Global setting: September 2017

Many atmospheric and oceanic indicators in the tropical Pacific are on the La Niña side of neutral, although not yet strong enough to reach La Niña thresholds.

The Southern Oscillation Index has been positive for the past three months and is currently about +0.5. Tropical convection patterns show a La Niña pattern of enhanced rainfall over Indonesia and suppressed rainfall around the Dateline and eastwards. Sea surface temperatures in the central and eastern tropical Pacific have recently cooled to below normal, with the central Pacific NINO3.4 index at -0.3°C. Sub-surface tropical ocean temperatures are now more than 3°C below normal at about 100m depth and 140°W.

International forecast guidance has changed markedly in the past month. There is now a 50:50 split between the likelihood of La Niña and neutral conditions extending through to March 2018, with very little chance of El Niño development through this period.

Sea Surface Temperatures

Coastal waters remain generally warmer than average all around the country, although the anomaly in the “NZ box” (160°E-170°W, 30-45°S) has weakened from last month to its current value of about +0.3oC. Ocean waters are especially warm, relative to the average for this time of year, around the Chatham Islands and south of New Zealand. The dynamical models’ forecasts indicate that warmer than average SSTs around New Zealand are likely to persist over the October – December 2017 period. 

Differences from average global sea surface temperatures for 17 September - 14 October 2017. Map courtesy of NOAA Climate Diagnostics Centre (https://www.esrl.noaa.gov/psd/map/images/sst/sst.anom.month.gif)
Monthly values of the Southern Oscillation Index (SOI), a measure of changes in atmospheric pressures across the Pacific, and the 3-month mean (black line). SOI mean values: September SOI 0.6; July-September average 0.6.
Differences from average September surface temperatures in the seas around New Zealand