Sudden sea level rise — tied to El Niño — is about to hit the California coast

By Andrew Freedman, CNN

(CNN) — The sea level along California’s coast will increase by about a foot by mid-October due to the influence of El Niño, which is driving a large volume of water northward from offshore of Mexico and Central America.

This water is coming in the form of a coastal “Kelvin wave,” and it will mark the arrival of one of the first significant impacts of the powerhouse El Niño taking shape in the tropical Pacific. According to Dillon Amaya, an oceanographer at North Carolina State University, the wave is a clear signal that El Niño has arrived in California.

“Typically, we think of El Niño and its impact on precipitation, but some of the first indications that El Niño is really occurring are the arrival of these coastally trapped waves,” he said.

🌊 Follow the CNN Weather app for the latest El Niño impacts.

A coastal Kelvin wave is not one that a person typically sees at a beach, breaking and crashing. Instead, it’s barely perceptible to the human eye as it slithers northward from offshore of Mexico’s Baja Peninsula at about six miles per hour. It is detectable, however, using satellites that measure changes in ocean heights.

You can’t surf a Kelvin wave, but it does affect surfing. These waves are important because of their role in raising coastal sea levels, which increases the likelihood of damage from astronomical high tides and winter storms that are also affected by El Niño, scientists say. In essence, one El Niño-related phenomenon — sea level rise — is compounding the damage caused by others.

Kelvin waves in the open ocean are massive, slow-moving and slosh up and down, like water in a bathtub. Once they hit land, Kelvin waves can shift direction, moving north and south, parallel to the coast — raising ocean levels by 6 to 12 inches as they go.

The ongoing El Niño, which is characterized by much hotter-than-average sea-surface temperatures in the tropical Pacific, is forecast to be the strongest such event on record since at least 1950. El Niño temporarily increases water levels on top of long-term rising sea levels from human-caused climate change.

Water levels along the California coastline have already been increasing in other ways tied to El Niño, Amaya said, including an earlier coastal Kelvin wave that reached California in May and June. The elevated sea levels that will result from the upcoming wave will further raise water levels and put the state at greater risk of coastal flooding, he said.

These large, undulating Kelvin waves are actually part of what helps El Niño form, transporting heat from the western Pacific Ocean to its central and eastern regions, near the equator. These waves help push huge quantities of ocean heat under the sea surface, in a process known as downwelling. When they reach the central and eastern tropical Pacific, the undersea ocean heat rises, or upwells, increasing ocean temperatures there.

In about 120 days, a Kelvin wave can travel from the western Pacific to the Pacific Northwest coastline of the US, or even as far as the Gulf of Alaska, raising ocean heights as it goes.

“It’s an epic transit,” Amaya said.

In addition, sea levels are also increasing along the West Coast because the hotter waters associated with El Niño are expanding to take up more space.

“The wave would add to what is already there,” Amaya said. The total sea level height, including this latest wave, is anticipated to be 10-to-14 inches higher than normal along the California coast, he said.

Patrick Barnard, research director at the Center for Coastal Climate Resilience at the University of California, Santa Cruz, said the ocean height increase puts many coastal residents at risk of flooding.

“Because so many of our communities are built right at sea level on the West Coast, and our extreme water levels don’t vary widely, every 2 to 4 inches of sea level doubles our flood risk,” Barnard said.

Amaya noted that the background sea level rise since the preindustrial era along parts of the California coast has been about one foot, so these El Niño-related changes will double that century-long trend all at once, albeit temporarily.

“El Niño has arrived. This is some of the first evidence that El Niño is impacting the United States.”

The-CNN-Wire
™ & © 2026 Cable News Network, Inc., a Warner Bros. Discovery Company. All rights reserved.