Island Climate Update 240 - September 2020

During August, the NINO3.4 Index anomaly (in the central Pacific) was 0.42˚ C. The NINO 1+2 Index (eastern Pacific) was 0.85 C. Upper oceanic heat content remained lower than normal in the eastern and central equatorial Pacific with a substantial decrease observed in the east central part of the basin. Overall , the pattern is well aligned with a developing La Niña event.

Trade winds during August were stronger than normal across the equatorial Pacific, particularly in the central part of the basin. This contributed to upwelling, which led to cooling sea surface temperatures, representing an oceanic trend in the La Niña direction. This pattern is expected to continue into austral spring, with enhanced trade winds occurring over the central equatorial Pacific (e.g. Kiribati, Nauru) this may mean that the coolest ocean temperatures, with respect to average, end up occurring there.

Rainfall and convection continued to be below normal across much of the equatorial Pacific during August. This was surrounded by isolated areas of above normal rainfall to the north, south and west, including a more active than normal South Pacific Convergence Zone, particularly to the east of the International Dateline. The pattern was La Niña like in nature, although it is apparent that the atmosphere has yet to fully couple to the ocean with some variability continuing.

Based on the consensus from international models, the probability for La Niña conditions is 57% for the September November period, an increase of 3% from last month. For the December 2020 February 2021 period, the probability for La Niña is 46% and 43% for ENSO neutral conditions.

Based on the observations and forecast guidance, a La Niña Alert is in place as of September 2020.

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