Extreme weather events from El Niño Southern Oscillation phases disrupt agroecosystems, causing reduced yields, pest outbreaks, and food insecurity. Gugulethu Zuma-Netshiukhwi, an agrometeorologist at the Agricultural Research Council (ARC) suggests climate-smart agriculture, weather forecasting, and proper management to mitigate these impacts.

The occurrence of weather events such as floods, droughts, heatwaves, hailstorms and many other climate-related disasters differs among years with La Niña, neutral or El Niño phases of the El Niño Southern Oscillation (Enso). The Enso cycle has significant irregularity in amplitude, duration, temporal evolution and spatial structure. The effect of La Niña tends to be opposite that of El Niño, bringing wetter than normal conditions to southern Africa.

Gugulethu Zuma-Netshiukhwi, agrometeorologist at the Agricultural Research Council. Photo: Supplied/ARC

The observed Enso diversity has a significant potential effect on climate change. Enso is the most prominent interannual climate variability with noticeable agroecological and societal impacts. El Niño conditions prevail in the central-eastern equatorial Pacific, with important oceanic and atmospheric indicators positioning with the current El Niño phase that is diminishing gradually, as of mid-March 2024.

The Enso-neutral conditions are most likely to phase in between April and August of 2024. La Niña is the highly probable predicted phase from July to September 2024 through to January 2025 (IRI, 2024). The Enso categories have a major influence on agricultural productivity, agroecosystem conditions and biodiversity.

An understanding of Enso is important to the agricultural fraternity because, in seasons with El Niño or La Niña conditions, the timing of extreme weather events leading to hazards may be unpredictable, preventing farmers, decision-makers and policy-makers from making informed decisions for such events. La Niña and El Niño can persist for up to a year or longer.

For example, the 2020-2023 La Niña was a rare triple-dip, which had numerous natural disasters such as the KwaZulu-Natal floods in July 2022, Eastern Cape snowfalls in April 2023, Limpopo heatwaves in November 2023, and the cut-off low weather system which resulted in heavy rains across most of South Africa in April 2024. This simply indicates that, whether it is El Niño or La Niña, extreme weather events are prone to occur.

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Challenges for rain-fed farmers

The intensification of the frequency of El Niño events in southern Africa is higher than average, resulting in extreme weather patterns that significantly impact agroecosystems and food security in most of the South African agroecological zones. The effects of El Niño contribute to lower crop yields, poor pasture conditions and disturbed agroecosystems.

El Niño seasons (e.g. 2023/24) have a negative effect on agricultural outcomes, indicating the strong relationship between its effects on crop yield and livestock productivity reduction. The reason for this is the extended dry spells and drought conditions in South Africa. Rain-fed farmers are the most affected, so they need to be knowledgeable to safeguard their livelihoods and improve preparedness for droughts, floods or other extreme weather events.

Cut-off low weather systems, which are upper air low pressure systems at around 300 hectopascal (hPa) / 3000 ft, stem from the main westerly trough systems of cold air that develop south of the continent. These are cold-cored weather systems that are unstable and associated with strong convergence and upward motion while deepening in their development. Likewise, the cut-off low system usually prevails over a wide area and can persist for up to 6 days (Figure 1a).

The slow movements of this system occur between 20° to 45° south. The cut-off low pressure system observed over most of South Africa in April 2024 led to widespread heavy rainfall. In southern Africa, cut-off low systems are relatively frequent in March, April and May and may lead to heavy rainfall and very cold conditions.

The highest frequency occurs in the month of April, as was the case in April 2024 (Figure 1b). The low-pressure weather system is associated with overcast cloud cover, heavy rains, widespread flooding, damaging winds and severe thunderstorms (Xulu et al., 2023), while snowfall may even occur in some areas. The aforementioned events cause hazards in the agroecosystems.

Figure 1(a) Cut-off low pressure system (SAWS, 2024), and (b) Cut-off low monthly frequency (EUMeTrain, 2024; EUMETSAT, 2024).

