After an unusual three-year run of La Niña - and all of the warm, wet and wild weather it’s delivered us – New Zealand might be about to see what its flipside looks like in a climate-heated world. Science reporter Jamie Morton explains.
It’s looking increasingly likely that, after a six-year hiatus, 2023 will bring the return of the “Little Boy” - El Niño.
While models are still clouded with variability, scientists are warning an El Niño could spell an especially warm 2024 for the planet - perhaps pushing our climate past the symbolic warming threshold of 1.5C.
For Kiwis, an El Niño would bring a distinctly different flavour to the La Niña-influenced weather we’ve been living under this decade – with a higher risk of wildfires and drought on the East Coast.
But let’s look at the basics first.
What are La Niña and El Niño?
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.
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.
Just as we’ve seen this summer, La Niña has traditionally 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.
In an El Niño state, however, we get the opposite set-up.
Ocean water from off the coast of South America to the central tropical Pacific warms above average, while trade winds weaken or even reverse, blowing warm water from the western Pacific toward the east.
As a result, sea temperatures in the far western Pacific can cool below average.
This unusually warm water in the eastern Pacific then influences what’s called the Walker Circulation, acting as a focal point for cloud, rainfall, and thunderstorms.
It’s this change in the Walker Circulation that impacts weather patterns around the world – and here, we’re more likely to see warm westerlies in summer, cold southerlies in winter and south-westerlies the rest of the time.
Why are we talking about El Niño when we’re still in La Niña?
Ask anyone who’s holidayed in Coromandel or Gisborne over the last few summers, and they’d probably tell you that a dry and settled regime for the north-east seems a world away.
But La Niña - something that helped make 2021 and 2022 our warmest (and among our wettest) years to date – is predicted to finally fade this autumn, transitioning to an Enso-neutral state.
“Then, as we go toward winter, the odds for El Niño start to climb,” Niwa meteorologist Ben Noll said.
“As of last month’s guidance, the odds exceeded 60 per cent – so there’s a better chance than not that we could see El Niño conditions arrive in the second half of this year.”
However, Noll pointed out we wouldn’t have a clearer picture until we passed through what climate modellers call the “spring predictability barrier” - or a period of uncertainty over the Northern Hemisphere spring.
This came about as Enso events tended to peak over our summer time (or winter in the north) - and didn’t typically persist beyond northern or boreal spring months, making it more difficult to pick trends and effects in the equatorial Pacific.
“So, predictions made during this period do have somewhat lower accuracy than at other times of the year,” Noll said.
“But, nevertheless, you can kind of see the direction of travel, which is moving toward ENSO-neutral, then potentially to El Niño, which would have a variety of effects, not just locally, but across the globe.”
So what might we expect?
“The first thing to point out is that, if we do indeed get an El Niño forming at some point over winter or spring, our weather in summer 2023 will be a lot different than what we had in late 2022,” Noll said.
Last November happened to be the country’s warmest on the books, but also a stand-out for rainfall, with most of the North Island receiving above or well above normal levels.
“It was exceedingly wet, with atmospheric river after atmospheric river – and it could be that we get a noticeable change to local patterns.”
It was important to note that each event came with a unique set of climate characteristics - and therefore can be expected to influence the weather differently.
But Noll pointed back to those broad-brush trends.
In El Niño winters, for instance, winds tended to blow more from the south, causing colder temperatures across the country, while in spring and autumn, southwesterly winds were more common.
And summer came with stronger, more frequent westerly winds - leaving conditions wetter in the west but drier in the east, parching soils and raising fire danger.
Some of New Zealand’s biggest droughts have played out under El Niño - including a horror event in 1997-98 that cost hundreds of millions of dollars, and major drys in 1972/73 and 1982/83.
While our most recent strong summer El Niño, in 2015-16, also brought south-westerly winds, a switch to more northerly-quarter winds later that season eased farmers’ fears of a disastrous repeat.
Globally, El Niño’s influence was much more dramatic – and in 2016, it drove the planet’s warmest year yet.
As warmer waters tended to nudge the Pacific jet stream south of its neutral position, vast areas of the northern US and Canada quickly dried out – while the US Gulf Coast and Southeast saw more floods.
In all, the 2015-16 event affected more than 60 million people – with populations across East Africa, Southern Africa, the Pacific Islands, South East Asia and Central America hit hardest by extreme weather.
That came with lower crop yields, forced displacement and disease outbreaks, with 23 countries issuing humanitarian appeals totaling billions of dollars.
What difference will climate change make?
That’s one of the biggest questions that climate scientists are already weighing up.
At home, we’ve already seen how a climate more than 1C hotter than it was a century ago worsened La Niña’s three-year reign, contributing to record-breaking downpours – and insurers’ costliest years yet for extreme weather claims.
“At some level, I think we’ll be learning in real time with the next El Niño, because we haven’t had the temperatures that we have now in some of those past events,” Noll said.
“But we do know that a warmer atmosphere holds more water vapour, so you can have some really potent atmospheric rivers,” he said, adding that this mightn’t necessarily be the case for New Zealand, but other parts of the globe.
“The flip-side of that is we could be seeing higher heat extremes across the US than we had in previous events.”
Another worrying question was whether years of La Niña-fuelled vegetation growth in Australia would now pose a huge wildfire danger, given El Niño’s record of bringing warmth and dryness to the country’s east.
Even though La Niña tended to dampen down global temperatures, Noll said recent years had still been some of the planet’s hottest ever observed.
“That’s largely because of the overwhelming aspect of long-term warming,” he said.
“If you combine that with El Niño, from a climate science perspective, I’m sitting here with some pretty serious concerns for what global temperatures are going to be like, from 2023 through 2025.”
Globally, scientists already expect this year to be warmer than 2022 – and there are warnings that the extremes this period could bring may push the planet over the 1.5C warming mark.
While New Zealand and other countries have set that threshold as an aspirational target for climate policies, the UN last year warned there now appeared to “no credible pathway” to keep within that bound.
Is climate change pushing us toward La Niña or El Niño?
That was something still being debated among climate scientists.
“But what we can say is that, over recent decades, there’s been a trend toward more La Niña episodes,” Noll said.
“So, yes, while it’s very much still up for discussion, regardless of what the long-term climate change guidance suggests, this is what we’ve already seen from the past.
“That’s not such a good thing for New Zealand, because unlike many parts of the globe, we tend to see warmest conditions during La Niña.”
Many climate scientists are also suggesting an emerging La Niña skew, including the University of New South Wales’ Professor Matthew England, who commented on our three-year run in Nature last year: “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.
Broadly, he said, we could expect extreme rainfall under La Niña to grow yet more intense, while droughts under El Niño would become more severe.
But as for climate change potentially tipping Enso’s balance, Renwick still considered the jury to be out.
It also wasn’t clear whether either El Niño or La Niña – typically lasting up to a year or two – would linger longer over time.
“Whether or not we see more prolonged events in the future? Maybe,” Renwick said.
“But there’s just not enough evidence in either direction.”
Boxing Day shoppers in Rotorua and Tauranga are happy to be spending.