By Isabelle Gerretsen, BBC

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.

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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.

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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.

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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.

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