The extreme weather conditions caused by the low-pressure system over South Africa in April 2024 led to several environmental and socio-economic impacts. The slow-moving nature of cut-off low weather systems results in high impact, as it produces periods of extreme rainfall, resulting in cropland and pasture conditions being disturbed and damaged. This further leads to the interruption of the agricultural supply chain and thus may cause food shortages and insecurity.

Of several rainfall-producing systems occurring over South Africa, the cut-off low is the most devastating to the agricultural sector. Its impacts include a reduction in crop yields and agricultural productivity caused by damage at critical crop growth stages, crops being washed off fields, activation of fungi which affect leafy vegetables, growth of mycotoxins, water- and vector-borne diseases, surface run-off and waterlogging, soil degradation, disruption of farming operations and transportation, and delays in harvesting and market supply.

Farmers experience extreme weather events with intensified frequency and severity. The consequences of weather extremes on agroecosystems vary with the intensity and timing of the extreme events and their combination in space and time.

For example, in most of South Africa, the April 2024 cut-off low negatively influenced the agroecosystem in terms of food sustainability by worsening weed invasions, pest outbreaks and plant diseases. The observed consequences of this cut-off low were the following:

Reduction in crop production.
Weed invasion which contributed to reduced crop growth.
Outbreak of locusts and other insects and pests.
Waterlogging and highly saturated soil profiles.
Negative impact on production supply and costs.
Production quality alteration and increased production costs.

Impact of delayed harvesting

In the central parts of South Africa, farmers who were severely impacted by the April 2024 cut-off low experienced crop failure that was instigated by the outbreak of fungal diseases that are harmful to humans and livestock. Mycotoxins are chemicals produced by fungi, posing several significant food safety concerns. The rainfall outbursts resulted in delayed harvesting of maize, which is conducive to the maize cobs being exposed to mycotoxins (Figure 2a).

The leafy vegetables were damaged by Cercospora leaf spot, caused by the fungus Cercospora beticola, which is one of the most important diseases affecting the Chenopodium plant family (Figure 2b). C. beticola exists between crop cycles in infected crop residues, in weed hosts and on seeds, and can survive in the soil for up to 2 years (Dicklow et al., 2018).

Cycles of infection and fungal production may occur with favourable environmental conditions such as frequent rains, high humidity and temperatures of ≤23°C. Pathogen spores are transported by wind, rain splash, irrigation water, insects, farm workers and types of garden equipment.

Leaf wetness during the night and dry conditions during the day encourage the disease. Successive plantings allow disease to spread from one planting to the next, and weed hosts can provide green bridges between crops. The spots turn brown, round to occasionally angular spots with purple to red borders (Visagie et al., 2024) (Figure 2b).

Figure 2(a) Maize cobs affected by mycotoxin, and (b) Cercospora leaf spot which occurred after the cut-off low occurrence. Both samples were collected from farms located in Mangaung Metro Municipality in the Free State Province of South Africa.

Recommended management practices are as follows:

Bury infected crop residues and weed hosts and administer fungicides.
Practise crop rotation with non-Chenopodium species for 2-3 years.
Avoid overhead irrigation to minimize leaf wetness conditions.
Use treated and certified seeds.

It is therefore fundamental for farmers to adhere to agricultural production guidelines for strategic, tactical and operational decision-making. Climate-smart agriculture techniques and tools are crucial for the sustainability of agroecosystems and habitats. The importance of equipping farmers with information on weather and climate, climate variability, weather forecasting, seasonal prediction, and supplying guidance on weather systems and their impact on agroecosystems, is clear. Many agricultural enterprises in South Africa do not perform optimally due to the failure to adopt weather and climate-related knowledge to implement suitable preventative measures. The Agricultural Research Council (ARC) plays a major role in keeping the agricultural community informed about the applications of agricultural meteorology to counteract the adverse consequences of extreme weather events.

Gugulethu Zuma-Netshiukhwi is an agrometeorologist at the Agricultural Research Council (ARC). The views and opinions expressed in this article are those of the authors and do not necessarily reflect the views or positions of Food For Mzansi.

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