Still waiting for spring to be sprung?
You can blame a weird event that opened Antarctica's freezer door upon New Zealand and effectively put the chill on September – with more cold blasts to come this month.
Since late August, meteorologists have been watching something called a stratospheric warming event, or SSW.
They're remarkably rare in our part of the world – just two have been recorded since the 1950s – and this event likely would have proven the largest ever seen in the Southern Hemisphere.
The troll from the pole
So how do they happen?
Every winter, westerly winds – often up to 200km/h – develop in the stratosphere high above the South Pole and circle the polar region.
These winds develop as a result of the difference in temperature over the pole, where there is no sunlight, and the Southern Ocean, where the sun still shines.
As the sun shifts southward during spring, the polar region starts to warm.
This warming causes the stratospheric vortex and associated westerly winds to gradually weaken within a few months.
However, in some years, like this one, this breakdown can happen much faster than usual.
Waves of air from the lower atmosphere warm the stratosphere above the South Pole, and weaken or "mix" the high-speed westerly winds.
Very rarely, if the waves are strong enough they can rapidly break down the polar vortex, actually reversing the direction of the winds so they become easterly.
This is the technical definition of "sudden stratospheric warming".
Although we have seen plenty of weak or moderate variations in the polar vortex over the past 60 years, the only other true sudden stratospheric warming event in the Southern Hemisphere was in September 2002.
Usually, the swirling, freezing air mass that is the polar vortex is effective at keeping harsh, wintry conditions locked up around Antarctica.
But an SSW can weaken it or displace it into the stratosphere, sending cold masses filtering down on to the tropospheric polar vortex, where it could then influence our own weather patterns.
In contrast, in the Northern Hemisphere, SSWs occur every other year or so during late winter, largely because of stronger and more variable tropospheric wave activity.
Up there, one recent SSW, dubbed the "beast from the east", unleashed a series of cold blasts from the North Pole to Western Europe and the UK, along with the east coast of the United States.
At its peak, our latest SSW pushed temperatures in the stratosphere above Antarctica to 70C above average – to the point where they were approaching 0C.
Incredibly, this meant it might have been warmer 30km above the South Pole than atop a mountain in the Southern Alps.
As at the end of the month, stratospheric temperatures were sitting at around -25C – still a solid 20C to 25C above what's typical in that layer of the atmosphere.
A "remarkable" event
"It's been quite a remarkable event, and it has definitely made some noise in the weather space here and internationally," Niwa meteorologist Ben Noll said.
While the impacts hadn't been as direct as the last big SSW that played with New Zealand's weather back in the spring of 2002, Noll said the indirect effects had still translated to some of the cooler spells felt around the South Island last month.
It was one of the coldest Septembers in recent years.
More significantly, coastal waters around New Zealand came in at below average for the first time since January 2017 – closing off a balmy 32-month stretch that featured two marine heatwaves.
"Ocean temperatures, across the board, have had negative anomalies and that's been seriously impressive," Noll said.
"People have been commenting that, hasn't this all been typical for spring? Well, spring of course does come with its ups and downs, but if you look at how we have compared this year against the 30-year baseline, it hasn't been typical at all."
But scientists say there has been one positive that's come with phenomenon – a reduction of the spring Antarctic ozone hole, potentially to its smallest size in decades.
The #ozone hole over #Antarctica is unusually small, one of the smallest seen in the last 30 years. The reason for this is sudden stratospheric warming, which occurs regularly in the Northern Hemisphere but is extremely rare in the Southern Hemisphere. https://t.co/Au8nL8v4Cl pic.twitter.com/2QguWQLKKh— Ilmatieteen laitos (@IlmaTiede) September 23, 2019
This had happened for two reasons: first, the rapid rise of temperatures in the upper atmosphere meant the super cold polar stratospheric ice clouds, vital for the chemical process that destroyed ozone, were prevented from forming.
Secondly, the disrupted winds carried more ozone-rich air from the tropics to the polar region, helping repair the ozone hole.
And while the SSW was winding down, it wasn't finished with us yet.
More cold to come
As spring gradually merged into summer each year, meteorologists observed a natural climatological occurrence called the "final warming" – or the last stratospheric warming of the season, making way for summer.
Only this year, Noll said the SSW would be "super-imposed" on to this yearly event.
"So I'm not sure that we will return to normal, quote unquote, this year."
In the short-term, at least, we could expect chilly bouts like that which has sent the mercury plunging over the past few days.
"We've got another snap coming around Saturday and Sunday, and then there might be more around October 10," Noll said.
"So it seems as though we are going to have more and more of these winter-like blasts as we go through the month of October."
And on the immediate horizon, MetService has issued heavy snow warnings for the South Island today, with the white stuff expected down as low as 200m in some areas – and accumulating to depths of 15cm to 20cm at certain places above 400m.
"The driver of the snow is a front that is moving up Aotearoa today which will see cold south-westerlies affecting the South Island today and the lower North Island tomorrow," MetService meteorologist Lewis Ferris said.
Areas in Canterbury below 200m could also see plenty of rain overnight.
It remained to be seen whether the past month's cooler trend would prove enough to knock back 2019's hitherto hot run in the record books.
The January to August period had come in at 0.96C above average – second only to the corresponding stretch in 2016, which was 1.10C above average.
At the same time, forecasters had been counting a run of more than 30 months of above average temperatures – something partly put down to the background warming of climate change.