The El Nino-Southern Oscillation set up over the Pacific Ocean is now officially neutral with the United States agencies and the Australian Bureau of Meteorology all agreeing. After a low in February around -15 when an El Nino pattern dominated, the Southern Oscillation Index is back close to zero.
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Most major models now expect that neutral conditions will continue at least until early August for most of winter, after which there are early signs that a La Nina might form in the Pacific Ocean during spring.
Accordingly, the bureau's official outlook is for a La Nina watch. When La Nina watch criteria have been met in the past, a La Nina event has subsequently developed around 50 per cent of the time. This means it is worth watching, but it is by no means certain, with the other well supported scenarios indicating the current neutral pattern will persist for the rest of the year.
There is little or no chance of an El Nino returning in the foreseeable future. A neutral pattern in the past has been linked to near average rainfall in eastern Australia, although such rainfall totals are often reached by the occasional significant short term event, interspersed by extended periods of stable conditions and this is likely to be the case over the coming months. If a La Nina does develop in spring, then there is a slightly increased chance of above average rainfall at that time.
However, as seems to be the case often in recent times, there are two significant features that have the potential to "throw a spanner in the works".
First - as I have been mentioning frequently in recent months is the unusually warm sea surface temperatures in the Tasman and Coral seas and parts of the tropical western Pacific. In fact, global SSTs have been the warmest on record for each month between April 2023 and April 2024.
Inevitably this is likely to affect the typical behaviour in both the ENSO and the Indian Ocean Dipole and because the current ocean temperature conditions have not been observed before, then how ENSO or IOD may respond during this year can no longer be based on past events. Warm waters off the east coast increase potential for rain events occurring when onshore winds dominate.
The Indian Ocean temperature patterns are becoming challenging to predict. The IOD is neutral again, after being positive for two months. This suggests that the expected development of a positive IOD may have stalled. The consequences of at least a neutral IOD would be to increase the chances for an occasional north west cloud band and its associated winter rain event for not only eastern but south east Australia, including Victoria.
