Island Climate Update 275 - September 2023
The monthly NINO3.4 Index anomaly (in the central equatorial Pacific) at the end of August was +1.34˚C. The weekly value reached +1.5˚C at the end of the month – the traditional threshold for a strong oceanic El Niño. Only August 1997 featured warmer conditions in the east-central Pacific (NINO1.2 and NINO3 Index) with records dating back to 1981.
The SOI was on the El Niño threshold (-1.0) during August. The SOI exhibited a negative, El Niño-like trend during August. Should this continue during September, the conditions needed to officially classify El Niño may be reached.
Trade wind strength was below normal in the west-central Pacific and near or above normal farther east during August. There is the potential for a major reduction in trades in the west-central Pacific during late September, which may spawn a downwelling Kelvin Wave. The Kelvin Wave would cause additional warming in the central and eastern Pacific, potentially resulting a very strong El Niño by the end of the year.
In the sub-surface eastern equatorial Pacific Ocean, remarkable anomalies of 5 to 7˚C above average were occurring in the upper 100 m as of late August.
The distribution of the anomalously warm water is consistent with the development of an east-based, canonical (classical) El Niño event. The abnormally water waters are predicted to surface and expand westward over the course of the next three to six months, culminating in a strong or very strong El Niño event that may rival records.
NIWA’s analysis indicates that oceanic indicators have reached El Niño thresholds and atmospheric indicators are likely to be sustained in the El Niño range over the next month. El Niño, once it develops, has a 95% chance of continuing through February 2024.