Three numbers that explain NZ's coming summer scorcher

Temperatures over summer are predicted to be hotter than usual for most of the country, with warmed-up westerlies to keep blowing from bushfire-ravaged Australia. Photo / Dean Purcell

Forecasters are picking a warmer summer than usual for New Zealand – with a few twists in the mix. Jamie Morton looks at three big numbers that tell the story.

40%

Niwa gives a 40 per cent chance of above average temperatures this summer – and 35 per cent for near average ones.

Put simply: summer is likely to be another scorcher, with little probability of a cooler season.

The agency projected that all regions would get temperatures sitting on the hotter side of normal, with the only exception being the South Island's west coast, which was just as likely to get average heat.

As far as Niwa meteorologist Ben Noll was concerned, summer kicked off a month early, with the westerly winds and extra dragged across from bushfire-ravaged Eastern Australia.

"We've had a lot of northwesterly flows pulling hot air across the Tasman Sea from Australia, which has been baking over recent years amid ongoing drought conditions," he said.

"So each time we get this kind of flow – which is just what we're expecting for the first half of the season – we're going to keep getting those warm masses from Australia.

"That means that across the North Island – and especially in the east of the South Island – it's going to get quite toasty."

Source / Niwa

Meteorologists have pointed to the fires across the Tasman for lifting temperatures in many parts of New Zealand to 8C to 10C above what normally would have been recorded.

Last week, the pre-summer swelter also drove down soil moisture levels across the upper North Island, as well as in Marlborough, Tasman and Westland.

Niwa's latest monitoring showed the driest areas for this time of year were coastal Wairarapa, eastern Marlborough and inland Southern Canterbury.

In all, the month was expected to finish somewhere in the top five hottest Novembers on record.

0.78C

Forecasters are closely watching another important driver of summer heat – warmed-up sea surfaces.

Throughout November, sea surface temperatures (SSTs) climbed to reach 0.78C above normal off the east coast of the South Island, and 0.39C above normal around the north of the country.

"In the last week of November, especially, the south-west Pacific has really started to cook, which is not surprising given the air flows we've had coming off the Australia," Noll said.

Source / Niwa

That also signalled the resumption of a 32-month pattern of above average SSTs only interrupted by a rare polar event in spring that hauled them back closer to historically normal levels.

Niwa predicted local SSTs would remain on the higher side throughout the season – which would have its own impact.

Not only did warmer seas create positive feedback effects that cranked land temperatures even higher, they could also modify cooler weather systems heading across water towards the country.

Noll pointed out warmer oceans were big factors in last summer becoming New Zealand's third hottest – and the summer before that proving the warmest ever.

Both came with "marine heatwaves" that drove the melting of glaciers, pushed warm water fish south and put the winegrowing season several weeks ahead of schedule.

Could a third marine heatwave engulf the country?

"I wouldn't rule it out – and to be honest I wouldn't be totally shocked if it did happen, either."

70%

Further out in the equatorial Pacific, oceans were expected to sit in an ENSO-neutral state – meaning in neither La Nina or El Nino – for the next three months at least.

But Noll still predicted summer would come with a hint of La Nina-like conditions, especially in the rainfall stakes.

Niwa's outlook projected rainfall levels to hover around near normal for most of the country, with the west of the South Island – just as likely to get a higher dose - again proving the exception.

Beyond that headline prediction was a more complex picture.

"Early in the season, we'll see lots of westerlies, which means those dry areas - that's from Hawke's Bay down to Wairarapa, along with other places like Bay of Plenty and Northland – will continue that way," Noll said.

Source / Niwa

"In the South Island, it's going to be quite a different story. It'll be stormy in the west as we go through December, with several rounds of heavy rainfall."

Other regions to the west – Tasman, Taranaki and Wellington included – could see some decent fronts bring downpours as they sucked up moisture from a balmier Tasman Sea.

But halfway through the season, the pattern could reverse, bringing rain to drier areas and drier weather to wet areas.

"The upshot is, if you're going camping this summer, you're best to do it early in the eastern parts of the country – or leave it until later if you're going in a western area."

Why the flip?

At mid-summer, the specific climate driver that's been influencing our weather since October – something called the Indian Ocean Dipole, or IOD – could switch places with another.

Having been fixed in a near-record positive phase, the IOD has helped create the unusual dryness that stoked Australia's fire danger, as well as New Zealand's own late spring warmth.

"As the IOD fades, it will possibly give way to the tropical western Pacific becoming a bit of a climate driver, overall giving us more of a La Nina-like flavour and encouraging more easterly or northeasterly quarter winds," Noll said.

"But that will all depend on how quickly the IOD fades, of course."

As it stood, the final year of the decade was on track to finish New Zealand's third hottest on record.

"I sound like a broken record here, but it's just more of the same that we've been dealing with for the last several years."

In fact, five of the past six years have been among the country's warmest.

Last year was New Zealand's second-equal hottest; only placing behind 2016, which had a record nationwide average temperature of 13.45C.

"Hotter temperatures sound good for holidaymakers, sure, but there is a much more important factor to consider here, and that's the warming planet that we live on."