Explained: The weird polar heatwave putting extra chill in New Zealand’s winter

Scientists have been watching temperatures climb rapidly over Antarctica in a rare phenomenon that could spell a cold early August for New Zealand. Image / Niwa

Rare patterns unfolding high over the South Pole are putting an extra chill in New Zealand’s mid-winter weather. How does that all work? Jamie Morton explains.

What’s going on down at the South Pole?

A lot of strange things that scientists are watching closely.

In July, temperatures over the East Antarctica coastline suddenly climbed by an incredible 50C.

Scientists have been tracking the formation of what’s called a “sudden stratospheric warming” (SSW) event - a rare phenomenon that has only been observed three times in the satellite era in the Southern Hemisphere.

This SSW happened when temperatures in the polar stratosphere – the layer of atmosphere 30km to 50km above ground – warmed by more than 25C in a week, while wind speeds dramatically decreased, as is predicted to occur soon.

“What’s going on is certainly noteworthy, as we don’t see a lot of these polar stratospheric disturbances down here in the Southern Hemisphere,” Niwa meteorologist Ben Noll said.

Also in the mix was an unprecedented “wave-2″ event, where the polar vortex split in two, with multiple centres of pressure.

This was expected to drive the SSW event and slow down winds in the stratosphere.

It was also forcing the Southern Annular Mode (SAM) – a ring of climate variability that encircles the pole and extends out to the latitudes of New Zealand – down to negative values not recorded in July in over a decade.

As we’ve seen in the past, lower SAM values typically come with stormy and chilly conditions here.

So what does that all mean for us?

Strong SSW events in the past have typically spelt bitterly cold weather in New Zealand: one in 2002 coincided with the coolest October in 20 years.

Even if this event proved minor, Noll says we are still likely to feel its icy influence over the first part of August.

Rare disturbances in the stratosphere have flow-on effects for the troposphere – the lowest layer of the atmosphere that contains most of our weather.

“When you have the stratosphere coupling with the troposphere – almost like the two are exchanging text messages – then you can get some unusual weather patterns.”

Particularly, the warmth is disrupting the polar jet stream: the fast-flowing band of stormy and freezing weather that swirls around Antarctica.

“We can start to see it buckle, become more wavy and extend to places it normally doesn’t,” Noll said.

“That can transport very mild conditions into Antarctica, as is forecast to happen, while displacing some of its typically cold weather into the mid-latitudes – including Australia and New Zealand.”

The south has already been hit with snow this week – the result of a low-pressure system colliding with freezing air blowing up from the Southern Ocean – and more cold snaps loom over the next seven to 10 days.

“Beyond that, it does look like the core of some of that unusually cold air might shift toward South America,” Noll said.

“But for weather right across the Southern Hemisphere, it does look to be the main show in town at the moment.”

What are the other pieces of the picture?

Commenting on X (formerly Twitter), Dr Amy Butler, of the US National Oceanic and Atmospheric Administration, said the rare polar pattern’s downstream impacts were still unclear.

It might “greatly reduce” the chances of a large Antarctic ozone hole - and hot and dry conditions in Australia often followed years when the polar vortex was weak.

Meanwhile, what will be our fourth La Nina climate pattern within a decade continues to develop in ocean waters near the equator - setting up an interesting wildcard amid the natural turbulence of spring weather.

“After watching these patterns that have been connected to the pole, we may soon find ourselves starting to look to the north for some of our climate action,” Noll said.

“So, we might be in for a bit of a bumpy ride as we watch these different climate drivers jockey for dominance.”

Niwa’s outlook for the next three months is due out later this week.

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.