap in. Die grootste deel van die Kalahari (noord van Upington) is ook nog baie droog, met ondergemiddelde reënval wat al vir byna ’n dekade lank voorkom.

Die noordelike binneland van die Wes-Kaap sowel as dele van die Oos-Kaap, is ook steeds in die greep van ’n droogte.   

Watervlakke
Die watervlakke in enkele groter damme is krities laag, soos die Vaaldam wat tans net 33% vol is teenoor 55% oor dieselfde tydperk verlede jaar. Die Tzaneendam, wat water aan die Letaba-besproeiingskema voorsien, het tot onder 10% gedaal en die vlak daal steeds vinnig.  

In die Oos-Kaap is die Kougadam slegs 8% vol teenoor ongeveer 40% dieselfde tyd verlede jaar, en die Nelson Mandela-metropool is spoedig op pad na ‘Dag Zero’, indien aanvulling nie binne die volgende weke gebeur nie. Dit bedreig ook die Gamtoos-besproeiingsgebied, wat ’n belangrike sitrusuitvoergebied is.

Die damvlakke in die Wes-Kaap is gunstig en die Theewaterskloofdam het vir die eerste keer in baie jare oorgeloop. Die belangrike Clanwilliamdam, wat water aan Citrusdal en omliggende dele verskaf, is ook 100% vol, nadat dit in Mei vanjaar onder 10% gedraai het.

Die watervlak van die Karibadam in Zambië staan tans op 30,5%, die Katzedam in Lesotho op 24,3% en die Hardapdam in Namibië op 35%. Dit bied groot rede tot kommer.

El Niño Suidelike Ossilasie en Indiese Oseaan
See-oppervlaktemperature in die El Niño-gebiede toon tans stewige La Niña-waardes. Die Australiese Buro vir Meteorologie voorspel ’n matige tot sterk La Niña vir die grootste deel van die 2020/21-somer.

Dié La Niña is goed gevestig en daar is meer as 80% sekerheid dat dit so sal bly tot ten minste die herfs van 2021. See-oppervlaktemperature in die belangrike Niño 3.4-indeksgebied is ongeveer 1°C koeler as normaal, met voorspellings wat aandui dat dit tot ongeveer 1,5°C koeler as normaal gaan daal, wat as ’n sterk La Niña beskou kan word.     

Die Suidelike Ossilasie-indeks (SOI), wat die maatstaf van die koppeling tussen see-oppervlaktemperature in die El Niño-gebiede en die oorliggende weersisteme is, is nou reeds vir die tweede agtereenvolgende maand in die positiewe fase van die SOI-stelsel (La Niña-neigend). Die teenwoordigheid van ’n La Niña-verskynsel met die SOI wat goed daarop reageer, is baie betekenisvol vir reën.   

Verdere afkoeling van oppervlaktemperature het in die suidwestelike Indiese Oseaan voorgekom, met verwarming in die sentrale tot oostelike Indiese Oseaan. Hierdie opstelling is gunstig vir die verlenging van die negatiewe fase van die Indiese Oseaan Dipool-indeks (IOD).

Reënval- en klimaatsvooruitsigte
Die teenwoordigheid van ’n La Niña-verskynsel saam met ’n negatiewe fase van die IOD, is baie positief vir somerreënval. Alhoewel ’n La Niña-verskynsel gewoonlik vir ’n effense later aanvang van die reënvalseisoen verantwoordelik is, is die mid- tot laatsomer gewoonlik nat tot baie nat.

Soortgelyke seisoene in die verlede, met die voorkoms van beide La Niña en ’n negatiewe fase van die IOD, sluit 1974/75, 1975/76, 1988/89 en 1998/99 in.

Gedurende die vorige sterk La Niña in 2010 tot begin 2012, was daar ook bogemiddelde reënval, alhoewel die IOD net in ’n swak negatiewe fase was. Gedurende hierdie soortgelyke jare het bogemiddelde reënval selfs oor die verre westelike binneland voorgekom. Die westelike binneland is alreeds vanaf ongeveer 2012 in ’n droogte, en dit lyk asof dit vanjaar gebreek kan word. 

Temperature
Minimum temperature gaan waarskynlik ondergemiddeld bly tot in die laaste deel van Oktober. Gemiddelde tot bogemiddelde temperature kan in die sentrale tot westelike asook noordelike binneland, vanaf die laaste deel van Oktober en November totdat die reënseisoen begin, verwag word.

