Weather: Heavy rain warnings for northern NZ; rare polar phenomenon could mean stormy conditions in weeks ahead

Every winter, the polar vortex forms high above Antarctica in the polar stratosphere. This year, the vortex is showing signs of being disturbed.

People across the upper North Island are being told to brace for severe weather, with gale-force winds, intense downpours, slips and flooding possible.

After a week of clear blue skies and record-breaking high pressure, a moisture-packed northeasterly flow is set to bring dramatically different weather to open the second half of the school holidays.

Heavy rain is forecast across Northland and Auckland from early Monday morning, with the possibility of severe northeast gales in some exposed northern locations.

But it’s the Coromandel Peninsula, Bay of Plenty, and west of Rotorua that’s square in the firing line, with the MetService having issued orange-level heavy rain warnings for those regions.

As much as 80mm to 120mm of rain – with peak hourly rates of 10mm to 20mm – was possible over 12 to 14-hour periods in those areas from Monday morning.

That amount of rain could cause streams and rivers to rise rapidly, while surface flooding and slips could make for difficult driving conditions, the MetService warned.

Ahead of the storm, Bay of Plenty Civil Defence was encouraging people to clear their drains and gutters, and to avoid low-lying areas during the event.

Heavy rain could also reach warning levels in the Tasman district and west of Motueka from 5pm on Monday, the MetService reported.

“It’s not going to be quite the scale of the last system we saw last month, but it is going to be quite a rainy day over the country,” MetService meteorologist Ngaire Wotherspoon said.

“It also isn’t the last of these: we’re expecting a less severe system coming on Tuesday night and into Wednesday,” she said, adding that yet another low could sweep in from the north on Friday night.

It comes as meteorologists are closely watching signs of a rare phenomenon deep below New Zealand, which could ultimately influence stormy conditions here in the weeks ahead.

What’s called “sudden stratospheric warming” (SSW) occurs when the temperature of the stratosphere – that’s the layer of the atmosphere 30km to 50km above ground – over the South Pole climbs by more than 25C.

This effect sometimes has the potential to mess with the polar vortex – a ring of stormy and freezing weather that encircles Antarctica - which is at its strongest at this time of year.

Right now, Niwa meteorologist Ben Noll said a “pattern of warming” was being observed in the stratosphere above East Antarctica – and it was possible it could reach the threshold of becoming a “minor” SSW event.

“When a SSW occurs, it can help to weaken or displace the polar vortex in the stratosphere, which then filters down onto the tropospheric polar vortex and influences our weather patterns,” Noll explained.

Such events were rare in the Southern Hemisphere, with only three – in 2002, 2010 and 2019 – recorded in the satellite era.

The 2002 event coincided with New Zealand’s coldest October in 20 years, with below-average temperatures covering much of the country.

In 2010′s minor event, a slew of rainfall records were broken amid well-below normal sunshine and freezing temperatures in parts of the South Island.

“This year, there are some signs that the developing anomalies in the stratosphere may indeed couple with the troposphere, ultimately affecting hemispheric weather patterns,” Noll said.

“The early indication is for above normal pressures to develop over Antarctica during late July.”

That could force lower pressures across the mid-latitudes of the hemisphere - including New Zealand and Australia - leading to stormier conditions.

“The effects can last for up to a month or two, so this unique climate driver may be with us through August.”

It’s not the only one forecasters are closely watching, with the decade’s fourth La Nina climate pattern increasingly predicted to form up over spring.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.