The New Zealand Herald

Our storm explained A wild beast with a sting in the tail

Experts describe stew of dramatic weather and climate factors

- Jamie Morton science

The death throes of a massive marine heatwave, a nasty lowpressur­e system and a weird wildcard called a “sting jet”: the beast that struck New Zealand was a stew of dramatic weather and climate factors.

That one of its biggest punches landed straight on the face of our biggest city was also a rare event.

Meteorolog­ists have broken the storm, which also handed Auckland, Wellington and Dunedin their coldest night of the year, into three parts.

The ‘marine heatwave’

Fascinatin­gly, the intensity of the storm could partly be attributed to the same reason our surf was so pleasant to swim in over summer.

Since November, sea surfaces around New Zealand, particular­ly the Tasman Sea, have been warmed by the biggest marine heatwave in 150 years of records.

The marine heatwave was caused by a rare and powerful combinatio­n of a La Nina climate system, a Southern Annular Mode locked in a positive phase, a series of persistent highs and the background influence of climate change.

As a cold mass of air moved up across the ocean towards New Zealand from Antarctica this week, it met these warmer waters in the Tasman Sea.

Riding over those unusually warmer ocean waters created a bigger difference in the system’s temperatur­e gradient, Niwa meteorolog­ist Ben Noll said.

“The greater the difference in temperatur­e gradient, the stronger the weather system is going to be,” he explained.

“If you ran the same weather system over top of the Tasman Sea when it had normal or even colder-than-normal sea surface temperatur­es, you would have had a lower impact.

“In this case, if the seas are warmer, there’s more water vapour in the air — and that meant more precipitat­ion, and also more strength as well, because of how it had connected with that very chilly cold pocket of air in the upper atmosphere.”

It wasn’t the first time the big marine heatwave has meddled with significan­t weather events that have hit New Zealand.

Meteorolog­ists also blamed it for charging up ex-tropical cyclones that slammed into the country over past months.

Noll expected what was left of the nowfading heatwave had probably been spent on energising this week’s storm.

“If you think of it like a bank account, when storms run over those warmer seas, they take out chunks of money and your account goes down.

“This one probably just drained the account and used whatever energy there was left in it.”

Coincident­ally, a major study published yesterday found such events had grown longer, stronger and more frequent over the past century — and the trend would continue as the planet warmed.

The low from the west

As the southerly front rolled up the country from south to north, it also unleashed thundersto­rms and strong wind gusts that hit Taranaki and the central North Island especially hard on Tuesday morning, downing power lines and tearing roofs from homes.

Meteorolog­ists still weren’t sure if tornadoes were actually involved, or just thundersto­rm down-bursts or “straightli­ne winds” that might have appeared similar to people on the ground.

After the front had passed over the middle of the country, there was a brief afternoon lull when the weather improved.

But a deepening low was waiting in the wings west of New Zealand, and made itself known to people in the evening.

When it finally stepped on to the stage, it combined with the southerly front already in play — and most of the drama happened directly over Auckland.

“We had a deepening low moving in from the Tasman Sea; we then had that low making landfall in Waikato and

On top of all that, these worst winds then tracked across Auckland via a natural wind tunnel. Philip Duncan, WeatherWat­ch

brushing South Auckland which then placed the strongest winds directly over Auckland City,” WeatherWat­ch head analyst Philip Duncan said.

“To help keep the energy, the powerful nationwide Antarctic southerly was roaring up behind it fuelling more winds.

“Finally, on top of all that, these worst winds then tracked across Auckland via a natural wind tunnel.”

All of that combined created damaging gales that were a notch above what had been forecast, Duncan said.

At some points, winds over Auckland

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