If you're looking to blame anything for the icy blast about to disturb what's been an otherwise pleasantly mild winter, point the finger at negative SAM.
And no, that's not a gloomy individual named Sam who somehow holds sway over our weather, but an indicator that's long helped climate scientists predict the patterns prevailing over the country from one week to the next.
MetService has just issued warnings for heavy snow in places in the South Island, with the white stuff accumulating at 400m in some parts.
"The driver of the snow is a front that is moving up Aotearoa today which will see cold southwesterlies affecting the South Island today and the lower North Island tomorrow," MetService meteorologist Lewis Ferris said.
Areas in Canterbury below 200m could see a lot of rain overnight into Wednesday as the cold front moved through.
A heavy rain watch was in force between 3am and 9am on Wednesday and people closer to the coast could be waking up to gale southerlies.
The system moving through the country today and tomorrow was the first of a few, with more unsettled periods arriving later this week and then again over the weekend.
Ferris said it would feel a particularly sharp change for those in the south.
"It's definitely a change from what we've been observing in the last month," he said.
"The area of Canterbury and Otago had two weeks of pretty consistent rain, and then last week it was basically pristine all week – now they've got this cold southerly push, with low snow accumulation."
Check out that cold southerly headed toward New Zealand! 🌬️
The open cell cumulus clouds out over the Tasman mark the leading edge of the chilly air. pic.twitter.com/0S7jMlMKnT
This switch was where the negative SAM – and in this case, it's extremely negative - came in.
THE HAND OF SAM
So what is the SAM?
The SAM, or Southern Annular Mode, can generally be described as a ring of climate variability that encircles the South Pole, but stretches far out to the latitudes of New Zealand.
First identified in the 1970s, it involves alternating changes in windiness and storm activity between the middle latitudes, where New Zealand lies, and higher latitudes, over the southern oceans and Antarctic sea ice zone.
In its positive phase, the SAM is associated with relatively light winds and more settled weather over New Zealand latitudes, together with enhanced westerly winds over the southern oceans.
Source / National Weather Service Climate Prediction Centre
It was a predominantly positive SAM that had contributed toward our past two unusually balmy summers, as well as two marine heatwaves that dramatically warmed up the oceans surrounding New Zealand.
In negative phase, however, the westerlies increase over New Zealand, with more unsettled weather, while windiness and storm activity eased over the southern oceans.
"The Southern Annular Mode is in the driver's seat here, there's no question about that – and it's doing everything we expect it to do in the negative phase, bring that cold, stormy, unsettled weather with rain and snow," MetService meteorologist Georgina Griffiths said.
"It's really the Southern Ocean showing us its nasty side, and here in New Zealand, we do tend to pay well for the settled weather we get."
The Southern Annular Mode is forecast to dip to levels that we haven't experienced in some time.
This climate metric, when negative, helps describe the storminess in the New Zealand region 📉
On a week-to-week basis, the SAM generally flipped between states – causing either windier or calmer weather over New Zealand latitudes – in an unpredictable way, and apparently at random.
Still, Griffiths said this was the time of the year when meteorologists expected the SAM to be flipping anyway, resulting in an overall more unsettled climate picture – as was the traditional norm in winter.
Though these phase changes of the SAM could not be predicted more than a few days in advance, once changed, the phases tended to persist for several weeks.
"I'd give this [negative phase] two weeks from today," she said.
"But to me, this week is looking probably the roughest, so batten down the hatches. This one does have a sting in its tail."
Griffiths added that, despite the negative change, the overall trend over recent years had been toward more periods of the positive phase, bringing strong westerlies over the southern oceans and lighter winds over the middle latitudes.
This trend appeared to be related to the Antarctic ozone hole, and the influence of the stratosphere on the weather lower down.
A BATTLE OF EFFECTS
Victoria University climate scientist Professor James Renwick said that, over the next 50 years, this hole was expected to fill in, which would warm up the atmosphere over the South Pole.
"Left to its own devices, the ozone hole recovering would give us a lot more of what we are about to experience – more negative SAM."
"Assuming that we keep putting greenhouse gases into the atmosphere, and keep warming things up, that will be delivering the opposite," Renwick.
"So it's a big question for the Southern Hemisphere and the climate over New Zealand: which of those effects is going to win?
"The answer we have from the literature out at the moment is that they are going to cancel each other out, and we are not going to see much of a trend in the SAM over the next few decades in the spring and summer.
"In the winter, however, we'd likely still see a weak positive trend overall, which means more sunshine, more lighter winds, less rain. And while you might think that sounds nice, farmers and hydroelectric companies are probably not so impressed."
Niwa climate scientist Dr Sam Dean similarly didn't expect climate change to have a dramatic effect on the SAM.
"That variability of our weather – the changes between wet and dry spells, on a two to four week basis – we don't expect it to change under climate change."
Meanwhile, Niwa meteorologist Ben Noll said it remained to be seen how the cold snap might nudge this month's place in the record books for unusual warmth.
"I'd say it's still going to finish in the top few, but whether that's the top three or top five depends on how much cold we get today and tomorrow."
