Moderate El Nino conditions persist in the equatorial Pacific east of the International Date Line, with important oceanic and atmospheric indicators continuing to indicate an ongoing El Nino event. However, there are a lot of signs that it is gradually diminishing. As a result, some models from the United States have already issued a La Nina watch for winter months (2024).
Almost all the models observed by the International Research Institute for Climate & Society forecast a continuation of the El Nino event during the rest of the autumn months, but rapidly weakens it after that. ENSO-neutral conditions become the most likely category over the period from April to June.
Then for the June to August period the La Nina becomes the most dominant pattern with the chances of its development reaching 60 per cent but by late winter. The chances of a La Nina increase further with the probability of such an occurrence exceeding 80pc by spring. Meteorologically speaking, this is highly significant so it is now almost certain that that will occur.
The ramifications of this for eastern and northern Australia are significant. If it occurs it will be the fourth year in the past five years with a La Nina dominating. During the three years to 2023, when the La Nina dominated for the whole period, Australia had close to 20pc above average rainfall. That might not seem significant, but it was sufficient to result in flooding in many major river systems. It also brought more cloud and consequently, lower summer temperatures than in the previous years. Then the 2023-24 El Nino brought higher temperatures again, but warm sea surface temperatures persisted around continental Australia, especially in the east and this resulted in further rain events.
It is worthwhile noting that just as El Nino events do not always result in warmer and drier weather, La Nina events do not always result in milder and wetter conditions. However, with a La Nina developing this year and the SSTs in the western Pacific remaining above average, it is likely that wetter and milder weather will dominate conditions in eastern Australia and probably north Australia.
What happens in the Indian Ocean will also be important. The situation there is unclear due to the unprecedented warmth of much of the ocean but the Indian Ocean Dipole is likely to be either neutral or slightly negative by later in autumn when it starts influencing Australian rainfall patterns and if maintained, this will also support the chance of slightly above average rainfall in winter and spring.
