Months out from its projected peak, El Niño is already helping push ocean temperatures into record territory.
Author
- Kevin Trenberth
Distinguished Scholar, National Center for Atmospheric Research; Affiliate Faculty, University of Auckland, Waipapa Taumata Rau
July brought the highest global sea-surface temperatures ever recorded for that month, partly fuelled by the fast-developing climate pattern. The latest forecasts suggest there is much more to come.
So, what exactly is El Niño, why is this one potentially so significant and what could it mean for New Zealand? Here are five things to know.
El Niño begins with a huge shift in ocean heat
El Niño is a natural climate pattern involving unusual warming of the central and eastern tropical Pacific Ocean, accompanied by changes in winds and atmospheric circulation.
Here’s how the process normally works. The sun heats the tropical ocean, while trade winds push warm surface water westward across the equatorial Pacific, causing it to pile up around Indonesia, northern Australia and Southeast Asia. Cooler water rises from below in the eastern Pacific.
This build-up means sea levels in the western Pacific “Warm Pool” average about 60cm higher than in the eastern Pacific off South America. Without the trade winds holding it there, that warm water would flow back eastwards. But there is a limit to how much can build up.
Exactly how an El Niño gets underway varies, with weather and tropical storms sometimes weakening the tradewinds and providing the trigger. Once it begins, warm water starts surging back eastwards across the Pacific.
It can take two to three months to cross the ocean, shifting thunderstorms and rainfall as it goes and influencing weather patterns far beyond the tropical Pacific.
El Niño events typically occur every two to seven years, most commonly every three to five. Sea temperatures typically peak around December, with the biggest atmospheric effects occurring around February the following year.
This El Niño will be one for the record books
By December 2025, the tropical Pacific was already primed for an El Niño. Westerly wind bursts associated with tropical storms then helped set things in motion, with subsequent surges reinforcing warming towards South America.
Signs of a major event emerged early this year. Former NIWA meteorologist Ben Noll raised the prospect in February , and by July was warning it could become a very strong – or “super” – El Niño.
Current forecasts suggest the event will continue strengthening towards the end of the year, with sea-surface temperatures in key parts of the tropical Pacific reaching more than a record 3°C above average.
It will push global temperatures even higher
El Niño is unfolding against a backdrop of human-caused warming, with global ocean heat content reaching record levels in every year from 2017 to 2025.
A 1997-98 event contributed to 1998 becoming the warmest year of the 20th century. The 2015-16 El Niño pushed temperatures to another level, while records were reset again during the 2023-24 event.
Strong El Niño events release some accumulated ocean heat into the atmosphere, temporarily boosting global surface temperatures. Global temperatures can therefore resemble a staircase , with major El Niños helping push them onto successively higher steps.
This developing El Niño could provide the next step, with record-high global temperatures possible in 2026 and even more likely in 2027.
Exactly how climate change is affecting El Niño remains uncertain. But there are indications warming may be altering the phenomenon itself, potentially increasing the risk of very strong events.
Its impacts will reach far beyond the Pacific
El Niño has profound consequences worldwide, influencing droughts, floods, storms and heatwaves, as well as agriculture, health, fisheries, wildfire risk and economic activity.
Some effects are already emerging . Dry conditions have disrupted crops in parts of Southeast Asia and Central America, while the UN World Food Programme estimates El Niño could leave nearly 50 million more people facing acute hunger by year’s end.
Much of El Niño’s global reach comes from its influence on rainfall. Exceptionally high sea surface temperatures in the tropical Pacific shift rainfall towards the ocean, often favouring drought on land. These changes alter atmospheric circulation and weather patterns much further afield, while increasing wildfire risk in dry areas.
Changes in temperature and rainfall can affect diseases such as malaria, dengue and cholera. El Niño also shifts tropical cyclone activity, typically increasing it in central and eastern parts of the Pacific while suppressing it in the tropical Atlantic.
Weakened upwelling off South America also reduces the supply of cold, nutrient-rich water, disrupting marine ecosystems and potentially causing dramatic declines in fisheries and seabird populations.
The economic effects can linger for years, with the potential to cause trillions of dollars in losses.
New Zealand could be in for a dry year
New Zealand is already experiencing some effects of El Niño, including the relatively sunny but cold southerly conditions seen through much of this winter.
Unlike many parts of the world, New Zealand often experiences cooler conditions during El Niño.
Strongest effects typically emerge later in the year. Across the Tasman, it can contribute to drought and wildfire risk in much of Australia, with smoke plumes from fires occasionally reaching New Zealand.
El Niño tends to bring drier conditions to northeastern parts of New Zealand, particularly the North Island, while the southwest – including Fiordland and the West Coast – can become wetter.
Given the potential strength of this event, there is a real risk New Zealand could experience a so-called ” dry year “, with consequences for agriculture and hydroelectric generation, and an increased risk of wildfires.
Farmers and others exposed to these risks should prepare for the possibility of drought as conditions develop.
