Global setting: November 2017

La Niña conditions are present in the tropical Pacific, as several conventional thresholds have been reached or are being approached.  As of earlier this month, NOAA declared that weak La Niña conditions have emerged, while the Australian Bureau of Meteorology escalated their ENSO tracker to the “La Niña Alert” category. Sea surface temperatures (SSTs) in the eastern Equatorial Pacific Ocean have continued to cool with the most anomalously cool temperatures from about 140°W east to the South American coast. 

The NIWA Southern Oscillation Index has been positive since July 2017 and is currently at +0.9.  Enhanced trade winds persist in the western equatorial Pacific Ocean and cooler than normal subsurface ocean waters present in the central Equatorial Pacific (at about 100m depth) have cooled further.

The consensus from international models is that the tropical Pacific Ocean will cool further for a portion of the next 3 months (December 2017 – February 2018), with La Niña conditions expected to continue over the same period (75% chance).  However, the models indicate that La Niña is most likely (72% chance) to transition to ENSO neutral conditions over the March – May 2018 period.  While this particular event is expected to remain in the weak category, it should be noted that the impacts of La Niña are not always proportional to its intensity.

For December 2017 – February 2018, the atmospheric circulation around New Zealand is forecast to be characterised by higher pressure than normal to the south and southeast of the country, and lower pressure than normal to the north. This pressure pattern is expected to be associated with easterly to northeasterly flow anomalies, a pattern which is consistent with regional conditions typically observed during La Niña events.

Sea Surface Temperatures

Coastal waters remain warmer than average around New Zealand, especially off the coast of the South Island, where the anomaly for the month of November (estimated using data to the 28th November) exceed +1oC. As a whole, warm anomalies have recently intensified, notably around the coasts of the South Island of the country.  It can be noted that warmer than normal surface ocean waters around NZ and in the Tasman Sea are often associated with La Niña conditions. 

Differences from average global sea surface temperatures for 5 November - 02 December 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: November SOI 0.9; September-November average 0.8.
Differences from average November surface temperatures in the seas around New Zealand