Global setting: October 2016

Sea surface temperatures (SSTs) in the central equatorial Pacific Ocean are below average, close to the threshold used to define La Niña events. Anomalously cold sub-surface waters are still present, but are now more confined to the central Pacific (near 140oW) than in previous months.  

The atmospheric conditions are also mixed: stronger easterly trade winds in the west and enhanced convection over the Maritime Continent (islands of Indonesia and Papua New Guinea) are consistent with a weak La Niña. However, the Southern Oscillation Index (SOI) has dropped to weakly negative values which are indicative of ENSO-neutral conditions.

International guidance still slightly favours La Niña conditions (53% chance versus 46% for neutral) over the next three month period (November 2016 - January 2017). However, neutral conditions are now much more likely than La Niña by February – April 2017: 74% chance for neutral, and only 22% for La Niña. In summary, La Niña conditions are only slightly more likely than not over the next 3-month period, and become less likely as we progress into 2017.

Despite the current borderline La Niña conditions and the only modest probability for La Niña to develop by the end of the year, the circulation pattern expected over the coming three months for the New Zealand region is broadly consistent with the typical La Niña signature: higher pressure than normal is forecast to the south and southeast of the country, while lower pressures than normal are forecast to the north of the New Zealand, leading to more persistent easterly or north-easterly airflow than normal.

Sea Surface Temperatures

Sea surface temperatures around New Zealand are forecast to remain near or above normal over the next three months.

Differences from average global sea surface temperatures for 2 October - 29 October 2016. Map courtesy of NOAA Climate Diagnostics Centre (http://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: October SOI -0.4; August to October average 0.5.
Differences from average October surface temperatures in the seas around New Zealand.