New Zealand is in for a windy time over coming days and weeks – and this year, it won’t just be the blustery arrival of spring to blame for persistent westerlies.

A forecaster says the annual spring equinox is coming in tandem with an El Niño climate pattern that could be formally declared within weeks, and which is already driving its signature westerly flows on to New Zealand.

WeatherWatch’s Philip Duncan said there was potential for strong westerly quarter winds over the country towards the end of the week, when gale or severe gale northwesterlies are expected about the eastern South Island and lower eastern North Island.

Duncan said those winds could strengthen over the weekend as another series of fronts arrived, making for especially choppy conditions at sea.

 “The pattern we’ll be seeing will be kind of off-and-on for the next couple of weeks, with a surge at the end of this week, followed by a bigger one around Sunday,” he said.

“I think we can expect severe weather warnings from MetService, and power lines and a few trees down in some parts across the country at times over the coming week or two.”

MetService has so far issued severe weather warnings and watches for Thursday and Friday, with a moderate risk of severe gale northwesterlies about Marlborough, Wellington, Wairarapa, Tararua and Central Hawke’s Bay on Friday morning.

“In the east of the main divide, anyone south of Napier is under a strong wind watch or warning during tomorrow,” MetService meteorologist Andrew James said.

The agency currently had “moderate” confidence of another period of severe northwest gales about northern Marlborough, Wellington, Wairarapa, Tararua and Central Hawke’s Bay on Sunday and Monday morning.

There was low confidence of these winds reaching gale-strength in exposed parts of southern Marlborough, Canterbury, Otago, Southland and southern Fiordland later Saturday and into Sunday or early Monday.

That was also the case for potential severe gale west or southwest winds about southern Fiordland, Southland and Otago later Sunday and early Monday – and for other North Island areas from southern Northland to Hastings District and Kapiti, and near Golden Bay.

Duncan didn’t view these westerly surges as extreme events, “as they’ll be so fast-moving that they’re gone as quickly as they move in each day”.

But, he added, they’d mark a noticeable change from the set-up Kiwis have seen under the last three years of La Niña.

“I think there’s been a lack of wind in spring, at least for the top of the country. Last year, we had a lot more low pressure that helped remove a lot of the windy westerlies,” he said.

“This year has more of a classic setup – but I think the El Niño component to it could keep it going for longer.”

Each year following the Spring Equinox – landing this month on September 23 – we notice the days grow longer, while extra warmth reaches the Southern Ocean.

This helps to activate the generally westerly quarter winds found there, and typically causes them to expand on to New Zealand, producing fronts and squally winds.

This period of “equinoctial gales” normally reaches its peak in October and November.

At the same time, the typical pressure set-up of El Niño - placing high pressure to the north of New Zealand and low pressure to the south – acts likes two cogs of a wheel that drives westerly flows onto the country.

For places like the West Coast of the South Island, that can mean frequent fronts and rain bands.

“Places like New Plymouth or Auckland can also end up getting weather that’s cloudier and windier over summer, with average temperatures,” Duncan said.

But, given the air parcels within these winds dry out as they move from west to east, long periods of warmth and little rain can become the theme for regions like Canterbury, eastern Marlborough, Hawke’s Bay and Gisborne.

Niwa anticipates a declaration of El Niño may come later this month, and there are early signs this system may be one of the strongest seen in decades.

Duncan said the modelling WeatherWatch used painted a picture of unsettled conditions over much of the country in spring, but with a drying trend on the East Coast.

“But as we go into summer, this dryness may become far more widespread across the North Island and the upper south, in places like Nelson, Marlborough and Canterbury,” he said.

“We’ve been talking about El Niño all year, and we’ve basically got this weather pattern now – but it gets more set-in during summer, and it could be winter before it goes away.”

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald reporting team in 2011 and has spent the last decade writing about everything from conservation and cosmology to climate change and Covid-19.