With international agencies having just declared the long-anticipated arrival of El Niño, climate scientists are now trying to assess how this ocean-driven pattern might play out in a fast-warming world. Science reporter Jamie Morton explains the potential implications for us - and the big uncertainties.
Weather: What Kiwis can expect from the coming El Niño
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During a La Niña event, ocean water spread 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.
This in turn churns cooler, deeper sea water up to the ocean’s surface, suppressing cloud, rain and thunderstorms in the eastern Pacific, while warming up sea temperatures in the far west.
Over the last three years, La Niña has helped build up the Western Pacific Warm Pool - which continues to be a potent source of subtropical moisture for the seemingly endless rain-makers that’ve come our way.
This animation shows how common atmospheric rivers (🟣) have been near New Zealand this year!
— NIWA Weather (@NiwaWeather) June 18, 2023
The exceptional amount of moisture plumes are connected to a reservoir of hot ocean water in the SW Pacific.
Sub-tropical moisture will (again) feature in our weather in the next week. pic.twitter.com/X82GBI4zEN
They’ve been steered here by another classic pattern of La Niña - more northeasterly winds - all while New Zealand’s south and southwest have seen drier conditions.
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Now that La Niña has all but faded, and with the first El Niño in eight years arriving, climate scientists are observing the opposite global shifts starting to play out.
These happen as 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 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.
📈 El Niño continues to develop in the equatorial Pacific Ocean & is expected to influence the climate across Aotearoa/New Zealand in the seasons to come...
— NIWA Weather (@NiwaWeather) June 9, 2023
The central equatorial Pacific is currently warming at a pace that could result in a strong El Niño event later this year. pic.twitter.com/f7pChOUODf
The influence of these changes also tended to reinforce the jet stream and make it stronger to the east, said Dr Kevin Trenberth, a distinguished scholar of National Centre of Atmospheric Research in Boulder, Colorado, and honorary academic with the University of Auckland.
“Normally it is strongest over and just east of Australia, and so that extends farther east and carries with it disturbances and storms in the atmosphere,” Trenberth said.
“Some of those are a bit like what is happening with the low pressure systems coming from the west rather than the north.
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“More generally, in winter, one expects more southwesterly prevailing flow, often with showers over New Zealand, behind cold fronts.”
Has El Niño really begun?
We’ve already seen one half of the equation come together, with ocean temperatures in a key monitoring part of the central equatorial Pacific recently bursting through the El Niño threshold.
“The second piece of the puzzle involves the atmosphere consistently responding to the warming seas, via shifting rainfall and thunderstorm activity in the tropical Pacific Ocean,” Niwa meteorologist Ben Noll said.
“We’re still waiting on that.”
Following NOAA's announcement of El Niño, here's a look at the recently released ECMWF outlook through November 📈
— Ben Noll (@BenNollWeather) June 8, 2023
Nearly 90% of ECMWF ensemble members predict a strong El Niño will develop by November 2023. pic.twitter.com/bSS5tbtR87
Based on recent and expected trends, he said El Niño might officially arrive toward the end of winter or in early spring - and the latest guidance suggested an over 80 per cent chance of the pattern lingering on through summer.
In trying to pick what might come, we could compare its current trajectory with historically strong events.
“Right now, the central equatorial Pacific Ocean is warmer than it was at the same time in 1997 and 1982,” Noll said.
“If the pace of warming were to continue, it would put 2023 in the conversation of being a strong or very strong event.”
And the stronger the El Niño, the bigger the impact it will have on the global climate system.
Wow!
— Prof. Eliot Jacobson (@EliotJacobson) June 20, 2023
The Australian Bureau of Meteorology has had both the most extreme and most accurate forecasts of the developing El Niño. But this forecast, just released, is so extreme it's hard to believe I'm seeing it.
3.0°C by October.
3.2°C by November.
Wow!https://t.co/XwGtgMurnP pic.twitter.com/t85I7ixl68
The World Meteorological Organisation (WMO) has already issued some dramatic predictions.
In at least one of the next five years, the agency gave a 66 per cent chance of the annual global surface temperature temporarily exceeding 1.5C above pre-industrial levels - a symbolic threshold the United Nations’ Paris Agreement aspired to limit warming to.
The WMO also gave a 98 per cent likelihood that at least one of the next five years would be the world’s warmest on record - while Arctic heating was predicted to be more than three times higher than the global average.
“Globally, we’re likely to hear about many of the same weather storylines that New Zealand has had in the last year: record warmth, marine heatwaves, flooding rainfall, and droughts - but probably in different places than those that experienced it in the last year,” Noll said.
“These climate fluctuations will impact agriculture, public health, water availability, and commodity markets on a global scale.”
For New Zealand, he said, one risk that came to mind was around dryness and drought later in 2023 and/or 2024.
“It certainly isn’t set in stone, but the primary sector in particular should be aware of the possibility.”
