Meteorologists say a "significant" El Niño weather pattern could influence weather conditions in parts of the country this winter.
Earth Science New Zealand says climate models are showing a "clear consensus" for an El Niño event to arrive this winter, bringing more rain to parts of the South Island while leaving northern and eastern parts drier than usual.
"La Niña has only just ended, but there are indications that we’ll see a rapid flip to El Niño," said Earth Sciences New Zealand and MetService meteorologist Jon Tunster.
"Given the projected strength of this event, it will be very surprising if New Zealand doesn’t feel some impacts."
El Niño and La Niña are two phases of the El Niño–Southern Oscillation, a natural climate cycle driven by ocean temperatures and atmospheric pressure across the tropical Pacific.
During El Niño, sea surface temperatures become warmer than normal in the central and eastern Pacific Ocean, changing wind patterns shifting where rain clouds and storms tend to form.
There is a greater than 60% probability that the event will be classified as "strong" by spring, with the potential to intensify further later in the year.
What does this mean for New Zealand?
Tunster said the atmospheric changes caused by El Niño could strengthen the tendency for high-pressure systems to anchor themselves over Australia during winter, resulting in a south-westerly flow over New Zealand.
“This could bring increased rainfall to Southland, parts of Otago, and western facing areas, primarily of the South Island, but reduced rainfall for much of the remainder of the country," he said.
“Dryness could become an issue as spring approaches, for areas that see a reduction in their normal winter rainfall such as the east of both islands, or the upper North Island."
While south westerly flows are often linked to colder weather, Tunster said a nationwide colder-than-average winter was unlikely at this stage.
“We could see occasional cold snaps, lasting a few days at a time – except in inland parts of the South Island where the cold air can linger for longer periods. Or a colder-than-average month within an otherwise near-average season."
Tunster said there were "multiple indicators" pointing to the development of an El Niño event this winter, with conditions beneath the ocean surface aligning with past El Niño events.
“One of the most striking observations is a steady eastern movement of extremely warm water beneath the surface of the tropical Pacific Ocean," Tunster said.
"This has been a precursor of significant El Niño events in the past when these warm waters are eventually brought to the surface,”
The developing El Niño could also affect major international weather systems, including the Indian monsoon and the North Pacific storm track.
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