‘Sudden Stratospheric Warming’ event could bring icy weather to New Zealand
© NIWANIWA says a ‘Sudden Stratospheric Warming’ event could bring icy weather to New Zealand next month.
It's called a sudden stratospheric warming event - and, unlike the name might suggest, the rare phenomenon could spell a burst of bitterly cold weather for New Zealand over coming weeks.

A sudden stratospheric warming (SSW) event kicks off when the temperature of the stratosphere - that's 30km to 50km above ground - over the South Pole climbs by more than 25C. Meteorologists think it's likely this is about to happen next week.

Importantly, it has the potential to mess with a ring of stormy and freezing weather that encircles Antarctica, which is at its strongest at this time of year - and which we know better as the polar vortex that's been dubbed the "beast from the east" - threatening to send a series of cold blasts from the North Pole to Western Europe and the UK, along with the east coast of the United States.

While this swirling, freezing air mass is usually effective at keeping harsh, wintry conditions locked up close to the pole, an SSW can help weaken or displace it in the stratosphere.

This sends these cold masses filtering down on to the tropospheric polar vortex, potentially influencing our own weather patterns.

Niwa meteorologist Ben Noll said that, during a major SSW, the winds in the stratosphere reversed from westerly to easterly.

"For up to about a month after the SSW, polar air masses, known as streamers, can break off from the weakened vortex and move towards New Zealand," he said.

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

How rare is it?

For New Zealand, extremely. There have been only two in New Zealand in recorded times, or since the late 1950s - one in September 2002 and the other in September 2010.

After the SSW in 2002, New Zealand experienced its coldest October in 20 years with below average temperatures covering much of the country and frequent ground frosts.

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

Plenty of unsettled weather is signalled for New Zealand in September and October with frequent chilly spells.

Noll said that next week the weather will feature 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.

The SSW was likely to peak between next Thursday, August 29 and Monday, September 2.

"Again, there is no immediate effect, as it can take between weeks to a month or two for these flow-on effects to play out.

"And also again, it also doesn't mean that the weather that comes from the event will land in New Zealand. It could go to South America, or it could go to Australia.

"Or it could just go over the ocean region in the Southern Hemisphere, which makes up about 80 per cent of it. We are just a tiny landmass within it, so it's tough to pin-point."

Do they happen in the north?

Yes - SSWs are actually a lot more common in the Northern Hemisphere.

That's partly because of differences in the distributions of mountains, land, and sea, which help to drive temperature contrasts.

These contrasts cause the formation of major planetary waves that help set SSWs into motion.

The Southern Hemisphere, meanwhile, is characterised by a cold Antarctic continent surrounded by relatively warm seas.

This leads to a more stable circulation around Antarctica than in the Arctic where a relatively warm Arctic Ocean is surrounded by cold continents in winter.

Noll said people might recall a major SSW - dubbed "the beast from the east" - that sent a series of cold blasts from the North Pole to Western Europe and the UK, along with the east coast of the United States.

"That was a good example of what a stratospheric warming event can bring."