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 dry spells in 1972/73 and 1982/83.
While our most recent strong summer El Niño, in 2015-16, also brought southwesterly winds, a switch to more northerly-quarter winds later that season eased farmers’ fears of a disastrous repeat.
We also might experience stronger-than-normal westerly winds in late winter, spring, and summer, which could dry out soils, increase the risk for wildfire development and bring occasional hot air masses from Australia.
But Noll stressed that El Niño didn’t always follow a tidy script.
“While we know the average outcome of El Niño, no El Niño is average - each event comes with a unique set of climate characteristics.”
Across the Tasman, there was a risk that El Niño - coming after a years-long build-up in vegetation under La Niña - could compound the threat of major bushfires.
“It’s not out of the question that New Zealand sees wildfire smoke in its skies in the next six to nine months,” Noll said.
In the Pacific, meanwhile, El Niño could bring heavy rainfall, increased sea levels, and a coral bleaching risk to island groups near the equator - but dryness, drought, and water stress to other island groups like Papua New Guinea, the Solomon Islands, and New Caledonia.
What are the wildcard factors?
“The transition from one climate driver to another can grey a long-range forecaster’s hair,” Noll said.
That was because the tug-of-war between our previous La Niña and the incoming El Niño could linger for months, leaving us in an awkward middle ground of variable weather patterns.
Noll pointed back to the Western Pacific Warm Pool.
“Recently trends have seen the warmest water shifting to the central and eastern Pacific, but not all of it – and three years of La Niña, and the warming of the ocean through climate change, allowed a very substantial amount of warm water to accumulate,” he said.
“This has meant that La Niña has had a ‘long tail’ in a New Zealand context, with La Niña-like weather patterns persisting from late autumn into early winter.”
And even after an event was officially designated, it could take weeks to months for the impacts to materialise as the event strengthened and peaked.
If we were looking for a clear cue, Noll said a sustained atmospheric response to the warming ocean in the central and eastern equatorial Pacific would tell us we were turning a climatic corner.
“And that’s forecast for the back half of winter.”
Noll said long-range forecasters focused on tropical climate drivers like El Niño and La Niña because of their higher levels of long-term predictability, which was tied to “ocean memory”, or the persistence of ocean temperatures over long time periods.
Another such driver was what’s called the Indian Ocean Dipole (IOD).
“Much like cousins enjoy spending time with one another, the positive phase of the IOD and El Niño can ‘collaborate’ on climate patterns,” he said.
“The positive phase of the IOD involves cooler-than-average sea water to the northwest of Australia, near Indonesia, and can turn the tropical moisture tap off to large parts of Australia and even extend as far afield as New Zealand.”
Auckland’s 2019-2020 drought - the worst in a century - happened to be kickstarted by a strongly positive IOD event.
Hovering above all of these ocean-driven influencers was the giant elephant in the forecasting room: global climate change.
“We often think about how climate change is impacting our weather patterns, but I think the conversation here needs to shift one rung down the ladder,” Noll said.
“How is climate change impacting our climate drivers, like Enso, which influence the weather that we experience?
“Right now, the entire equatorial Pacific Ocean basin is warmer than average. As is much of the Atlantic.
“Parts of both oceans are experiencing temperatures that are outside the modern climatological record.”
🔥 The current state of play across the Atlantic Ocean: more like September than June...
— Ben Noll (@BenNollWeather) June 19, 2023
With the first tropical storm(s) of the season expected to form this week, the Atlantic Main Development Region (MDR) is the warmest it's been during the month of June on record.
The… pic.twitter.com/p2p8nX154M
That meant that, when climatologists looked back for historical years to compare as “analogues”, they were left with very few, if any, good matches.
“Forecasting in a warming world means that are increasingly confronted with patterns that have little historical precedent,” Noll said.
“Fortunately, we have a multitude of tools on our belt for this difficult job - the ever-improving seasonal forecast models take into account all of the excess warmth in our seas and shrinking sea ice concentration when they make their forecasts.”
Niwa was also using the data-crunching power of artificial intelligence to improve its models, such as with New Zealand’s recently-launched, first-ever drought forecaster.
“It may be put through its paces next summer.”
Is climate change more La Niña or more El Niño?
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.
But whether climate change was nudging us toward either end of the Enso scale was still being debated.
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.
As far as Victoria University climate scientist Professor James Renwick was concerned, the jury was still out.
“The current consensus is still ‘we’re not sure’ and models produce all manner of results,” Renwick said.
“Certainly, the rainfall anomalies associated with ENSO are increasing, just because that’s what’s happening with rainfall generally.”
In a major study published last month, researchers from Australia’s national science agency CSIRO suggested La Niña and El Niño events were becoming more frequently strong as our planet heated.
Again, though, Renwick said there was much uncertainty.
“There is some indication of more strong events, but I wouldn’t bet the farm on it.”
