Island Climate Update 205 - October 2017

Download the PDF: Island Climate Update 205

La Niña-like signals in the ocean—atmosphere system in the tropical Pacific Ocean have become more prominent and coherent in September 2017, although have not yet reached the thresholds required for a La Niña event to be declared.

Sea surface temperatures (SSTs) in the central and eastern equatorial Pacific Ocean cooled substantially during September. In mid-August, NINO3 and NINO3.4 were near zero, and NINO4 positive. By mid-September, NINO3 and NINO3.4 were 0.3 to 0.4 degree cooler than normal, and NINO4 close to normal.

The Southern Oscillation Index (SOI) for the month of September 2017 was positive at +0.6, on the La Niña side of neutral. Zonal wind anomalies along the equator have become more negative in the central and western equatorial Pacific during September, indicating enhanced trade-winds and a pattern which is consistent with a positive SOI.

Moreover, rainfall and convection anomalies in the tropical Pacific have become more La Niña-like than they were in August.; that is, enhanced convection over the maritime continent and drier than normal conditions about and east of the Dateline. 

International forecast guidance has changed markedly in the past month. Previously, there was a strong consensus for continuation of ENSO-neutral conditions. Now there is a an approximate 50:50 split between La Niña and neutral conditions being forecast through to March 2018, with very little chance of El Niño development (<5%) through this period.