Island Climate Update 300 - October 2025

ENSO-neutral conditions currently remain in place in the tropical Pacific, but they are trending toward La Niña. There is a 60% chance that La Niña conditions will develop during October-December 2025, with a 60% chance that La Niña conditions will continue during November 2025-January 2026.

As of 22 September 2025, the 30-day NINO3.4 Index (in the central equatorial Pacific) was -0.38˚C, in the neutral range. The 30-day relative Niño 3.4 Index (RONI) was -0.70˚C, at the La Niña threshold and reflective of the central equatorial Pacific being cooler than the average of the global tropics.

The Southern Oscillation Index (SOI) was on the La Niña side of the neutral range during July-September (+0.4), while the September value was 0.0 (in the neutral range). 

Temperatures in the subsurface equatorial Pacific remain above average in the western part of the basin with cooler than average temperatures continuing to develop in the east.

Notably cooler than average temperatures are currently located in the eastern equatorial Pacific at depths of 50-200 metres. These cooler water temperatures reaching the surface will move conditions in a La Niña direction.

Upper oceanic heat content is also moving in a La Niña-like direction.

During October-December, model guidance favours an enhancement in convective forcing over the western Pacific and parts of Melanesia, co-located with the warmest sea surface temperatures. This may lead to enhanced rainfall for island groups such as Palau, western Federated States of Micronesia, and Papua New Guinea east to parts of French Polynesia.

Conversely, drier than normal conditions are likely to occur for most island groups along and north of the equator, including much of the Marshall Islands, Nauru, Kiribati, Tuvalu, Tokelau, northern Cook Islands, and northern French Polynesia (see pages 6-7 for more information).

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Island Climate Update - October 2025 [PDF 5MB]