The Southern Oscillation Index is expected to be neutral this winter, increasing the likelihood of an El Niño system developing next summer.
The El Niño-Southern Oscillation (ENSO) refers to the phenomenon of varying ocean surface temperatures in the central Pacific Ocean with the associated response by overhead climate systems. It consists of two components, namely El Niño and La Niña, as well as in-between or neutral periods. The long-term average expected occurrence of both El Niño and La Niña systems is between 20% and 30% of years each, with neutral periods in about 40% to 50% of years or seasons.
Johan van den Berg, an independent agricultural meteorologist, says the current La Niña system, which was the last in a series of three consecutive La Niña episodes, is now almost in a neutral period since the middle of February. However, the different Niño areas are still sending mixed signals with the closest area to South America already leaning more towards El Niño (0,8°C warmer than normal), and the Niño area closest to Australia (Niño-4) leaning more towards La Niña (0,6°C colder than normal).
“The Southern Oscillation Index (SOI), which represents the climate’s reaction to the underlying ocean temperatures, is reacting very slowly, however, and was leaning more towards La Niña at the end of February. That means the overhead climate systems are still leaning fairly towards La Niña, which makes predictions very unsure.
“From April, ocean surface temperatures and other indexes like the SOI should be neutral until late winter. From August it should move over to El Niño conditions with a high likelihood for El Niño to possibly be present from September for the entire 2023-2024 summer season.”
Winter rain
Johan says although frontal systems have already started moving in over the southwestern Cape, it appears as though the rain conditions for April to June will be average, especially based on the neutral state of the SOI.
“However, it is the second part of winter, from July to September, where chances are very good for below-average rainfall. This is also the period during which the most dam level replenishment usually occurs.”
What can El Niño mean for summer rainfall?
The system’s implications for rainfall in the spring and early summer (especially the last part of August until October) include possible average to above-average rainfall over the central to eastern parts of the country that could expand southwards around September to the beginning of November. “From November until at least March 2024 rainfall appears to be below average over the majority of the summer rainfall area.”
Johan says it should be taken into consideration that below-average rainfall does not mean it will not rain at all, but just below-average rain totals. “The biggest deviations will be present in the central to western parts especially, between 20-50% below average. The more eastern parts’ deviations for a season like this is approximately 10-20%. However, El Niño seasons are often characterised by the spread over time along with a lack of rain during critical periods and fairly long dry periods in between rain showers. Keep in mind that the small number of clouds and rain during midsummer could result in very high temperatures.”
