An "atmospheric river" that caused disastrous flooding in Nelson and other spots last month was one of the strongest ever measured in New Zealand. Photo / Tim Cuff
An "atmospheric river" that caused disastrous flooding in Nelson and other spots last month was one of the strongest ever measured in New Zealand. Photo / Tim Cuff
New Zealand has again experienced its warmest winter – but also its wettest.
Niwa this afternoon confirmed what many Kiwis would have suspected over a relentlessly wet winter, reporting that the season was the standout in rainfall records stretching back to 1971.
Eighteen places - spread from Te Puke to Lauder in Otago, and including Blenheim, Wellington, Oamaru and Greymouth – observed their highest winter rainfall totals on the books.
In two dozen other spots - including Auckland, Christchurch, Palmerston North, Queenstown and Nelson – the season's rainfall came close to record levels.
Winter 2022 also finished as the warmest observed in more than a century of records, finishing up with a national average temperature of 9.8C - or 1.4C above the 1981-2010 normal.
That meant New Zealand had had three consecutive record-warm winters, with each coming in slightly hotter than the last.
That's never happened before: and the last time New Zealand had two consecutive record warm winters was in 1970 and 1971.
"Winter 2022 was also the first winter on record where the temperature anomaly exceeded +1.2C for all three months of the season, relative to the 1981-2010 long-term average," Niwa added in its just-issued summary.
Of the 10 warmest winters on record, six had been since 2013.
"In line with the overall national record, 50 individual locations experienced record warm temperatures in winter 2022 and a further 33 locations experienced near-record warmth."
A Nelson resident walks by a broken stormwater pipe during a flood-making rain event on August 18. Photo / Tim Cuff
The season kicked off with our eighth-hottest June, and rainfall in the west that exceeded 200 per cent of annual levels in some inland parts of the South Island.
Then came out wettest July, smashing rainfall records in 20 spots from Auckland to Akaroa.
Late that month, Christchurch was drenched with more rain than the city's entire monthly average – and more than anything measured over a single day in 157 years.
The season eventually finished with a dramatic climax: the strongest "atmospheric river" measured in an August month, the second-most powerful observed over winters going back to 1959.
That system, which funnelled tropical humidity and moisture directly into New Zealand, ultimately left more than 130 homes in the Nelson-Marlborough region red-stickered from flood damage.
Niwa said the title of our wettest winter was the culmination of numerous extreme rainfall events, "which affected almost every part of the country at some point".
The combination of warm and wet conditions led to variable snow conditions around the country.
A prolonged period of low pressure during the second week of June delivered considerable snowfall to mountainous areas in the South Island, as well as to lower elevations.
That timing was ideal for ski areas, Niwa said, with several opening for the season in mid-June.
In the North Island, however, the warmer and frequently humid conditions resulted in poor cover – eventually forcing Mt Ruapehu's skifield operators to close Turoa early, and send more than 100 workers home.
A Nile St, Nelson, resident checks their neighbour's river-side property last month. Photo / Tim Cuff
What was behind the wetness and warmth?
Niwa singled out a mix of local, regional and global climate drivers, from warmer coastal waters to La Niña and climate change.
Winter's overall air pressure pattern saw higher than normal pressures to the east of the country, and was associated with more northerly quarter air flows.
"This allowed for frequent low-pressure systems to approach from the northwest, transporting warmth and moisture from the tropics," Niwa said.
"The distinctive lack of southerlies during winter replaced by more warm and wet airmasses is characteristic of La Nina – a key driver of our climate for the third consecutive record warm winter."
The influence of La Nina also contributed to the warmest winter sea surface temperatures in New Zealand's coastal waters on record since at least 1982.
Marine heatwave conditions occurred offshore of many regions – and was still ongoing for more than six months in the eastern Tasman Sea.
The source region for several of New Zealand's atmospheric river events, the Coral Sea, also experienced a marine heatwave.
"These warm waters not only contributed to warmer temperatures on land but also provided extra moisture to low pressure systems approaching the country.
The extra warmth in the Southwest Pacific culminated in a sub-tropical jet stream whose average position was farther south than normal, more commonly passing over the North Island.
Another influential climate driver later in the season was the developing negative phase of the Indian Ocean Dipole, marked by warmer than average seas in the tropical eastern Indian Ocean near Sumatra and cooler seas in the west near Africa.
"This was yet another moisture source for New Zealand, with pipelines of water vapour streaming across Australia into the Tasman and Coral Seas at times," Niwa said in its summary.
The excessively warm seas meant that another driver, the Madden-Julian Oscillation - a tropical pulse of rain and cloud - was commonly active across Australasia.
"This was a boon to many a northerly low pressure system that affected New Zealand."
Finally, the Southern Annular Mode (SAM) - a proxy for the location of the polar jet stream and storm track - was in its negative phase for much of June, with the 30-day running average becoming negative for the first time since early September last year.
The negative phase of the SAM tended to be associated with more active weather around New Zealand. July and August were marked by a variable SAM.
"These various drivers occurred against the backdrop of a warmer atmosphere due to climate change which is strongly contributing to New Zealand's temperature trend," Niwa said.
"Because a warmer atmosphere means water will be in its vapour state, which is fuel for rain, it is likely that the extreme rainfall events of winter 2022 were more intense because of climate change."
Fortunately, Niwa is picking a general shift to sunnier, drier conditions in many spots over spring.
Temperatures over the next three months were "very likely" to be warmer than average in the North Island, along with the north and west of the South Island – and "most likely" to be warmer in the southeast, according to Niwa's just-issued seasonal outlook.
It was tougher to precisely predict how rainy spring would prove, thanks to a "battleground" of pressure features hovering around New Zealand – ranging from the odd sub-tropical low to strong belts of high pressure.
But the calm, sunny weather Kiwis have been enjoying for the past few days could become increasingly more common, with periods of high pressure and easterly flows bringing long dry spells, especially in the west of both islands.
