Global setting: July 2017

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

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. On the other hand, the Southern Oscillation Index is currently positive (+0.8, 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.

International guidance favours a persistence of ENSO-neutral conditions over the next three-month period (59% chance for August – October 2017), a likelihood comparable to the one indicated 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.

Sea Surface Temperatures

Coastal waters remain generally warmer than average all around New Zealand, with anomalies still exceeding +0.5oC along the west coast of the South Island. Anomalies in the “NZ box” (160°E-170°W, 30-45°S) are currently reaching about +0.5oC. Ocean waters are still much warmer than average in the southern part of the Tasman Sea, but these anomalies have weakened compared to last month and are less extensive.

The dynamical models’ forecasts indicate that warmer than average SSTs around New Zealand are likely to persist over the August – October 2017 period. 

Differences from average global sea surface temperatures for 2 July - 29 July 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: July SOI 0.9; May-July average 0.0.
Differences from average July surface temperatures in the seas around New Zealand
Research subject: Climate