Polar blast: The science behind NZ’s deep freeze
It’s a bit chilly out there, right?
New Zealand is in the grip of a polar blast, delivered straight from the Antarctic ice shelf, that’s brought snow to low levels across the country, closed highways, and grounded flights.
But, rather than being any rare phenomenon, a Niwa meteorologist says this system is just what a traditional New Zealand winter should feel like from time-to-time.
It just happened to feel all the more cold because the country is tracking toward its warmest June ever recorded – something that’s owed to a warm blob of ocean above us, a locked-up pole, and the background trend of climate change.
“Just because it’s been a warm June, the perception is that this is an epic, polar blast,” Niwa Weather forecaster Ben Noll said.
“Yes, it’s colder than average in some places and it’s snowing to low levels, but that’s actually something we’d expect to see a couple of times through a winter season.
“What’s really abnormal is what’s happening in the last three weeks, where temperatures have been so far above average.”
Another factor was wind chill.
While much of New Zealand was sitting at temperatures in the mid-single digits today, wind chill made Gisborne’s lunchtime 5C feel like 2C, Dunedin’s 6C feel like 2C, and Wellington’s 7C and Christchurch’s 6C feel like 1C.
But some of today’s weather truly was unusual – namely snow falling in places like Wellington, Bay of Plenty and, for the first time in a decade, the Taranaki town of Stratford.
What brought the cold?
Noll said this week’s snow-maker was a “classic pattern” for cold in New Zealand.
That was a large high-pressure system to the west of Tasmania and Eastern Australia, and a deep, swirling low to the east of the Chatham Islands.
Noll said these could be thought of as two large “cogs” of air pressure, which flowed anti-clockwise around the high, and clockwise around the low.
Squeezed between these two “cogs”, isobars – or lines on meteorological charts connecting points of equal atmospheric pressure – appeared like train tracks tidily stretching across the Southern Ocean between New Zealand and Antarctica.
“This means your airflow is being dredged straight up from the deep in the Southern Ocean,” he said.
“In this case, these air parcels were straight over the ice shelf. Although they’ve been modified as they’ve travelled thousands of kilometres across the Southern Ocean, that gives you a picture of why we feel the way we do right now.”
The tidy alignment of air pressure over the oceans below us had also helped waves sweeping up to gather more height and energy as they crossed the sea – and the effect of that is already being shown across Wellington’s south coast today.
Noll said there also happened to be a much wider picture at play.
“When you’re talking about the seedlings of a polar blast, how far do you look?”
He said it was a possible link to a phenomenon playing out on the other side of the globe, in the Atlantic Ocean basin – something climatologists have dubbed “Atlantic Nino”.
Like the better-known El Nino, that was characterised by warmer-than-average sea surface temperatures in the eastern equatorial basin and weaker-than-average trade winds throughout the east-central equatorial Atlantic.
But it was different from El Nino in that it tended to peak around the time of New Zealand’s winter, rather than summer, and brought much smaller, and more localised, climate impacts.
What did that have to do with us?
“Although it’s fleeting, it seems to be associated with what we’re experiencing here in New Zealand, which is a pulse of southwesterly-to-southerly winds, and which is what we’d actually expect to see quite a bit of during a typical El Nino winter here,” he said.
“So there’s probably an element of international climate forcing at play.”
That El Nino-like set-up may have had an indirect effect on New Zealand exposure to Antarctica’s polar vortex, a large area of low pressure and cold air that sweeps around the bottom of the globe.
“This vortex has been stronger than normal this year, and been pretty effective at keeping that cold locked up,” he said.
“It just so happens that, at a time this piece of cold air has been dislodged and sent our way, the bottom of South America, around Argentina, is looking comparatively pretty warm over the next week.”
High and dry
As a series of tropical-flavoured deluges have shown over the past few weeks, warmer air parcels often carry more water vapour.
“So it’s not surprising you’d expect a freezing air mass like this to be associated with less moisture and drier air,” Noll said.
“The fact this air is so dry illustrates again that this mass can be traced right back to the South Pole.”
Interestingly, that lack of moisture in the atmosphere was reaching right up the country, killing humidity in places where it’s ever-present.
“In Auckland tonight, the dew point, which is a measure of humidity, is expected to dip down to around zero, or maybe even below,” he said.
“We record dew point in Auckland every 10 minutes, and to have it go below zero is pretty atypical.
“While we know that the South Island can get very dry, especially across the interior, it’s notable that this dryness is going to be pushing as far as Northland.
“And as we go through to the end of this week, New Caledonia, Vanuatu and maybe even Fiji might feel this pulse of cold air. The train isn’t just stopping in Auckland, by any means.”
Once the polar blast blew through, however, Noll said Kiwis could expect a resumption of New Zealand’s unseasonably mild weather conditions.
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