The monthly NINO3.4 Index anomaly (in the central equatorial Pacific) at the end of July was 1.11 ˚C . At the same point during the developmental phase of strong past El Niño events , it was +1.35˚C in 2015, +1.31˚C in 1997, and +0.26˚C in 1982.
The SOI was on the La Niña side of neutral (+0.5) during June. Although changes in the ocean temperature are suggestive of El Niño, the atmosphere has yet to demonstrate a consistent, El Niño-like response to those warming seas.
June trade winds were above normal in the eastern Pacific and near or slightly below normal in the central Pacific during June. July is expected to have enhanced trade winds west of the International Date Line during the first half of the month, then enhanced trades east of the Date Line during the second half of the month with weakening trades to the west.
In the sub-surface equatorial Pacific, the most unusually warm waters were located in the east with localised anomalies of 4-6˚C above average. The western Pacific remained anomalously warm (1-2˚C above average), but the warm pool has shown an eastward progression. This shift is expected to continue in the months ahead, leading to an atmospheric response that becomes more El Niño-like over time.
The persistence of warmth in the West Pacific Warm Pool may allow La Niña-like patterns to occur from time-to-time over the next month or so, lengthening the transition window to El Niño.
NIWA’s analysis indicates that oceanic indicators have reached El Niño thresholds, but atmospheric indicators remain neutral. Because El Niño is a coupled ocean-atmosphere phenomenon, the ocean and the atmosphere need to be working in tandem before an event is classified. NIWA is now at El Niño Alert with an event likely to begin by August or September. Overall, El Niño has a 95% chance of developing by September, with air and sea temperatures, rainfall, and sea level anomalies tending in an El Niño-like direction.
