Two giant warming patterns are brewing in the Pacific and warping US weather

A rare but powerful combination of an immense Pacific Ocean heatwave and a "super El Niño" are set to have profound effects on US weather and marine life this autumn. Here are five changes to expect.

In the north-east Pacific Ocean, an immense and persistent heatwave is wreaking havoc on marine life and weather patterns.

"It's a really bright red eyeball in the Pacific," says Andrew Leising, a research oceanographer at the US National Oceanic and Atmospheric Administration (Noaa). "It's huge, it stretches across the whole [ocean]." The Pacific heatwave, which formed in May 2025, is already bringing surface temperatures 3-4C (5.4-7.2F) above average for this time of year, according to Leising.

Add to that: a massive warming pattern known as El Niño is brewing further south. El Niño is a weather phenomenon that pushes up global temperatures, and it is also supercharging Pacific Ocean warming. It is expected to intensify this autumn to become the strongest El Niño in living memory.

This combination of a massive marine heatwave and a "super El Niño" is rarely seen. Here are five impacts we can expect to see – and where they will happen.

California, which has been in the grips of a marine heatwave since December, is bracing for more storm surges and flooding over the autumn and winter due to warmer coastal waters. Coastal water levels are already 15cm (5.9in) above mean sea level, says Patrick Barnard, research director of the Center for Coastal Climate Resilience at the University of California, Santa Cruz. During this El Niño, that already high sea level will soar further still: to at least 15-30cm (5.9-11.8in) higher than normal, says Barnard.

"Every 5-10cm [1.9-3.9in] of mean sea level rise doubles the flooding frequency for most coastal communities along the US West Coast, so with all these expected sea level increases peaking this winter, we will have historically high flood risk," he says.

Warmer ocean surfaces also evaporate more heat and moisture into the air, which can cause heavier rainfall, says Leising. This can lead to more atmospheric rivers – huge, invisible ribbons of water vapour in the sky – in the coming months. These raise the risk of heavy inland flooding and mudslides in California too.

Further north in the US and in Canada, the combined impact of El Niño and marine heatwaves is expected to reduce snowfall, says Larry O'Neill, professor at Oregon State University and director of the Oregon Climate Service. The last huge Pacific marine heatwave, coined "the Blob", led to a record low snowpack in the Pacific Northwest in 2015.

"It may have modified the jet stream and sent storms further north than usual. The storms we did get were much warmer than usual, so the precipitation that would usually come as snow came as rain at higher elevations," says O'Neill.

Reduced snowfall is a concern as many parts of the Pacific Northwest are currently experiencing drought conditions, he adds. "It could take multiple winters for us to recover."

Dry conditions and above-average warming in the north-west US, exacerbated by El Niño, increase the risk of wildfires across northern California, eastern Oregon, southern Idaho and Utah this autumn. This is projected to reduce air quality for millions and result in billions of dollars of economic impact.

Meanwhile in California, warmer coastal waters have already led to warmer, more humid conditions across the state over the spring and summer this year, according to Leising.

In inland California, temperatures also tend to be "a little warmer", during a marine heatwave, as the winds off the ocean pick up a little extra heat before moving away from the coast, says Nick Bond, a climatologist at the University of Washington.

Over in the Atlantic Ocean, El Niño tends to suppress hurricane development as it makes thunderstorms less likely to become hurricanes. But tropical storms in the eastern Pacific are stronger as warmer ocean waters provide additional energy, with heightened risk for Mexico.

These tropical storms can help weaken marine heatwaves during the autumn, says O'Neill. "Evaporation from the winds and the cold temperatures tends to cool [them] off."

A 2020 study by O'Neill found that strong storms fully destroyed marine heatwaves in the Northeast Pacific in a matter of days between 2013 and 2016.

The heatwave off California is already "basically cooking the animals along the coast", says Leising.

During a marine heatwave, nutrients don't rise to the ocean surface as often as they usually do and oxygen levels in the water are lower, says O'Neil. "A lot of fish and shellfish will not have enough oxygen, so they'll either die or they'll go further offshore." Fisheries are likely to feel the effects of this damage to the ecosystem, he says.

The only marine heatwave to beat the current one for size led to "really big changes in the marine ecosystem", says Bond. "The Blob", which lasted from 2013-16, stretched for 1,000 miles (1,609km) between North America and Asia.

"Warm water species thrived at the expense of the cooler water species," says Bond. This was detrimental for juvenile salmon, for marine mammals and some sea birds, he adds.

The duration of these heatwaves is a major concern, says Leising. "Marine animals will have been exposed to this warm water for a really long time over a large area, and now they're just going to roll right into the warm waters from El Niño."

Some species, such as rockfish and squid, may benefit from warmer waters, "but overall the balance is more losers as the productivity is down for the whole system", he says. "There's less food out there for everyone."

Large marine heatwaves have occurred each of the past seven years and six of these were among the biggest on record for the eastern North Pacific since records began in 1982.

The increase in these marine heatwaves is directly linked to climate change, says O'Neill. "Marine heatwaves are occurring more frequently now, because the ocean has been warming unrelentingly over the last 40 years."

More like this:

• The couple who married in typhoon floods

• How do you cool a 100F city?

• The most extreme hurricanes in history

Since the 1970s, there has been a marked acceleration in sea surface temperatures and the ocean has absorbed more than 90% of the excess heat from climate change. Today, ocean temperatures are hotter than they have ever been.

The heatwave in the northeast Pacific is "starting to become more or less a permanent feature of the climate system," says Bond.

It is only the second time in recorded history that a marine heatwave of this magnitude has occurred alongside an El Niño, says Leising, noting that the relationship between these two events is not fully understood. Scientists are still working out how climate change influences El Niño, but agree that the frequency of these events is likely to increase this century due to global warming.

Met Office forecasts suggest that sea surface temperature anomalies could exceed 3C (5.4F) later in 2026 as El Niño reaches its peak. El Niño events typically lasts 9-12 months, with this one expected to dissipate around spring.

It is unknown how long the marine heatwaves will last. "They certainly do have some real persistence to them. The Blob had legs," says Bond.

For as long as the waters of the Pacific Ocean remain excessively hot, we can expect the weather and marine life to feel the effects.

--

For essential climate news and hopeful developments to your inbox, sign up to the Future Earth newsletter, while The Essential List delivers a handpicked selection of features and insights twice a week.

For more science, technology, environment and health stories from the BBC, follow us on Facebook and Instagram.