Global setting: March 2018

Weak La Niña conditions continued in the tropical Pacific during March 2018, but trends in low-level winds and in sub-surface ocean temperatures during the month indicate that the event is coming to an end. 

The consensus from international models is for the tropical Pacific to transition to an ENSO-neutral state over the next 3-month period (75% chance over April – June 2018). ENSO-neutral remains the most likely outcome over the late-winter season (July – September 2018). The forecast models predict about an equal chance of the Pacific remaining neutral or transitioning towards El Niño over the spring (September – November 2018).

Even though La Niña’s influence will wane over the next three-month period, New Zealand’s regional climate over April – June 2018 is expected to be driven by persistence of more northeasterly airflow than normal, and by the persistence of warm ocean waters that are present around the country. The northern Tasman Sea will remain unsettled, with lower pressures than normal, bringing the likelihood of significant rainfall events to the North Island and the upper South Island.

Sea Surface Temperatures

Coastal waters remain much warmer than average all around New Zealand, in the Tasman Sea, and in the Southwest Pacific. However, the anomalies for coastal New Zealand continue to weaken from the peak reached in December 2017. The latest vertical profiles from the Argo floats in the Tasman Sea show that the surface layer is more well-mixed down to 60m in March (and about +1.0 oC warmer than average), as opposed to the strong stratification and extreme warmth at the surface (+2.5 oC) in the January 2018 profiles.

The warmest coastal anomalies (for the past three months) are around the west of the South Island, where they remain at least +2.0oC above normal. According to the dynamical models’ forecasts, warmer than average SSTs are likely to persist for at least part of the next 3 months (April – June 2018).

Differences from average global sea surface temperatures for 4 March - 31 March 2018. Map courtesy of NOAA Climate Diagnostics.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: March SOI 1.1; January-March average 0.5.
Differences from average March surface temperatures in the seas around New Zealand