Winter or summer? Why there’s more than spring behind NZ’s wild temperature swings

A meteorologist says El Nino is adding to the wild temperature swings that've been observed across New Zealand this spring. Photo / 123rf

If the wild swings we’ve seen lately have left you unsure whether to break out the winter duvets or summer sheets, you won’t be alone: a meteorologist says dramatic weather shifts have been observed up and down the country. And this year, there’s more than just spring to blame. Science reporter Jamie Morton explains.

Has this spring been as much of a weather rollercoaster as it’s felt like?

Indeed – and there’s no shortage of crazy numbers to prove it.

“We’ve seen temperatures that are as much as 10C to 12C above average, followed by temperatures sitting 5C below average,” National Institute of Water and Atmospheric Research (Niwa) meteorologist Ben Noll said.

“A good example of this happened at Mt Cook on Thursday, where the maximum reached nearly 25C – making for the fourth-warmest October temperature on record there.

“Ahead of this weekend, however, we were forecasting a maximum of 8C – that’s a swing of 16C or 17C in less than 48 hours.”

Further north, Wairoa registered a summery high of nearly 29.6C on September 21 – but a southerly change saw the maximum crash below 15C just a few days later.

Last Monday, Auckland saw a high of 18.4C and a comfortable overnight low of 13.6C: on Wednesday night, those overnight temperatures plunged to 5.2C.

With these swings in temperature have come seesaw-like shifts between balmy sunshine and rainy, bitterly cold southerly flows that’ve made it feel like we’re back in the throes of winter - even amid what was our country’s warmest start to spring on the books.

So what are we expecting for the rest of the month?

More of the same, unfortunately.

“We probably have another three or four dramatic swings coming in just the next three to four weeks – and the first week back to school is looking a bit more like August or September temperatures, rather than what we’d usually get in October,” Noll said.

“So, we’re in for a bit of a dip. But then we poke back out of it around next weekend, when we’re expecting another flow of air from Queensland.”

Straight after that, however, forecasters are picking another week of cool temperatures.

“We see this up-and-down pattern continuing right through the rest of October, and I wouldn’t be surprised if we see another spike in warm weather right at the end of the month.”

If anyone was surprised, this set-up was just what Niwa picked in its recently issued seasonal climate outlook, which picked “little middle ground” between spells of unseasonable warmth – courtesy of Australian air masses – and bracing winds blown up from the Southern Ocean.

This is just typical spring variability, right?

Yes and no.

Noll explained the changeable weather we typically see in spring owed to the shifting of the jet stream.

“As we go into spring, we usually see the polar jet stream start to weaken off a little bit, which brings in more of that warmer air coming in from the north,” he said.

“But even so, over this period, the polar jet can still come up into our region and clash with the warmer air sitting off to the north.

“This usually makes for a kind of shoulder season where the angle of the sun is increasing in the sky, and while we can have those days of warmer, almost subtropical days, we’re still exposed to that wintry air from the south.”

As well, no Kiwi spring is without its blustery winds.

Each year following the spring equinox – which landed this year on September 23 – we notice the days grow longer, while extra warmth reaches the Southern Ocean.

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

This period of “equinoctial gales” normally reached its peak in October and November: only this spring, those westerly winds been particularly wild.

A Wellingtonian battles through the gale-force wind and rain this month. A meteorologist says dramatic weather shifts have been observed up and down the country this year - and there’s more than just spring to blame. Photo / Mark Mitchell

Noll pointed to the key culprit, and one also partly to blame for those extreme temperature swings: El Nino.

While the climate driver didn’t fully make its arrival known in New Zealand until those scorching days on the East Coast at the back end of last month, El Nino-like patterns have been influencing our weather for months.

That’s come with the way the pattern has been configuring high pressure to our north and low pressure to our south – in effect setting up two opposing cogs of a wheel driving constant westerlies on to the country.

El Nino also tended to coincide with negative phases of an indicator of storminess in the Southern Ocean – something called the Southern Annular Mode.

That meant more cold flows making their way up here – as was seen in New Zealand’s coldest August in seven years - yet El Nino’s arrival and those warm westerlies from Australia also made for the country’s warmest September on record.

“Now, we see El Nino adding an extra layer to the variability we see in spring, as is another climate driver, which is a positive phase of the Indian Ocean Dipole,” Noll said.

“So, if you were to attribute the swings we’ve seen over the past month only to spring, you’d barely be scratching the surface.”

Is everywhere in New Zealand exposed to these big swings?

While there’s no part of the country that hasn’t seen conditions yo-yo between winter and summer over the past month, some regions had been less exposed to the extremes than others.

“Going forward, I suspect those regions least affected by them will be the northeastern quadrant of the country,” Noll said.

“So, Northland, Auckland, Coromandel, Bay of Plenty, Hawke’s Bay and Gisborne are probably the most sheltered, even though they will see their fair share of swings over coming months.

“These regions happen to be closest to a subtropical ridge of high pressure that’s expected to be anchored just north of New Zealand over the next while.”

For the rest of the year, at least, Niwa expected temperatures in the east of both islands to be above average – largely owing to that El Nino pressure pattern delivering drier, warmer, westerly flows to those regions – with the rest of New Zealand equally likely to see near or above average temperatures.

Rainfall, too, was likely to be below normal in the north and east of the North Island over the next three months – but above normal in the west of the South Island, making for the opposite of what New Zealand experienced over the last three years of La Nina.

Will our summer weather be just as changeable?

“That’s a good question – and that level of detail will become clearer as we get closer to summer,” Noll said.

“But, in saying that, there’s now a broad indication that this southwesterly pattern we’ve been seeing through early and mid-spring will become more west to northwesterly over time.”

That should prevent, or at least reduce, the frequency of those more dramatic swings, he said.

“At the same time, what we’re experiencing now is going to give us some clues as to what summer will bring – and that likely could include more variability.”