Seasonal Climate Outlook

September – November 2020 air pressure is forecast to be higher than normal to the east and lower than normal to the northwest of New Zealand. This is expected to be associated with La Niña-like northeasterly air flow anomalies.

During periods of northeasterly winds, the threat for sub-tropical low pressure systems capable of producing heavy rainfall, similar to those experienced in late June and mid-July, is elevated, particularly in the north and east of the North Island.

Rainfall is about equally likely to be near normal or below normal in the west of the South Island, about equally likely to be near normal or above normal in the east of the North Island, and most likely to be near normal in all other regions.

Oceanic ENSO-neutral conditions will very likely persist over the next three months. The long-standing climate drivers that have contributed to dryness over much of NZ are expected to influence our weather for at least the first half of the winter season.

Sharp cold snaps are possible during the month of May as fronts move onto the country from the Tasman Sea and Southern Ocean.

Warmer than average ocean waters in the Coral Sea and west-central Pacific Ocean are expected to have some influence on New Zealand’s climate in the first half of the three month period.

Temperatures are mostly likely to be above average in the North Island and about equally likely to be above average or near average in the South Island.

Temperatures are expected to be near or above average for all regions of New Zealand except the north of the North Island where above average temperatures are most likely.

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.