From Toronto to Minneapolis, Chicago to Omaha many places in North America are seeing a surge in summer heat well into fall. But one of the drivers is a weather phenomenon occurring thousands of miles away, in the Pacific Ocean.
A marine heat wave, stretching from the coast of Japan to North America, is contributing to higher temperatures—and is expected to have a crucial impact on winter forecasts.
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Here’s what to know.
What is a marine heat wave and does climate change impact it?
Marine heat waves occur when sea surface temperatures exceed 90% of typical regional temperatures, the equivalent of an increase of about one to three degrees Celsius. “That doesn’t sound like a lot when you’re over land, but when you’re over water, it makes a big difference,” says Paul Pastelok, senior meteorologist at AccuWeather.
Just as climate change is causing warmer temperatures on land, our oceans are heating up too. The 2025 Copernicus Ocean State Report found that, in the Northwest Atlantic, marine heat waves have increased in frequency and intensity over the past three decades. Large marine heat waves have occurred in the Pacific during each of the last six years—five of them were the largest heat waves on record for the eastern North Pacific since monitoring began in 1982.
“All the evidence today shows that the frequency and the scale of these marine heat waves is substantially greater in the last decade than recorded time, and all indicators point to that being a direct cause of climate change,” says Ben Halpern, director of the University of California, Santa Barbara National Center for Ecological Analysis and Synthesis.
How a marine heat wave creates a warmer fall
Though it’s a little-known phenomenon, a marine heat wave can have major impacts on weather.
The ocean plays a big part in cooling down our coasts. “The ocean serves as sort of like an air conditioner for us. As you go inland, it can be 100 degrees in the summer, but on the coast, maybe it’s only 70 degrees,” says Halpern. “The water creates humidity [and absorbs heat] that helps keep temperatures down.”
Marine heat waves can also impact the jet stream, the bands of strong wind in the upper levels of the atmosphere that help shape weather patterns. “The temperature and the speed of how this [air] flow will happen across or through the jet stream will be impacted by the temperatures coming out of the ocean,” says Frederic Bertley, president and CEO of the Center of Science and Industry. The heat pushes the jet stream further north, paving the path for unusually warm air to flow.
Scientists are still studying the full connections between marine heat waves and the jet stream, but this year’s marine heat wave was linked to the hottest summer on record in Japan and eastern China. The ocean anomaly has also been associated with extreme weather events, including droughts; warmer ocean temperatures can also generally help hurricanes develop more rapidly, and exacerbate fires, says Pastelok.
How will this impact winter weather?
The phenomenon is expected to influence our forecasts into the winter, says Pastelok—leading to more storms on the west coast. “If this marine heat wave holds in the eastern Pacific, some of these storms early on in the fall, late in early, winter season could be pretty strong and produce heavy mountain snows and heavy rainfall,” he notes.
It’s also bad news for marine life that faces sweltering temperatures. “Marine creatures don’t have the luxury of an air conditioned room,” says Halpern. “They are stuck and it can be weeks or months that they’re struggling to survive through this marine heat wave. So the impacts are much longer lasting in the ocean than they are on land.”