Seasonal Climate Outlook

The weakening of the Southern Annular Mode and the Indian Ocean Dipole and the forecast for ENSO to remain neutral over the next three months indicate the absence of strong, persistent climate drivers for New Zealand.

Temperatures for the summer season are expected to be above average for New Zealand, apart from the west of the South Island where there are about equal chances for near average or above average temperatures.

For November to January, air pressure is forecast to be lower than normal to the southwest of New Zealand and higher than normal to the north. This is expected to be associated with a westerly quarter air flow anomaly.

For October to December, air pressure is forecast to be lower than normal in the New Zealand region, especially south of the country. This is expected to be associated with a westerly quarter air flow anomaly, particularly during November and December. Occasional easterly quarter winds are possible during October.

The central Pacific El Niño event that arrived in March 2019 has ended, giving way to ENSO neutral conditions, owing to cooling sea surface temperatures (SSTs) in the tropical Pacific and a neutral Southern Oscillation Index (SOI) during August.

Despite a sharp cold snap in early August, seasonal temperatures are forecast to be near average or above average for all regions of New Zealand, owing to warmer than average coastal and regional sea surface temperatures.

A weak, central Pacific El Niño continued during June as sea surface temperatures (SSTs) remained more than 0.7˚C above average (i.e. the El Niño threshold) for the fourth consecutive month.

Although El Niño is forecast to continue during the upcoming three-month period, it may weaken later in 2019. Winter temperatures are forecast to be above average in the east of the South Island and to be above average or near average in all remaining regions.

A weak, central Pacific El Niño continued during April, as patterns of enhanced rainfall persisted in the vicinity of the International Dateline. More westerly quarter winds than normal. Tasman Sea surface temperatures may influence several spells of unseasonable warmth through the season, particularly in eastern areas, contributed to by frequent westerly air flows.

A central Pacific El Niño event continued during March as the ocean and atmosphere remained weakly coupled. Sea surface temperatures warmed across the equatorial Pacific during March and El Niño is expected to continue during the upcoming three-month period.

A central Pacific El Niño is now occurring as the ocean and atmosphere have been weakly coupled for a third consecutive month. Traditionally, this occurs farther east toward South America and during the early summer season. Mean temperatures are forecast to be above average for all of New Zealand.

The atmospheric circulation around New Zealand is forecast to be characterised by slightly higher than normal pressure to the southwest and southeast of New Zealand and lower pressure than normal to the northeast of the country. Weak easterly-quarter air flows are favoured.

The atmospheric circulation around New Zealand is forecast to be characterised by pressures slightly higher than normal over and to the north of the country. For New Zealand, this pressure pattern will be accompanied by weak south-westerly quarter flow anomalies.

Mean summer temperatures are about equally likely to be near or above average. Near normal summer rainfall is likely for most regions; however, the north of the North Island has about equal chances for below normal or near normal rainfall and the west of the South Island has about equal chances for above normal or near normal rainfall.

The November 2018 – January 2019 atmospheric circulation around New Zealand is forecast to be characterised by higher pressure than normal around the country and lower pressure than normal to the southwest.

The consensus from international models is for the tropical Pacific to transition toward El Niño over the next three-month period (68% chance over October – December 2018). The probability for El Niño conditions being established remains high until autumn 2019, with a 71% chance for El Niño conditions over the April – June 2019 period.

The consensus from international models is for the tropical Pacific to transition toward El Niño over the next three-month period (65% chance over September – November 2018). The probability for El Niño conditions being established increases as we reach into and beyond the Southern Hemisphere summer, with a 78% chance for El Niño conditions over the March – May 2019 period. Indications are that the event – if it eventuates – will not be in the strong category. The type appears to lean toward the central Pacific event, although dynamical models indicate that warm SST anomalies may gradually transit eastward later in 2018 into early 2019.

ENSO-neutral conditions continued across the tropical Pacific during July 2018. The Southern Oscillation Index (SOI) was in the neutral range, with a preliminary value of +0.2 for July 2018. Surface ocean waters in the central and eastern equatorial Pacific continued to warm during July but remained in the neutral range. The subsurface ocean remained warmer than average in the eastern equatorial Pacific during July, but anomalies weakened slightly compared to June.

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.

ENSO-neutral conditions persisted in the tropical Pacific during May 2018. The Southern Oscillation Index (SOI) was positive at about +0.3 during the past 30 days. Sea surface temperatures (SSTs) in the central and eastern equatorial Pacific Ocean warmed for the second consecutive month and are now near or slightly above average for the time of year.

Temperatures are forecast to be above average in the north of the North Island and average or above average for all remaining regions of New Zealand. Despite the prospect for average or warmer than average temperatures, frosts and cold snaps will become more common, with some cold snaps possibly quite sharp.

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.

With La Niña’s influence waning over the next three month period, New Zealand’s regional climate over March – May 2018 is expected to be driven by the warmer than average ocean waters that are present around the country.

Apart from waning La Niña conditions, New Zealand’s regional climate over the next three month period is expected to be dominated by the very warm ocean waters present around the country, in the Tasman Sea, and in the Southwest Pacific Ocean.