Daar is ’n groot waarskynlikheid dat ’n ondergemiddelde aantal hitte-eenhede gaan voorkom vir die somerseisoen as geheel, en produsente word aangeraai om somergewasse vanjaar nie te laat aan te plant nie. Moontlike oormaat reën in Januarie tot Maart kan latere aanplantings meer benadeel as vroeë aanplantings.

Winterreënvalgebied
Daar word verwag dat die reënvalpatroon in Oktober verder vanaf die Suidwes-Kaap, nader aan die suidelike, oostelike en noordoostelike dele van die land, gaan skuif. Reën is egter steeds moontlik oor die winterreënvalgebied, met kouefronte wat die land beïnvloed. Ligte reën is moontlik rondom 7 en 8, en 15 en 16 Oktober, asook in die laaste week van Oktober.   

Namibië
Die huidige La Niña-ontwikkeling, tesame met die negatiewe fase van die IOD, is baie positief vir bogemiddelde reënval in die mid- tot laatsomer, met ’n groot risiko vir vloede in laagliggende gedeeltes.

Kort- tot mediumtermynvooruitsigte dui daarop dat ligte reënneerslae in die tweede week van Oktober moontlik is, maar dat die werklike aanvang van die reënseisoen waarskynlik eers in die tweede deel van November gaan wees.

Die verwagting is dat die baie droë suidelike tot suidwestelike dele van Namibië ook voldoende reën gaan kry om die droogte te breek, maar waarskynlik eers in Desember óf in die tweede deel van die somer. 

Samevatting
Ondergemiddelde temperature met ryp het vanjaar weer later as verwag voorgekom en rypskade in die sentrale tot suidelike binneland veroorsaak. Ondergemiddelde temperature kan steeds tot in die laaste week van Oktober voorkom, maar verdere ryp word nie verwag nie.

Beide die El Niño Suidelike Ossilasie (ENSO) en IOD gaan verantwoordelik wees vir gunstige reënvaltoestande vir die 2020/21-somer, veral vir die sentrale tot westelike somerreënvalstreke. Daar is ’n bogemiddelde vloedrisiko in die tweede deel van die somer.

Oppervlakwatertoestande in die Wes-Kaap is vir die eerste keer in ongeveer vyf tot ses jaar weer in ’n gunstige situasie, maar die vlakke van die Vaaldam, damme in die Oos-Kaap en groot damme in die buurlande is steeds rede tot kommer. 

Climate and agricultural conditions - October 2020

Important Issues:

  • The winter of 2020 continued and frost damage occurred in the last week of September. Below average temperatures are still possible until the end of October but frost is not expected any more for this season.  
  • La Nina and favourable sea surface temperatures in the Indian Ocean are still on track for favourable summer rainfall conditions.

1. Current conditions

Frost and low temperatures continued to occur over much of South Africa. Mild to severe frost in the last week of September caused damage to grapes, fruit, nuts, potatoes and early planted winter wheat in parts of the Free State, Northern Cape and Northwest. These occurrences are on the later side of the expected spectrum for frost occurrences.

The first significant summer rainfall was recorded in the first days of October with Vrede in the Free State with more than 40mm, Warden with more than 30mm, Bothaville 20mm. Rain was also recorded in Marble Hall 17mm, Witbank 10mm and Bethal 15mm as well as Mokopane in Limpopo with 30mm. Grahamstown in dteh Eastern Cape recorded about 30mm Port Elizabeth 25mm and Graaff-Reinet in the Karoo 22mm. Up to 50mm was recorded in parts of KZN.

Significant rainfall still occurred in the Winter Rainfall Area in September, resulting in very favourable production conditions for winter grain production. The relative low temperatures that were experienced were also positive for favourable production conditions for winter grains.

The intense drought conditions continued in parts of the Northern Cape although very good falls of rain in the south western parts in districts like Springbok, Garies and Kamieskroon occurred and released these areas from the grip of the drought. Areas that are still experiencing disaster drought conditions are Pofadder, Kenhardt, Prieska, Van Wyksvlei, Carnavon and other districts in the central to southern parts of the Northern Cape. Most of the Kalahari (to the north of Upington) are also very dry with below average rainfall for nearly a decade. The northern interior of the Western Cape as well as parts of the Eastern Cape are also still in the grip of the drought.       

