Explained: The ‘loopy’ reason behind our polar blast

October snow falls in Christchurch for first time in 53 years; polar blast sweeps through South Island, Wellington – cold temperatures in Auckland. Video / Supplied

What’s going on with the weather?

Temperatures in parts of the country this week crashed by 15C to 20C in a matter of hours this week – but the mid-spring polar snap is soon expected to rebound back to warmer temperatures within days.

This chilly turn, which has brought snow and ice from the deep south to the plains of central Taranaki, can be explained by a complex chain of events that originated not in Antarctica, but thousands of kilometres away in the Indian Ocean.

“If you go right back, you can actually trace the seedlings of this event back to the western part of the Indian Ocean,” Niwa meteorologist Ben Noll said.

An anomaly observed here led to the polar jet stream – that’s a band of fast-flowing, narrow air currents circulating high in the atmosphere – being pushed further south, toward Antarctica.

“It’s taken a very loopy pathway, all the way toward the South Pole, and then directly up into New Zealand,” he said.

“In order for this to happen, you have to have a very, very large area of high pressure south of Australia – or what we call anticyclonic wave-breaking – which basically forces the jet stream to move around.

“In this case, it had to travel right down to the ice sheets just to manoeuvre its way around this massive high, before finding its way back up to New Zealand – so it’s quite a sequence.”

Closer to home, Noll said it was possible warmer ocean temperatures in our coastal waters – sea surfaces around Otago and Southland have been 1C above average – may have played some part.

In gauging the overall strength of a very cold air mass, meteorologists often look about 1.5km above the Earth’s surface to get an indication.

“If that part of the atmosphere is extremely cold – as it has been in this case – and then it runs over relatively warmer seas, you then get instability in the atmosphere,” he explained.

“That instability manifests in these cellular areas of rain and snow that we’ve seen quite a bit of in Southland, Otago, eastern and coastal Canterbury, and even Wellington overnight.”

With Christchurch seeing snow in October for the first time in 53 years, Noll considered this mid-spring snap to be a “stand-out event” in New Zealand’s climatology for the month.

The last time the country saw an October of below average temperatures was back in 2009.

Still, Noll it was unlikely this event, dramatic as it was, would greatly skew this month’s overall average.

Some colder weather last month resulted in September proving the coolest month recorded, relative to average, since September 2021 – yet it still finished up more than 0.4C above normal.

For that matter, it’s been more than five years – or nearly 70 months – since any month proved colder than normal, relative to the 1981-2010 baseline.

“So I don’t think this will be enough to bring October into the negatives,” Noll said.

“And if we look at what’s coming, we’re talking about a 10C to 15C turnaround in some areas, easily,” he said, pointing to a warmer, northwest flow developing in the south over Sunday and Monday.

Over the final three months of 2022, Niwa is forecasting a “very likely” chance of hotter temperatures across the North Island and west and north of the South Island – and “most likely” in the east of the South Island.

Widespread marine heatwave conditions – likely to escalate over late spring – will add to that warmth, as will the background influences of our third La Niña summer in as many years.

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