Island Climate Update 204 - September 2017

Download the PDF: Island Climate Update 204

ENSO (El Niño –Southern Oscillation) neutral conditions (neither El Niño nor La Niña) continued in the tropical Pacific during July 2017, mixed signals were again present this month. Some atmospheric patterns have been recently leaning more towards weak La Niña conditions.

The Southern Oscillation Index is currently positive (+0.9, i.e. on the La Niña side of neutral) and large-scale circulation anomalies along the Equator as well as rainfall anomalies in the western Pacific are consistent with patterns usually associated with a La Niña state.

On the other hand, Sea-surface temperatures (SSTs) in the equatorial Pacific Ocean are slightly above normal (i.e. on the El Niño side of neutral), but these positive anomalies weakened over the course of the month. The latest monthly SST anomaly (data ending 30th July 2017) in the NINO3.4region (in the central Pacific) is currently at +0.43°C (was +0.5°C last month).

International guidance favors a persistence of ENSO-neutral conditions over the next three months period (59% chance for August –October 2017), a likelihood similar to last month. ENSO-neutral remains the most likely outcome throughout the first quarter of 2018. Note however that the models used to build this consensus forecast have not picked up yet on the most recent changes shown by the La Niña-leaning atmospheric indicators.