Global setting: June 2018
El Nino – Southern Oscillation (ENSO) neutral conditions continued across the tropical Pacific during June 2018. The Southern Oscillation Index (SOI) was slightly negative at about -0.7 during the past 30 days (on the El Niño side of neutral). The central equatorial Pacific experienced warming sea surface temperatures (SSTs) for the third consecutive month and was slightly above average for the time of year.
SSTs in the eastern equatorial Pacific remained slightly cooler than average for the time of year. The current anomaly configuration (warmer west, cooler east) is consistent with the development of El Niño-Modoki (central Pacific warm event).
The consensus from international models is for the tropical Pacific to persist in an ENSO-neutral state over the next three-month period (52% chance over July – September 2018). However, El Niño conditions become favoured in the following three-month period (54% chance over August – October 2018), peaking in December 2018 – February 2019 at a 71% chance.
The atmospheric circulation around New Zealand is expected to be characterized by mixed flow patterns along with periodic easterly quarter flows. Consequently, the disturbed circulation pattern is anticipated to produce episodic southerly quarter flows with sharp but short-lived cold snaps and snowfall. Periodic easterly quarter flows will act as a counterbalance, bringing milder temperatures and heavy rain events to the northern and eastern North Island and eastern South Island.
Sea Surface Temperatures
Sea surface temperatures (SSTs) remain warmer than average to the east and north of New Zealand while an area of near or below average SSTs persisted in the Tasman Sea. Anomalies within New Zealand’s immediate coastal waters persisted near average during June. Coastal anomalies for the past three months are between 0.30-0.50°C above average for all regions of the country.
In the absence of large-scale climate drivers and near-average regional SSTs, forecast predictability outside of the tropics, including New Zealand, may be lower than normal over the coming three month period.