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Coming 'La Nina-like' pulse could prove big rainmaker for NZ

Jamie Morton,
Publish Date
Tue, 5 Mar 2024, 8:48PM

Coming 'La Nina-like' pulse could prove big rainmaker for NZ

Jamie Morton,
Publish Date
Tue, 5 Mar 2024, 8:48PM

After a chilly start to autumn, forecasters say a “La Nina-like” pulse of subtropical moisture in the Pacific could bring a big dose of rain to New Zealand next week.

Early modelling points to the potential for a subtropical low above New Zealand, to the east of Australia, early next week – around the time a globe-circling pulse of rain and thunderstorms is due to roll through the region.

Whether the system translated into stormy weather here depended on how well a ridge of high pressure forecast to move in this week held against it, MetService meteorologist Ngaire Wotherspoon said.

“It’s definitely something to keep an eye on, but it’s very hard to say anything at this stage about how it will play out.

“If it does get over the country, then it could be a potential rainmaker for the North Island. We’ll see some warmer temperatures, an uptick in thunderstorm activity, that sort of thing.”

There were also heightened odds for a tropical cyclone to form in the west Pacific around the middle of the month, Niwa meteorologist Ben Noll said.

“You could probably peg that [probability] at 50 per cent, give or take, at this point – but as for any impacts of it to New Zealand, the odds on that are obviously significantly lower.”

The 2023-24 season has so far brought just one cyclone system that’s affected New Zealand: Lola, which formed last October ahead of the November-to-April season’s traditional start.

Noll said the potential low coincided with what’s called the Madden Julian Oscillation (MJO) passing through the west Pacific and bringing more active weather.

While this happened every one to two months, next week’s event could bring a major departure from the conditions New Zealand has been experiencing under a now-fading El Nino climate pattern.

“This is going to be a La Nina-like pulse of climate variability that goes against the grain of the pre-existing El Nino,” Noll said.

“That’s interesting, because it will generate weather in the wider region not typically associated with El Nino – and it may very well be the first domino to fall, in relation to an oceanic trend back toward La Nina,” he said.

“In any case, it’s going to generate an easterly wind-burst event in the equatorial Pacific that will move us out of El Nino.”

New Zealand has spent most of this decade in a La Nina state, with three back-to-back years locked in the pattern culminating in the North Island’s extreme summer of 2022-23.

In the shorter term, Noll said there was a possibility of active weather in the Southern Ocean and Tasman Sea at the same time the tropical low could be deepening next week.

“So, any feature that develops to our north and siphons moisture southward will be well worth watching, because it might be occurring with other pressure systems in the region.”

For particularly parched parts of New Zealand – notably regions between Wairarapa and Marlborough – another big rainfall event could be welcome.

“That mid-month potential for activity is something that could alleviate some of those ongoing issues for the driest areas.”

Niwa’s just-issued summer summary showed the season was the ninth warmest on record in New Zealand, with more north-westerly winds and marine heatwave conditions helping dry out some regions.

Its outlook for autumn forecast above average temperatures in most regions, along with more northwesterly winds and below-average rainfall in the upper North Island.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

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