The levels of surface water are critical in some larger storage dams like the Vaal dam with only about 33% of full capacity compared to about 55% last year the same time. The Tzaneen dam supplying water to the Letaba irrigation valley is at less than 10% of full capacity and the level is dropping rapidly. In the Eastern Cape is the Kouga dam at about 8% compared to nearly 40% last year the same time and the Nelson Mandela Metropole is rapidly closing in on Day Zero if replenishment is not taking place within weeks. It also threatens the water for the Gamtoos irrigation area that is a very important citrus exporting area. The level of storage dams in the Western Cape is favourable with the Theewaterskloof dam overflowing for the first time in many years. The important Clanwilliam dam supplying water to the Citrusdal and surrounding irrigation areas is also at 100% of full capacity after being below 10% in May 2020.    

The Water level in Lake Kariba in Zambia is at 30.5%, the Katze dam in Lesotho at 24.3% and Hardap dam in Namibia at 35%.  

2. ENSO and Indian Ocean

2.1 ENSO (El Nino Southern Oscillation)

Sea surface temperatures in the Nino-areas were all indicating solid La Nina values at the end of September. The Australian Bureau of Meteorology favours a medium to strong La Nina event to last for most of the Southern Hemisphere summer of 2020/21. The la Nina is now well established and more than 80% sure that it will remain in place until about the fall of 2021. Sea surface temperatures in the important Nino3.4-area are about 1°C lower than average with forecasts indicating that it will probably further cool down to about 1.5°C below average.    

The Southern Oscillation Index (SOI) that is the measure of the interaction between surface conditions is in the consistently positive phase of the SOI system. The La Nina-development and the SOI reacting to the development this early in the summer season, is highly significant for rainfall.

2.2 Indian Ocean

Further cooling of the south western Indian Ocean took place in September 2020 with warming in the central to eastern parts of the Ocean. This is favourable for the continued development of a negative phase of the Indian Ocean Dipole (IOD).     

3. Rainfall and Climate

3.1 Summer Rainfall Area

Rainfall: The presence of a La Nina phenomenon associated with a negative phase of the Indian Ocean Dipole is positive for summer rainfall. Although a La Nina event is usually responsible for a normal to later than normal start of the summer rainfall season, is the mid- to second part of the summer wet to very wet.    

Similar seasons in history with both La Nina and negative IOD were:

  • 1974/75
  • 1975/76
  • 1988/89
  • 1998/99

The previous strong La Nina from 2010 to 2012 resulted also in above average rainfall although the Indian Ocean Dipole was in a weak negative to neutral phase.

The similar years as mentioned above resulted also in above average rainfall in the extreme western interior of the country. These parts of the country are currently still in the grip of a drought since about 2012 and it is likely that the drought will be broken in these areas in the months to come.

Temperature

Minimum temperatures are likely to remain below average until about the end of October. Average to above average temperatures can be expected over the central to western as well as extreme northern interior for the last part of October and November until the start of the rainy season.  There is a high probability for a below average number of heat units and farmers are advised not to plant summer crops too late. Expected excessive rainfall in January to March can also favour early plantings.     

3.2 Winter Rainfall Area

It is expected that the rainfall pattern will further shift from the Southwestern Cape towards the southern, eastern and north eastern parts of the country in October. Rain is still possible over the Winter Rainfall Area with colds fronts visiting the southern parts of the country. Light rain is possible on 7-8, 15-16 October and in the last week of October.          

3.3 Namibia

The current status of the La Nina and Indian Ocean is also very positive for above average rainfall for the mid– to late summer with a high risk for flooding in lower lying areas. Short to medium term outlooks for rainfall indicates light falls of rain possible in the second week of October but the start of the rainy season is likely only to occur in the second part of November. 

It is expected that the very dry southern to south western parts of Namibia will also experience sufficient rainfall to break the drought but only towards December or in the second part of the summer.  

4. Summary and conclusion

  • Below average temperatures with late frost still occurred and caused damage to crops in the central to southern interior. Below average temperatures likely to remain dominant until the end of October but further frost is not expected.
  • 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 for the first time in about 5 to 6 years again in a favourable state for the Western Cape but the Vaal dam, dams in the Eastern Cape as well as large storage dams in neighbouring countries are still reason for concern with very low levels.


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