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El Nino has been officially declared, and Earth Sciences New Zealand (ESNZ) expects the climate phenomenon to intensify into one of the strongest on record.
The naturally occurring cycle originates in the tropical Pacific Ocean, and significantly alters weather patterns in New Zealand, typically bringing stronger westerly winds and increased rainfall to the west and south of the country, and drier and warmer weather to the east and north.
ESNZ forecasting principal scientist Chris Brandolino said El Nino conditions had now been reached in the tropical Pacific atmosphere and ocean, but the event was still in its early stages.
He said the warmer-than-average subsurface ocean temperature anomalies would continue to intensify and expand in the tropical Pacific.
"This means that a significant El Nino event is increasingly likely.
"There is about an 80% chance for El Nino to reach or exceed strong intensity over the outlook period [July-September 2026].
"It will have significant impacts on New Zealand’s weather."
On the West Coast and in the Southern Alps and foothills, inland Otago, and Southland, temperatures were about equally likely to be near average or above average, he said.
Rainfall totals were also most likely to be above normal, and soil moisture levels and river flows were most likely to be near normal.
"Chances for active weather — heavy rain and strong wind — are expected to increase later in the winter season, and especially into spring."
Along eastern Otago, temperatures were about equally likely to be above average or near average.
"Chances for increased temperature variability — like large temperature swings — and unusually windy periods, are likely to increase later in the winter season and especially during spring," he said.
"Rainfall totals are about equally likely to be below normal or near normal for the outlook period as a whole, but there is a risk of some significant rainfall occurring early in July."
Soil moisture levels and river flows are about equally likely to be below normal or near normal, he said.
"The prospect of below normal winter rainfall in several regions is likely to translate to below normal groundwater recharge, creating challenges for water-reliant sectors."
He said El Nino conditions were expected to peak during the austral summer of 2026-27.
