EARLIER this week, the Bureau of Meteorology (BOM) decided to shift its climate outlook to La Niña watch, despite the El Niño–Southern Oscillation (ENSO) currently being neutral.

With some early signs that a La Niña might form in the Pacific Ocean later in 2024, the BOM said when the watch criteria have been met in the past, a La Niña event has subsequently developed around 50 per cent of the time.

The BOM said there was about an equal chance of neutral ENSO conditions in the same outlook period.

Sea surface temperatures (SSTs) in the central Pacific have been steadily cooling since December 2023

This surface cooling is supported by a significant amount of sub-surface cooling in the central and eastern Pacific.

Recent cloud and surface pressure patterns are ENSO-neutral.

Jane’s Weather website meteorologist, Jane Bunn said the Pacific Ocean has been transitioning from El Niño to La Niña “for a few months now” and it was important to register it as an objective observation from BOM, not a forecast.

The BOM’s modelling suggested that ENSO will likely remain neutral until at least July 2024, but Ms Bunn said three global models are forecasting that the La Nina threshold could be crossed in August.

“Another two leave it until October/November,” she said.

“The remaining two don’t quite cross the threshold (but are nowhere near El Nino). So, that is five out of seven models that have us in La Niña by the end of spring - with some getting us there in late winter.”

However, she said conditions in the Indian Ocean were different, displaying a positive Indian Ocean Dipole (IOD).

“This acts to limit the amount of moisture available to Australia from the Indian Ocean (like the ‘El Niño of the Indian Ocean’),” Ms Bunn said.

“However, this may not be a typical positive IOD. There is no pool of cool water off Indonesia yet, so the amount of moisture may not be as low as usual. The last El Niño in the Pacific was the same, no cool pool off Queensland to limit the moisture.”

The BOM said the most recent two weeks have seen the IOD index within neutral thresholds and follow seven weeks of the index being above the positive IOD threshold (+0.40 °C).

The current global sea surface temperatures (SSTs) observations suggested that recent development of a positive IOD may have stalled, so if a positive IOD eventually developed, it would be earlier in the calendar year than is typical historically.

The BOM said SSTs have been the warmest on record for each month between April 2023 and April 2024, with April 2024 SSTs warmer than April 2023.

The global pattern of warmth is affecting the typical historical global pattern of sea surface temperatures associated with ENSO and IOD variability.

As the current global ocean conditions have not been observed before, inferences of how ENSO or IOD may develop in 2024 that are based on past events may not be reliable.

The Southern Annular Mode (SAM) is currently neutral (as at  May 13) and forecasts have indicated the index is mostly likely to remain neutral or become positive in the coming fortnight.

The Madden–Julian Oscillation (MJO) is currently weak with most models indicating that the MJO will remain weak before re-strengthening over the eastern Indian Ocean or Maritime Continent region from mid- to late-May.

Ms Bunn said when the MJO is near Australia it enhances our supply of moisture and the climate driver SAM was also important.

“We also have cold fronts crossing the south, but without help from … SAM, they lack the power and strength to spread any rain inland,” she said.

“While we can’t see what SAM and MJO are likely to do after the short term, we can see what the Pacific and Indian Oceans are likely to do over the next few months.”