A pulse of warm water hitting the Pacific coast could mean erosion and flooding

SEATTLE (AP) — Sea levels along the West Coast will begin to rise by up to a foot this month as a series of pulses of warm water called Kelvin waves hit the shoreline and move north, sparking concerns about beach erosion and flooding in areas already impacted by climate change and El Niño.

Kelvin waves are not like tsunamis, and they don't curl over and crash on the beach. A better image is water sloshing in the bathtub or a swimming pool during an earthquake. In the ocean, they are bulges of water that lift the sea level as they move along the coast. They're common during El Niño events, but this year’s super El Niño could intensify the impacts, experts say.

Over the next week, the 6-inch to foot-tall surge will move north along the California coast at about 6 miles per hour (9.6 kilometers per hour), or 150 miles per day (241 kilometers per day), bringing higher sea levels to Oregon and Washington state. Experts predict the waves will continue to pass through from October until February.

“It’s not that this Kelvin wave is going to come and flood anything on its own,” said Michael Jacox, a research oceanographer with National Oceanic and Atmospheric Administration Fisheries’ Ecosystem Science Division.

Instead, it raises the baseline.

If you start with sea levels that are 6 inches higher than normal, when you have really high tides, big storms or storm surges, the water levels are now 6 inches higher, he said. “The effects can be amplified,” he said.

The waves also increase ocean temperatures and push cold, nutrient-rich waters deeper below the surface, which has an impact on marine life.

Here's what to know about Kelvin waves.

Kelvin waves are a “fact of life” during an El Niño , a naturally occurring climate pattern involving the warming ocean waters , said Dillon Amaya, an assistant professor in the Department of Marine, Earth, and Atmospheric Sciences at North Carolina State University.

They form when the trade winds along the equator change direction.

Winds blowing east to west along the equator push water in the Pacific Ocean west toward Asia, Jacox said. When those winds weaken or reverse direction, that bulge of water gets pushed back toward the east along the equator until it hits South America.

When the surge of water hits the coastline at the equator, it moves north and south along the North and South American coasts.

The wave isn't visible to the naked eye and they won't cause boats to go up and down, Amaya said. It's more like a big pancake. It's wider than it's deep: The bulge extends 12 to 20 miles (19 to 32 kilometers) off the coast and about 200 meters deep, Amaya said.

Many coastal communities are already grappling with the impacts of rising sea levels, like the coastal erosion eating away at the beach in San Clemente, California. Waves are also getting bigger , experts say.

The higher sea levels could exacerbate those problems.

“People are on edge,” Amaya said. “They think about more flooding happening, more beach erosion so when you hear elevated sea states coastal inundation risk, I think that raises a lot of anxiety.”

The higher sea level combined with larger than average waves from North Pacific swell, which are expected during the El Niño , plus large spring tides from October to February, can cause exceptional erosion on the West Coast, said Sean Vitousek, a research oceanographer with the U.S. Geological Survey.

Generally, the largest El Niño events of 1982-83, 1997-98 and 2015-16 caused West Coast beaches to experience their worst erosion on record, Vitousek said.

“The loss of protective beaches also increases wave impacts and coastal flooding on property and infrastructure,” he said.

Things could intensify over time as more waves arrive over the winter.

Typically El Niños are associated with five or six of these waves, Amaya said. During the last major El Niño in 2015-2016 there were five Kelvin waves.

One already came through in May or June and there’s evidence of a third Kelvin wave forming, he said. Experts expect it to continue up the coast over the next month or so.

There's concern the waves could worsen challenges along the Southern California coast, said Alyssa Napuri, deputy chief of staff for Orange County Supervisor Katrina Foley. Already, waves are approaching a seaside rail line and oceanfront homes were damaged during swells last month tied to Hurricane Marie .

Authorities are placing emergency sand on beaches as quickly as possible, Napuri said, adding that forecasts show the Kelvin Wave could send water levels between six to 12 inches higher.

“We’re already struggling as it is,” Napuri said.

California Gov. Gavin Newsom, a Democrat, proclaimed a state of emergency in late September to prepare the state for what will likely be a historic El Niño season. The proclamation will help the state “move faster to protect communities, stage critical resources, and strengthen readiness before severe weather intensifies,” the governor said in a statement.

Experts with the Washington state Department of Natural Resources are ready to respond to any disasters that might come with the Kelvin waves, said spokesperson Courtney James. They regularly coordinate through the state Emergency Management Division but they have not been asked to do any special preparation so far, she said.

Actual damages from the Kelvin waves will depend on the storms and tides magnified by El Niño, and if they're stronger, the damage could be significant, Amaya said.

“It’s too too early to be sure whether this particular winter season will be a record breaker in terms of this El Niño, but this El Niño itself is certainly on that trajectory of being one of the largest if not the largest El Niño we’ve ever seen," he said.

Associated Press reporter Amy Taxin contributed to this story from Santa Ana, California.