Marufish World of Disaster PreventionBOM said 2025 Negative Indian Ocean Dipole event continues, La Nina developing

Bureau of Meteorology, Australia.

Negative Indian Ocean Dipole event continues; tropical Pacific remains cooler than average

Since July 2024, sea surface temperatures (SSTs) in the Australian region have been the warmest or second warmest on record for each respective month. Similarly, global SSTs remain substantially above average, with September 2025 the third warmest on record.
The sea surface temperatures (SST) analysis for the week ending 26 October 2025 shows warmer than average waters across much of the Australian region, particularly along the east coast. Near-average waters surround Victoria and Tasmania.
Forecasts for November to January show warmer-than-average SSTs are likely around most of Australia, especially to the south-east.
The negative Indian Ocean Dipole (IOD) event remains active, with the latest index value reaching −1.61 °C for the week ending 26 October—the most negative IOD value recorded since the Bureau’s dataset began in August 2008.
The Bureau’s model predicts the negative IOD event to continue throughout spring, with a return to neutral in early summer. This is consistent with most international models assessed and the typical IOD life cycle.
The El Niño–Southern Oscillation (ENSO) remains neutral. While there are signs of possible La Niña development in the tropical Pacific Ocean and atmosphere, these signals have not yet been sustained at a sufficient strength or duration to meet Bureau criteria for an active event.
The latest relative Niño3.4 SST index value for the week ending 26 October 2025 is −0.86 °C. The weekly values of the relative Niño3.4 index have been fluctuating around the La Niña threshold since mid-to-late September. Index values between –0.8 °C and +0.8 °C are considered ENSO-neutral while sustained values below –0.8 °C (for at least 3 months) are considered indicative of La Niña.
Some atmospheric indicators, such as trade winds, pressure and cloud patterns over the equatorial central Pacific, show some signs of La Niña development. The 30-day Southern Oscillation Index (SOI) has just recently exceeded +7, although the 90-day SOI remains neutral. The Madden–Julian Oscillation is currently, as at 26 October 2025, enhancing trade winds over the western Pacific, which may provide a short-term boost towards La Niña conditions.
Under Bureau criteria, La Niña is considered established when sustained cool waters in the central tropical Pacific (relative Niño3.4 index values below –0.8 °C for at least 3 months) are accompanied by a consistent atmospheric response.
The Bureau’s model currently predicts that tropical Pacific Ocean temperatures are likely to just meet La Niña levels during November and December, before returning to neutral. This timing aligns with most international models assessed, although they generally show slightly more cooling, compared to the Bureau’s model.
The Southern Annular Mode (SAM) index is negative as at 25 October. It is forecast to increase in the coming days to neutral SAM levels and remain so for the at least the next fortnight.