Oriental Bay beach, Wellington. File photo. Photo: RNZ / Soumya Bhamidipati
New Zealand can expect a bumpy ride through September storms before conditions calm down as October settles, shifting towards the sunny start of a La Niña summer, a climate scientist says.
La Niña - a climate pattern driven by cooler-than-usual ocean temperatures along the equator - is returning for the fifth time in just six years, an unusually frequent run only seen once before since 1900.
It shapes global wind and rainfall patterns and will play a major role in New Zealand's spring and summer weather.
Earth Sciences New Zealand's Chris Brandolino said the signs have been building for months.
Earth Sciences New Zealand's Chris Brandolino. File photo. Photo: NIWA
"Conditions in the tropical Pacific are trending toward La Niña, and that will influence weather across the Pacific, including New Zealand," he told RNZ.
But don't expect a quick change.
Brandolino said September would remain "exceptionally unsettled, stormy, dare I say," with strong westerlies dominating thanks to a sudden stratospheric warming event high above Antarctica.
That rare atmospheric jolt has helped fuel the relentless run of storms, and the impact will be felt, particularly in the South Island.
"We have to watch out for potentially some very heavy rain and strong, damaging wind later this weekend into next week," Brandolino warned.
"The top of the South Island and the West Coast are worth watching."
While September will stay rough, a shift should begin in early to mid-October.
"As we enter the month of October… maybe week two we'll start to see this transition to a more La Niña-like pattern," Brandolino said.
That means more settled, drier, warmer "happy weather," especially for the South Island heading into November and December.
What La Niña will actually deliver depends on where the high-pressure systems position themselves. If they sit closer to the Chatham Islands, the North Island could be more exposed to rain-bearing systems from the tropics, raising the risk of floods.
If the highs park over the South Island, they act like a shield, blocking those systems and favouring drier, sunnier conditions.
Brandolino pointed out that not all La Niña summers look the same. The 2020-21 pattern brought dry, settled weather, while 2022-23 delivered the opposite: the Auckland Anniversary floods and Cyclone Gabrielle.
This year, Brandolino said the middle and latter part of spring should be more settled, while the New Year may bring new challenges.
"As we move past the Christmas and New Year period… we could see the north becoming more active.
"That could mean better chances for particularly heavy rainfall events, as we work our way through January and especially February.
"Whereas the South Island, especially the lower South Island and the western South Island, Otago, Fiordland, west coast of the South Island, they may see a continuation of dry weather, which sounds pretty good right about now.
"But if dry weather becomes, I guess, persistent and extended and prolonged, that can create things like drought… that's also something worth watching."
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