Island Climate Update 253 - October 2021

The NINO3.4 Index anomaly (in the central Pacific) during September was -0.23˚C. Compared to this time last year, conditions are about 0.5˚C warmer. The Southern Oscillation Index (SOI) was +0.8 during September, on the La Niña side of neutral. Notably, the three-month average SOI was +1.0, in La Niña territory.

During September, upper-oceanic heat content again decreased across the equatorial Pacific. Importantly, the cooler conditions relative to average remain displaced toward the west-central part of the basin, aligned with a non-traditional type of La Niña.

Trade winds were enhanced in a widespread fashion across the tropical Pacific during September, which contributed to additional cooling at the ocean’s surface. Enhanced trade winds are expected to continue during the next three months as the ocean continues to progress toward La Niña thresholds.

In a La Niña-like fashion, patterns of reduced rainfall that established earlier this year have persisted for several island groups near and extending southeastward of the equator during September. Some areas around Nauru, Kiribati (Gilbert, Phoenix & Line Islands), Cook Islands, Tuamotu/Gambier Islands, Marquesas, Pitcairn Islands and New Caledonia saw over 25 dry days during September.

On the balance of evidence, La Niña conditions are favoured to develop during October to December period (60% chance), peaking at a 65% chance between November and January.

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