One of the first major signs of this year's "Super El Niño" has reached the U.S. It's called a Kelvin wave – a deep ocean wave that's currently raising sea levels along the West Coast.
Scientists have long known about Kelvin waves, but this year, the phenomena grabbed the public's attention with video explanations receiving thousands of views across TikTok and X.
"It does surprise me – I mean, here it's been around all this time," says Mark Cane, who created one of the first models to forecast and predict El Niño. "I'm sure Lord Kelvin is happy in his grave."
(Lord Kelvin, the British physicist who also developed the Kelvin temperature scale, first described the wave back in the late 1800s.)
Kelvin waves can develop along the equator during El Niño years, when a vast region of water in the Pacific Ocean becomes much warmer than usual.
Some of this warm water, in the form of a Kelvin wave that stretches thousands of miles, moves slowly east beneath the ocean's surface until it reaches South America. Then it splits, traveling north along the West Coast and as far south as Chile.
"It's like an enormous bathtub sloshing around," Cane says, professor emeritus of earth and climate sciences at Columbia University. "But the slosh is very peculiar, because it only goes west to east along the equator in a Kelvin wave."
These are not the massive, highly visible waves that crash into shorelines during major storms and hurricanes.
"It's not a wall of water, it's not a tsunami, it's not a wave you can surf," says Dillon Amaya, an assistant professor of atmospheric sciences at North Carolina State University. "It's a planetary-scale wave."
A Kelvin wave's impact is gradual, raising sea levels as it moves up the coast, and bringing with it an elevated risk of floods and warmer water temperatures.
What's the impact?
Kelvin waves can increase the risk of flooding and coastal erosion.
By themselves, Kelvin waves don't cause flooding, Amaya says. Instead, they load the dice, making flooding more likely by raising sea levels.
"It makes the background conditions more favorable towards coastal flooding, but what you need are triggering factors," Amaya said, such as a major storm or very high tides.
Josh Willis, a climate scientist at NASA's Jet Propulsion Laboratory, says Kelvin waves raise the chances of a "triple whammy" along the California coast: "El Nino, the high tide and the storm surge all coming at once."
That's exactly what some Southern California communities experienced in early October. Sea levels were already higher than normal because of the Kelvin wave – about six to eight inches, Amaya says. Then, when Tropical Storm Rachel hit, a storm surge flooded neighborhoods and left oceanfront homes partially submerged.
"That storm surge was almost certainly made more devastating because of the Kelvin wave," Amaya says.
Warmer water temperatures
Kelvin waves also bring warmer water up the coast, which can be disruptive and put stress on marine life.
"We've seen tropical fishes in places that we don't normally see them," Amaya says. "Like a whale shark was spotted off the coast of San Diego."
The Kelvin wave can push down nutrient-dense cold waters that support productive marine ecosystems. Amaya says researchers have already seen evidence of sea birds dying in response to warmer water temperatures along the coast.
When is the next Kelvin wave?
There are usually five to six Kelvin waves during a strong El Niño year, Amaya says.
Amaya predicts the next Kelvin wave will reach San Diego in late October or early November. Several more waves are expected through the fall and early winter, he says. Kelvin waves usually subside before the peak of El Niño.
The El Niño pattern typically peaks late in the year or early January. This year, NOAA researchers predict the El Niño will remain strong through early 2027.
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