Island Climate Update 252 - September 2021

The NINO3.4 Index anomaly (in the central Pacific) during August was -0.17 ̊C. The Southern Oscillation Index was +0.5. While both remained in neutral territory, other indicators trended toward La Niña.

During August, upper-oceanic heat content decreased across the equatorial Pacific for the third consecutive month. Cooler than average equatorial sub-surface waters (-0.5˚C to 1.5˚C) progressed toward the surface in the central Pacific while a layer of warmer than average water persisted near the surface off the west coast of South America. The distribution of anomalies is currently closest to a central Pacific type of La Niña, similar to conditions that occurred this time last year.

Trade winds continued to be enhanced in the equatorial central Pacific, which is where the cooling of the sea surface was focused. Enhanced trade winds are expected to continue through September, particularly in the central equatorial Pacific, which should result in additional cooling of the sea surface. In a La Niña-like fashion, patterns of reduced rainfall affected several island groups near and extending southeastward of the equator during August.

La Niña or ENSO “cool” neutral conditions are about equally likely (45-50% chance each) from September-November before the chance for La Niña peaks at around 60% during December - February.

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