Is wild spring weather the new normal?

Explainer - Much of New Zealand is still cleaning up from heavy winds, rains and even snow over the past week.

States of emergency were declared around the country and many properties damaged in the series of wind-driven storms.

Does this mean spring is just going to be wilder and windier in the future? It depends, but warming temperatures driven by climate change do make a difference.

Climate scientist and 2024 New Zealander of the Year Dr Jim Salinger told RNZ the wind set several records.

"It's pretty unusual - you just have to look at some of the wind gusts we've had," he told Afternoons' Jesse Mulligan.

"We see areas like southwest cape having a gust of 190km/h - That's the strongest wind gust ever recorded at that location."

Why was that wind so strong, anyway?

It's all about pressure differences. Very basically, high pressure systems - where air is descending - usually bring settled weather while low-pressure - where air is rising - tends to cool and cause unsettled and wetter weather.

"Spring is always an unsettled time of year, of course there is typically some nice big areas of high pressure that visit, this coming weekend for example," said MetService meteorologist Lewis Ferris.

"But it's the time of year when the thermal gradient between the equator and Antarctica is at its largest. When there's a large thermal gradient there's going to be wind. This wind does a great job at transporting weather systems around the mid-latitudes."

Winds were supercharged by clashes between warm air masses from Australia and colder air spreading north from Antarctica, with little New Zealand plonked right in the middle.

The warming in Antarctica in September was what's called a sudden stratospheric warming event, which were considered relatively rare in the Southern Hemisphere until recently.

"That rapid warming very likely played a significant role as to why we've experienced what we've experienced," Earth Sciences New Zealand meteorologist Chris Brandolino said.

He said that sudden warming made forecasts more challenging, too.

Ferris said that air pressure levels were extraordinary last week.

"Last week we saw the pressure difference between Auckland and Invercargill exceed 50hPa (hectopascals, a measurement unit of pressure), there's only been one event in the last 30 years where it exceeded 40hPa!

"So in a nutshell, the last couple weeks haven't been normal, even by spring's standards."

Ferris said that there had been more low pressure than normal to New Zealand's southwest, as seen in data from the US National Oceanic and Atmospheric Administration.

"This does a simplified job of explaining the unsettled west/northwesterly storms we've seen this month."

The jetstream - fast flowing air currents in the upper atmosphere - has also been a factor.

"The westerly component of the jetstream has been much stronger than normal over the Tasman Sea - this goes some way to explaining why it's been so windy and why so many low pressure systems approached from the southwest of us."

So is spring just going to be like this now?

"The short answer is I would say there's a bit more uncertainty than usual," Brandolino said.

Earth Sciences New Zealand are looking at what long range forecasts for the next three months or so may hold. An event like the sudden stratospheric warming is considered a climate driver, he said.

"So climate drivers, they're very helpful for identifying how our weather may shape up when you take a medium to especially longer-term view, so when we're trying to understand what the next two, three, four months may be like in terms of themes and concepts.

"We're trying to say right, well, who's going to be at the steering wheel of Mother Nature's car? Because if you know who's driving the car you kind of have an idea of which direction to go. It's not perfect but it's certainly very helpful."

New Zealand is also heading into a La Niña cycle, the climate pattern driven by cooler-than-usual ocean temperatures along the equator. It'll play a big part in how spring shakes down.

"Every La Niña is different. We know the average outcome, but no event is average."

High pressure tends to favour areas in the east and southeast of the country and low pressure up in the seas near New Caledonia and Queensland.

Current indications are a drier season for eastern parts of the country, while others like Auckland, Coromandel and Northland may be wetter.

"That sort of set-up is favourable to bring warm moist air from the subtropics and tropics, moisture-laden air which obviously would be fuel for heavy rain but also things like tropical lows or ex-tropical cyclones so that will be something we'll have to watch."

Is global warming playing a part in these storms?

Temperatures have been increasing worldwide, breaking 1.5C above pre-industrial levels in 2024.

Salinger said October has shown that happening in this part of the world.

"Hastings had its highest October temperature ever at 32 degrees and this relates back to what's happening in Australia where Birdsville in Western Queensland broke its October record for all of eastern Australia with 46 degrees centigrade."

Much of Aotearoa ran between 1 to 2 degrees above average for October, he said.

The rising temperatures can play havoc with broader weather patterns, Salinger said.

"If we look at climate models, they show that the westerlies in winter and spring strengthened and this is simply because when you have Australia being very warm and the Antarctic being very cold, you're actually increasing the temperature difference between the two and when that happens that increases the westerly winds in the Southern Ocean, which is exactly what's happened.

"All the models are showing you get more increased westerly winds in winter and spring and that's what we see this year. You might say it's typical with what's happening that we'd expect in a warmer world."

Despite snow across parts of Southland "for 75 percent of the country it's been a warm month," Salinger said.

Storms can also get more destructive in a warming climate, Salinger said, pointing to Hurricane Melissa which hit Jamaica as a powerful Category 5 storm this week.

"If we look at hurricanes - tropical cyclones are the same beast in our part of the world - simply the global warming scenarios show that you might have the same number or slightly less but when they do occur, they're stronger in intensity and this is what we're seeing with Melissa."

Ferris said it's hard to say if spring weather will always be so volatile as it's been in October.

"Broadly, just like weather varies within a season, seasons vary between years. Will we see the Auckland-Invercargill pressure difference exceed 50hPa again next spring? Statistically it's very unlikely and I sure hope we don't!"

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