Beachgoers enjoy the water at Auckland's Narrow Neck Beach on Christmas Day last year. Forecasters expect sea temperatures to be unusually high again this summer. Photo / Brett Phibbs
Beachgoers enjoy the water at Auckland's Narrow Neck Beach on Christmas Day last year. Forecasters expect sea temperatures to be unusually high again this summer. Photo / Brett Phibbs
Meteorologists say it's now likely New Zealand will see its third La Niña summer in as many years. Why is this "triple dip" a notable event – and is it a symptom of a warming climate? Science reporter Jamie Morton explains.
Remind me again, what exactly is La Niña?
Dependingon where in New Zealand you live, the last two summers may have been unusually warmer, cooler, wetter, drier, windier, or calmer.
We can blame La Niña - an ocean-atmosphere phenomenon that's been stubbornly churning away in the background – for much of this meddling with our holiday weather.
Under the El Niño-Southern Oscillation (Enso) - measuring the movement of warm, equatorial water across the Pacific Ocean, and the atmospheric response – our climate cycles between three phases.
We know them as La Niña, El Niño and Enso-neutral, when there's no dominant driver at our climate steering wheel, and weather variability is influenced by a mix of other, smaller-scale sources.
When we're sitting in an El Niño state, we're more likely to see warm westerlies in summer, cold southerlies in winter and south-westerlies the rest of the time.
Beachgoers enjoy the water at Auckland's Narrow Neck Beach on Christmas Day last year. Forecasters expect sea temperatures to be unusually high again this summer. Photo / Brett Phibbs
Some of New Zealand's biggest droughts have played out under El Niño - including a horror event in the late 1990s that cost hundreds of millions of dollars.
Traditionally, La Niña has delivered 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 the north-easterly winds, warmer temperatures also tended to play out over much of the country during La Niña, although there are always regional and seasonal exceptions.
One was 2020, when an odd-ball La Niña event delivered a somewhat unexpected flavour – and became among four of 17 La Niña events measured since 1972 that failed to bring near or above normal rainfall for Auckland.
The 2021-22 event behaved closer to script, contributing to a record marine heatwave near the North Island, several close calls with ex-tropical cyclones, and flooding in the east and droughts in the far south.
Why do meteorologists think we're in for another one?
We've never really shifted out of La Niña: while meteorologists earlier gave 50-50 odds of a move to Enso-neutral conditions, it's lingered on.
That staying power was made dramatically 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.
It was followed by a July surge in trade winds over the equatorial Pacific, where currently cooler waters were now expected to spread eastward over the next two months, and strengthen the oceanic La Niña signal.
In its latest outlook, Niwa gave a 70 per cent chance of La Niña conditions carrying on until October – and a 65 per cent chance of the system still hanging about between November and January.
📣 Aug-Oct 2022 climate outlook: gradually turning less wet
If that happened, we will have seen a rare "triple dip" of consecutive La Niña events – which has only ever been observed over 1973 to 1975, and most recently from 1998 to 2000.
"It's now clear we're headed back in a La Niña direction over late winter and early spring – this transition has already started," Niwa forecaster Ben Noll said.
"And this is continuing to influence our weather patterns here in New Zealand."
Lingering remnants of La Niña - something that'd also left our sea surface temperatures warm enough to energise an onslaught of low-pressure systems - were partly to blame for the country's wettest July on record.
Another 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.
"The combined effect of these two prominent climate drivers will continue to impact our weather in the coming weeks and months."
Will our third La Niña summer be like the last two?
"When we look ahead to this La Niña, that's one of the most – if not the most – important questions, because we have very few historical comparisons," Noll said.
As such, forecasters had to be particularly careful when predicting how this one might shape up – and whether it'd go down as a "moderate" event like the last two.
Under La Niña, we could at least expect added holiday warmth – its signal contributed to a maximum temperature of 39.3˚C in Ashburton during summer 2020-21, persistent dryness about the northern North Island, and our fifth warmest summer on record last summer.
La Niña conditions also contributed to New Zealand's hottest-ever summer – 2017-18 – which came in tandem with an unprecedented marine heatwave and dominant high pressure.
It remained to be seen whether much of the north would again miss out on the more traditional La Niña rainfall.
While Niwa's summer outlook won't be released until late November, Noll said that characteristics from each of the last two summers might help describe what's in store later this year into next.
"It's now clear we're headed back in a La Niña direction over late winter and early spring - this transition has already started," Niwa forecaster Ben Noll said. Photo / Michael Craig
"We're more likely than not to again see warmer-than-average sea temperatures, which influences on-land temperatures both day and night, along with higher humidity," he said.
"So, if you're in the North Island, you'd potentially be looking to having that air conditioning running more frequently – and perhaps sooner rather than later, given temperatures last year really started to creep up in late October."
Where does climate change fit in?
The fact our climate is changing as our planet warms should now be unmistakably clear to Kiwis.
Last year was our warmest on record, as were our last two winters.
Sophisticated models are increasingly enabling scientists to attribute the handprint of climate change in heavy deluges, like those just seen in our wettest-ever July, and other extreme events.
🔢 We’ve counted, collated and quality-controlled all the numbers in NZ’s National Climate Database for the exceptional month of July 2022. What are the key takeaways? 🧵 pic.twitter.com/unKYzHiUtF
Climate scientist Professor Jim Salinger said there was emerging evidence to suggest that La Niña-like conditions might become more likely as the climate warmed, pointing to a recent study out of the University of New South Wales.
Its co-author, Professor Matthew England, told Nature in June: "We are stacking the odds higher for these triple events coming along."
England and others have queried findings of IPCC models, which instead indicate a shift to more El Niño-like states.
Victoria University climate scientist Professor James Renwick said it was much more apparent that climate change was making weather patterns more extreme under Enso - itself among several "oscillations" in the Earth's climate, and a deviance of a much broader system called the Walker Circulation.
Extreme rainfall under La Niña would grow yet more intense, while droughts under El Niño would become more severe.
As for climate change potentially tipping Enso's balance, Renwick still considered the jury to be out.
"And maybe the jury will continue to be out," he said.
"Being such a huge thing that affects the whole planet, it's amazing how much [Enso shifts] comes down to small subtleties and gradients in temperature across the Pacific – and we may never get a consensus."
