As frustratingly familiar as this week’s wet and wild weather might feel to rain-weary Kiwis, Niwa says the coming months will be a “much different season” than the record-warm winters of the past three years.
That’s down to a pair of climate drivers working in tandem to bring cooler, drier days across New Zealand over winter, in what should be a sharp shift from the unprecedented warmth and rain the country experienced last year.
Niwa’s just-issued outlook for May to July predicted that, overall, temperatures were most likely to be above average for the north and west of the North Island and the north and west of the South Island.
In the east of both islands, they were equally likely to be above or near average for the period.
Rainfall, meanwhile, was equally likely to be normal or above normal in the north and west of the North Island, along with the north and west of the South Island.
Elsewhere, normal totals were expected in the east of the North Island - and either below or near normal in the east of the South Island.
The torrential downpours - which have lashed Northland, Auckland, Bay of Plenty and Coromandel and closed Karangahake Gorge with slips - weren’t unlike what the country experienced in the relentlessly wet winter of 2022, which broke rainfall records going back to 1971.
Going forward, however, a distinctly different flavour was likely to emerge.
Niwa forecaster Chris Brandolino explained that, in terms of those three-month trends in the outlook, the huge amounts of rain now drenching the country would “offset” dryness to come in June and July.
“I think it’s fair to say that what we’re experiencing right now, in this opening week of May, is not going to be congruent with the overall flavour we’ll get, as we work our way through the three-month period,” he said.
Source / Niwa
“As we go into June and July, we’ll see a transition to more southeasterly winds – then eventually to south or southwest winds.”
That shift was related to what was gradually unfolding thousands of kilometres away in the equatorial Pacific.
Over the first three years of the decade, our weather patterns have been shaped by La Nina: the ocean-driven climate system that set up New Zealand’s dramatic tale of two summers.
While its unusually long reign finally ended earlier in autumn, our currently warm, wet conditions could be thought of as La Nina’s “lingering cough” - or a lag effect in the ocean-atmosphere state.
In its place would come its climate counterpart El Nino - known to deliver cooler, but also drier winters to much of the country – along with another big influencer.
That was the positive phase of a far-off phenomenon called the Indian Ocean Dipole, associated with drier-than-normal conditions for parts of New Zealand, particularly in El Nino states.
Niwa further expected that the Southern Annular Mode - a ring of climate variability encircling the South Pole – would trend more negatively as winter ground on, spelling increased potential for colder temperatures.
Although New Zealand’s coastal seas were still running markedly warm – some regions were recording surface temperatures of more than 1C above average last month – local waters, too, were likely to cool.
Over the longer term, however, scientists expected “marine heatwaves” to grow longer, stronger and more frequent under global climate change – in step with warmer average temperatures and shifting extremes.
Heatwaves hit hard in NZ
Meanwhile, scientists have found heatwaves that hit New Zealand over recent summers are already having wide-ranging effects on our environment.
Climate scientist Professor Jim Salinger said summer heatwaves in 2017-18, 2018-19 and 2021-22 brought the warmest months on record, with many more days than usual reaching 25C or higher.
“These warm season heatwaves all produced dramatic climate impacts across New Zealand, including marine heatwave conditions and major loss of glacier ice volume in the Southern Alps,” said Salinger, an adjunct research fellow at Victoria University of Wellington.
“Combined, the three recent heatwaves peeled 17 per cent of the total ice off the Southern Alps glaciers, which have lost half their volume since 1949 — down from 65 to 32 km3 by 2021.”
Off northwest and southwest coasts, marine heatwave conditions were extreme during these summers.
Victoria University marine biologist Professor James Bell, who co-authored a newly published study led by Salinger, said the increasing frequency and strength of marine heatwaves was a major concern.
“The impacts on marine fauna of the 2021-22 heatwave were much larger than any that have been reported during previous heatwaves, although they were disproportionately felt among some marine organisms.”
The study suggested recent ocean heatwaves could be linked to the starvation and death of kororā, or little blue penguins, in the region, with rising sea surface temperatures potentially affecting the species’ ability to find food.
Heatwaves have also been linked to the widespread bleaching of marine sponges across the north and south of New Zealand, affecting millions of sponges.
A bleached sea sponge in Fiordland, where sea temperatures recently soared to 5C above normal. Image / Victoria University
“For most marine species in our waters, we don’t know their thermal thresholds — the temperatures they’re able to tolerate — so it’s possible that future, more intense marine heatwaves will have even bigger impacts than we’ve seen to date,” Bell said.
Another study co-author, Victoria University climate scientist Professor James Renwick, said heatwaves of this type used to be rare, occurring once every few hundred years.
“By the late 20th century, heatwaves were occurring once every 40 years. With 1.5C of warming, events such as we’ve seen in recent summers would have estimated recurrence intervals of two to three years,” he said.
“With 2C of warming, these summers would be considered cool relative to what we’d experience with [more than] 2C of warming.”
In all, New Zealand’s temperatures have risen by more than 1.1C since the 1870s.
