Niwa is picking above-average temperatures for the season, owing to fewer southerly winds, warmer seas and a La Niña climate system. Photo / Dean Purcell
It might be chilly now - but forecasters say it's possible the coming winter could bring record warmth for the third year in a row – and plenty more wet and wild weather with it.
Although the next 10 days are forecast to be chilly and snowy at times, Niwa is picking above-average temperatures for the entire season.
That's down to fewer southerly winds, warmer seas and a La Niña climate system that's still hanging about to meddle with our weather, despite earlier indications.
The other big theme of winter was rain: more than normal amounts of it are in store for the north of the North Island, with most other parts of the country expected to get either near or above-average amounts.
Niwa also singled out the potential for more low-pressure systems sweeping across the Tasman Sea – and pointing plumes of sub-tropical moisture toward the country.
Big deluges caused by these visiting systems have already made for the two costliest years yet for extreme weather claims – amounting to more than a billion dollars in insured losses.
New Zealand's past two winters - respectively registering at 1.1C and 1.3C above the 30-year average – also happened to set new records for seasonal warmth.
📣 Winter 2022 climate outlook: warmer, wetter overall
— NIWA Weather (@NiwaWeather) June 1, 2022
🌡️ Unlikely to be colder than average; warm seas continue
💨 More westerly & northeasterly winds, fewer southerlies
🌧️ Leaning wetter: plumes of tropical moisture may cause flooding
The details 👇https://t.co/SSqegyiA1s pic.twitter.com/UMWJZeHxWr
Niwa forecaster Ben Noll said our January to May period – which included our fifth hottest summer and second-equal warmest winter – had brought temperatures about 1.2C above average, putting the first five months of 2022 among the top five hottest starts on record.
He couldn't rule out winter 2022 breaking new records for heat – especially with many of the same characteristics of the last two winters back on the menu.
"If you're wanting to create another record warm winter, you don't have to go to the local supermarket – the ingredients are already in the pantry."
He explained three of the stand-out features.
La Niña, an ocean-driven climate system known to fuel warm and wet conditions, has constantly flavoured New Zealand's weather over the past few years.
Separate La Niñas helped push last year's average temperature to the country's warmest in more than a century of records – and another could arrive later in 2022, making for a rare, back to back "triple dip".
During a La Niña event, ocean water from off the coast of South America to the central tropical Pacific cools to below average - a result of stronger than normal easterly trade winds, which churns cooler, deeper sea water up to the ocean's surface.
This unusually cool water in the eastern Pacific then suppresses cloud, rain, and thunderstorms, as sea temperatures in the far west of the ocean warm to above-average temperatures.
Here in New Zealand, we can usually expect more north-easterly winds that bring rainy conditions to North Island's northeast, and drier conditions to the south and southeast of the South Island.
Thanks to the northeasterly winds, warmer temperatures also tended to play out over much of the country during La Niña, although there are always regional and seasonal exceptions.
New WMO Global Seasonal Climate Update has details on temperature predictions for June-Aug, which reflect the continuing #LaNiña and other #climate drivers (including warming impact of greenhouse gases).
— World Meteorological Organization (@WMO) June 10, 2022
https://t.co/vB5BP3FnVx pic.twitter.com/8ihxficzBW
One clear exception was 2020, when an oddball La Niña event delivered a somewhat unexpected flavour to New Zealand – and became among four of 17 La Niña events measured since 1972 that failed to bring near or above normal rainfall for Auckland.
Back at the start of the year, Niwa offered a 50 per cent probability for the current La Niña to have faded back to "ENSO neutral" conditions by July.
According to its latest outlook for winter, however, the system was still going strong – and favoured to linger on through into spring.
"In the context of other historical events, it's unusual to see it as strong as it is for this time of year, when conditions do usually trend back toward neutral," Noll said.
The staying power of this La Nina was made clear with the Southern Oscillation Index – one of the biggest measures of strength for La Niña events – reaching near-record values over April and May, against data stretching back to 1876.
