Another year, another warm, wet and wild winter.
But we can't put this steadily soggy season down to La Niña alone – nor can we point the finger solely at the inexorable march of global warming.
Rather, a meteorologist says, we need to cast our moist and mild winter against a backdrop of local, regional and global climate drivers, which also include marine heatwaves and a jet stream that's nudged closer to us.
"We're still getting annual and decadal variability here, that's true," Niwa's Ben Noll said.
"But, then, the characteristics of the players on the field, our climate drivers, are changing with all of this unnatural, underlying warming that we now have in the background."
While the final figures won't be ready until next month, Noll expected the season to sit somewhere in the five warmest, alongside the last two record-beating winters.
It 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 July, our wettest ever, smashing rainfall records in 20 different 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.
It also finished up as the fourth-warmest July on the books, or 1.3C above average, and Noll said that pattern had only carried on into August.
As of Tuesday, this month's average had been tracking above 10C.
"The 1981 to 2010 norm for August is about 8.8C ... so you can see the difference there."
🗣️ Kaitaia's minimum temperature of 18.2˚C is provisionally New Zealand's new national minimum temperature record for the month of August! 🌡️
— NIWA Weather (@NiwaWeather) August 18, 2022
Mother Nature is running a fever 🤒
Whangārei & Auckland set records for the warmest August night - for the third consecutive night... pic.twitter.com/sCfL8yt4Xf
Despite a cooler start, a single climatological event changed the trend line.
That was last week's "atmospheric river", which funnelled tropical humidity and moisture directly into New Zealand, and ultimately left more than 130 homes in the Nelson-Marlborough region red-stickered from flood damage.
That rain-packed plume was this week found to be the strongest atmospheric river ever recorded in an August month - and the second most powerful seen in any winter.
"There were a few days during that event where our national average temperature nearly reached 15C – that's not unlike what we see in November," Noll said.
"All of those warm days and nights we've had recently ... that's been an influence of having more cloud-keeping heat trapped, which has been one of the many unusual hallmarks of this winter."
But to understand winter's entire climate puzzle, Noll added, we had to look at each piece separately.
In announcing the temporary closure of the Tūroa ski area this week – and more than 100 job losses with it - operators of Mt Ruapehu's skifields cited "unseasonable" winter weather for a poor snowpack.
Specifically blamed was La Niña: an ocean-driven climate phenomenon that's been meddling with our weather for more than two years running.
Models now favour a third system reforming for summer which, depending on where you live, will make the holiday season unusually warmer, cooler, wetter, drier, windier, or calmer.
La Niña represents one side of the El Niño-Southern Oscillation (Enso), a natural see-saw that measures the movement of warm, equatorial water across the Pacific Ocean, and the atmospheric response to it.
In direct contrast to El Niño, La Niña typically delivers more north-easterly winds that bring rainy conditions to North Island's northeast, and drier conditions to the south and south-west of the South Island.
Thanks to these north-easterly winds, warmer temperatures also tend to play out over much of the country during La Niña - but there are always regional and seasonal exceptions.
One was 2020, when an odd-ball La Niña event delivered a somewhat unexpectedly dry summer for Auckland, although the 2021-22 event behaved closer to script.
Another was the fact the South Island's West Coast has copped a seemingly relentless slew of deluges over the past few years.
Nevertheless, lingering remnants of the last La Niña - something that's also left our sea surface temperatures warm enough to energise an onslaught of low-pressure systems – were a big part of the reason for July's incessant downpours.
A similar-scale background driver was the positive phase of a far-off phenomenon called the Indian Ocean Dipole (IOD), which has contributed to a series of devastating floods across Australia in recent times.
We might expect that the warmth and wetness of La Niña has left all of our mountains starved of healthy snowpacks.
Yet, unlike in the north, southern skifields had actually fared reasonably well this winter, Noll said.
A big reason for that mismatch was another enormous contributor to our warm and wet winter, but also one intricately connected to this La Niña - the subtropical jet stream.
🌬️ Incursions from the sub-tropical jet stream (🟡) have been very common this winter!
— NIWA Weather (@NiwaWeather) August 24, 2022
Its more southerly position has been one of the main reasons that the season has been wet 💧 + warm 🌡️
The jet will affect us again tomorrow & Friday, followed by a well-deserved break! pic.twitter.com/mrAaj81eTI
We could think of it as a giant ribbon of fast-moving winds, sprawled high above regions of subtropical high pressure.
Over recent months, it's shifted further south of its normal climatological position in the South Pacific and closer to us here in New Zealand.
"That's certainly helped siphon down tropical air masses – but it's also formed up frequent low-pressure systems in the northern Tasman Sea and Coral Sea, helping to create a motorway of moisture down to us," Noll said.
"For the North Island, it's been overwhelmingly warm – hence the lack of seasonal snow in the Central Plateau.
"In the South Island, interestingly, it's been just cold enough there that this moisture has actually been able to fall as snowfall at elevation."
Fortunately for southern skifields, those pulses of moisture had coincided with cold air flows from the southeast.
"It's just that these colder outbreaks haven't managed to creep up the country, which has made for a big difference from island to island."
Another subtropical element playing with our weather was a regional "marine heatwave" to New Zealand's north – which had persisted to help send a slew of atmospheric rivers our way.
"We're talking about the profound effect from a pattern that's actually going on two or three thousand kilometres away from us."
At the same time, our own seas continued to run hot.
Coastal sea temperatures ranged from 0.5C to 1.3C above average over July, and regions like the Bay of Plenty were now experiencing their longest-lingering ocean heatwaves ever seen.
So where did climate change fit into all of this?
For one, a warming planet meant warmer oceans.
On top of what we're already witnessing, Kiwi scientists have warned that average sea temperatures could rise by 1.4C within four decades – and almost 3C by the century's end.
That would mean that, by mid-century, we could be facing 260 days of marine heatwaves per year – and 350 days by 2100.
Importantly, those warming seas also meant more water vapour.
As the air warmed, its capacity to hold water increased by about a 7 per cent per degree of warming, in turn making the kinds of deluges we saw last month more likely.
Scientists have observed a positive trend in atmospheric water vapour concentrations across more than three-quarters of the planet over the past three decades.
More water vapour in the air enhanced the greenhouse effect - trapping more heat at the surface and causing temperatures to rise – while also resulting in yet more evaporation or positive feedback.
While scientists don't necessarily expect the frequency of storms to increase under climate change, they do expect these storms to grow only more intense over time.
Niwa climate scientist Peter Gibson said climate change could have a particularly big influence on the future strength of atmospheric rivers like last week's.
"A warmer atmosphere can hold more water vapour, so it's likely that New Zealand will see more intense atnospheric rivers as climate change continues," Gibson said.
"The challenging part will be figuring out precisely which locations will see the biggest increases."
Extreme precipitation from atmospheric rivers was already being enhanced by climate change in New Zealand, Gibson said.
Scientists have estimated that warming made last year's disastrous deluge in Westport about 10 per cent more intense – with a similar influence also attributed to last winter's Canterbury floods.
But whether climate change happened to be raising the odds of La Nina-like conditions like these was less clear.
While some recent studies have made a case for this, IPCC models instead indicate a shift to the other end of Enso, favouring more El Niño-like states.
In any case, Noll said the hand of a changing climate in this year's winter – above and beyond everything else we'd expect with natural variability - was unmistakable.
"We shouldn't be seeing month after month of marine heatwaves in New Zealand's coastal waters, nor should we be seeing Noumea in New Caledonia nearly reaching temperatures of 33C at this time of year," he said.
"That's just not normal."
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