Global setting: December 2017

Weak La Niña conditions persisted in the tropical Pacific during December 2017. Below average sea surface temperatures (SSTs) are present across the central and eastern Equatorial Pacific Ocean, and have cooled further compared to November 2017, with the strongest anomalies currently measured off the coast of Ecuador and Peru.  Subsurface temperature anomalies are also consistent with weak La Niña conditions, however atmospheric signals have become less consistent over the course of December 2017: the enhanced trade winds that were present in the western Pacific Ocean earlier on have weakened, and in December 2017 the Southern Oscillation Index (SOI) weakened and is currently in the neutral range.

The consensus from international models is that weak La Niña conditions are likely (72% chance) to continue over the next 3 months (January – March 2018). Thereafter, the models agree, that La Niña will decay rapidly and a return to ENSO-neutral conditions is most likely (74% chance) over the April – June 2018 period.

 For January – March 2018, the atmospheric circulation around New Zealand is forecast to be characterized by higher pressures than normal east and south of the country, while lower pressure than normal is forecast over the Tasman Sea area, extending over the country. This pressure pattern is expected to be associated with unsettled conditions with periods of north-easterly quarter flow anomalies.

Sea Surface Temperatures

Coastal waters remain much warmer than average all around New Zealand and in the Tasman Sea, with anomalies currently exceeding +1.5oC for all coastal areas around the country. According to the dynamical models’ forecasts, this “Marine Heat Wave” is likely to persist for at least part of the next 3 months (January – March 2018), although the anomalies are expected to ease off slightly over the same period.

Differences from average global sea surface temperatures for 10 December - 8 January 2018. 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: December SOI -0.2; October-December average 0.6.
Differences from average December surface temperatures in the seas around New Zealand