"We're talking about a very, very strong atmospheric imprint here."
Climate scientist Professor Jim Salinger pointed to the World Meteorological Society's latest advisory, which gave a 70 per cent chance of current La Niña conditions extending into boreal summer (June to October) - and about 50-60 per cent during July to September.
"With sea surface temperatures running above normal we can expect it stay warmer – and also wetter in Auckland and other areas exposed to the north," Salinger said.
Our recent La Niñas have been accompanied by balmier coastal waters surrounding the country – often tipping into marine heatwave territory.
Over autumn, sea surface temperatures were the warmest on record in the west of the South Island – registering at 2.6C above average – and also in the north of the South Island (1.8C above average) and the east of the South Island (1.7C above average).
Those warmer waters – stoked by blocking anti-cyclones, fewer low-pressure systems and a climate indicator called the Southern Annular Mode (SAM) trending positive – in turn drove more heat on land, with often dramatic consequences.
Scientists have partly blamed marine heatwaves for big glacier melts, disrupted growing seasons and soaring summer temperatures.
While stronger westerly winds and more frequent lows forecast for this month might take the edge off, Niwa nonetheless expected seas to keep running warm throughout winter and beyond.
"We might see that anomaly drop off from where it was, say, in summer and autumn – but our eyes will still be on sea surface temperatures as we head further into the year," he said.
"Whether a triple-dip La Nina happens, and whether that spells another marine heatwave next summer, is uncertain – but if it did, that would be bad news."
Warmer waters also packed extra energy into damaging weather systems that swung into our neighbourhood: notably tropically-charged "atmospheric rivers" that have wrought havoc over the last 12 months.
Noll said it remained to be seen whether a cold spell over the next 10 days would deliver enough snow to higher elevations across the South Island to help offset some of the abnormal warmth.
"Will it provide a decent base of snow, and will that act as a bit of a freezer that might help keep things cooler in the South Island than the last two winters? We don't know yet, but it does look to turn warmer toward the end of June."
The third factor on our winter weather table was another ocean phenomenon – but one unfolding thousands of kilometres away from New Zealand.
Meteorologists have been closely watching what could be a strongly negative phase of what's called the Indian Ocean Dipole (IOD) - a gauge of relative sea-surface temperatures on the west and east of the ocean's basin.
Noll described the IOD as a seesaw-like system that typically had three phases: neutral, positive and negative.
Its strongly-positive state in 2019 – which created cooler waters in the ocean's east, and warmer seas in its west – fuelled dry conditions across Australia and worsened its devastating bushfires and played a role in Auckland's severe drought.
The current picture, however, was much the opposite: and eastern Australia has been bracing for more deluges to follow a slew of record-breaking floods.
While New Zealand was less affected than its neighbour by swings in the IOD, its influence could be enough to add yet more warmth and wetness to winter.
"As the IOD really starts to kick in during July, there's the potential for yet more moisture across the Australia-New Zealand region as warm and sometimes humid northwesterly winds extend across Australia's north.
That'd particularly be the case where the IOD's signal coincided with that of the Madden-Julian Oscillation (MJO): a pulse of rain and thunderstorms that circled the globe every 30 to 60 days on average.
"The IOD can influence the MJO to shift more to the maritime region north of Australia, leading to more warmth and rivers of moisture stemming from that part of the world."
As for the elephant in the forecasting room – global climate change – Noll said the effect of more heat in our oceans, and more moisture in our atmosphere, was already clear to see.
Scientists have already estimated the extreme rainfall that caused Canterbury's flooding last year was 10 to 15 per cent more intense because of human-driven climate change - with a similar-sized influence also observed in last year's Westport disaster.
And if that wasn't enough of an indicator, Niwa data shows it's been 65 months – or roughly five and a half years – since New Zealand saw a month of below-average temperatures, relative to the 1981-2010 baseline.
"We should be celebrating those colder than average, snowy days as they are becoming less common."
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