Weather: NIWA warns rare 'sudden stratospheric warming' to blast NZ with icy 'streamers'

Winter is set to end with a biting cold snap, thanks to a freak atmospheric phenomenon that'll see temperatures over Antarctica rise more than 25C.

Sudden stratospheric warming (SSW) events in the south have only beenrecorded twice in modern history, weather agency NIWA says, and both times saw temperatures in New Zealand plunge.

And the wind is to blame. NIWA says a SSW happens when the usual westerly winds in the stratosphere - 30 to 50km above the ground - reverse direction, disrupting the stable status quo.

"In the Southern Hemisphere during winter, a ring of stormy and freezing weather encircles Antarctica," NIWA explains.

"Known as the polar vortex, it is usually very good at keeping harsh, wintry conditions locked up close to the pole. When a SSW occurs, it can help to weaken or displace the polar vortex in the stratosphere, which then filters down onto the tropospheric polar vortex and influences our weather patterns."

The weakened vortex can let off "polar air masses known as streamers" towards New Zealand for up to a month, said forecaster Ben Noll.

"It doesn't guarantee unusual or extreme weather, but it can happen."

NIWA map
The stratosphere will be warm. Down where we are, not so much. Photo credit: NIWA

SSWs more common in the northern hemisphere. That's because the Arctic is relatively warm and is surrounded by colder land, while the Antarctic environment is colder and less volatile.

"These events are rare in the Southern Hemisphere," said Noll. "There have only been two in New Zealand in recorded times: one in September 2002 and the other in September 2010."

An SSW in the Arctic in 2018 resulted in a cold wave dubbed the 'Beast from the East' across Europe, Ireland and the UK. Nearly 100 people lost their lives in the ensuing cold and chaos.

When an Antarctic SSW happened in 2002 it resulted in the coldest October in 20 years. Historic NIWA data shows it was on average between 2C and 3C colder than normal from Timaru in the south to Auckland in the north. Invercargill saw its second-highest rainfall on record, and hailstones the size of golf balls fell in Christchurch. 

"In 2010 - which is classed as minor event - a number of rainfall records were broken, with well-below normal sunshine and very cold temperatures in parts of the South Island," said NIWA.

Unlike many extreme weather events, Noll says the SSW can't be linked to climate change at this stage. 

"It's difficult to pinpoint one particular event that would set the expected SSW next week into motion... This is a natural phenomenon that occurs from time to time, albeit rarely in the southern hemisphere compared to the northern hemisphere. There's no direct link I can establish at this point."

Instead, the wind's reversal of direction is likely to be the result of a butterfly effect sequence of events. Noll said it might have been something as simple as temperature differences in the atmosphere thousands of kilometres away in the tropics that kickstarted it.

If it happens, it'll peak between Thursday, August 29 and Monday, September 2. Noll says to expect "cool conditions for the time of year and the potential for an active spell of weather late in the week bringing rain, snow, and wind".

"Even though it's discussing a warming in one part of the atmosphere, it doesn't mean that's what we're going to feel at the ground. People see the maps and it looks all red and orange and it's like, 'It's going to get really warm.' But that's actually the opposite of what will occur."