What to Know About the Kelvin Wave Raising Sea Levels Along the West Coast

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

Kelvin waves are not like tsunamis, and they do not roll up and crash onto the beach. A better image is water flowing into the bathtub or swimming pool during an earthquake. In the ocean, they are bulges of water that raise sea levels as they move along the coast. They are 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 rise will move northward along the California coast at about 6 miles per hour (9.6 kilometers per hour), or 150 miles per day (241 kilometers per day), causing sea levels to rise toward Oregon and Washington. Experts predict that the waves will continue to pass from October to February.

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

Instead, it raises the baseline.

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

Waves also increase ocean temperatures and push cold, nutrient-rich waters deeper below the surface, impacting marine life.

Here’s what you need to know about Kelvin waves.

How is a Kelvin wave formed?

Kelvin waves are a “reality” during an El Niño, a natural climate phenomenon involving warming of 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.

WATCH: How a Super El Niño Fuels Extreme Weather

Winds blowing from east to west along the equator push the waters of the Pacific Ocean westward toward Asia, Jacox explained. When these winds weaken or reverse direction, this mass of water will be pushed eastward along the equator until it reaches South America.

When rising waters reach the coastline at the equator, they move north and south along the North American and South American coasts.

The wave is not visible to the naked eye and will not cause boats to move up and down, Amaya said. It’s more of a big pancake. It’s wider than it is deep: The bulge extends 12 to 20 miles (19 to 32 kilometers) off the coast and about 200 meters deep, Amaya said.

What are the potential impacts of Kelvin waves?

Many coastal communities are already grappling with the impacts of sea level rise, such as coastal erosion eating away at San Clemente Beach, California. The waves are also getting bigger, experts say.

Rising sea levels could exacerbate these problems.

“People are nervous,” Amaya said. “They’re thinking about more flooding and more beach erosion. So when you hear high risk of coastal flooding in the sea state, I think that causes a lot of anxiety.”

Higher sea levels, combined with larger-than-average waves from North Pacific swells expected during El Niño, as well as 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.

WATCH: Ocean at the Doorstep: Coastal Communities Fight Erosion

Overall, the largest El Niño events of 1982-83, 1997-98 and 2015-16 caused the worst erosion ever recorded on West Coast beaches, Vitousek said.

“The loss of protective beaches also increases the impact of waves and coastal flooding on properties and infrastructure,” he said.

Things could intensify over time as new 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 of them already arrived in May or June and there is evidence of a third Kelvin wave forming, he said. Experts expect this to continue along the coast over the coming months.

How are states responding to concerns about sea level rise?

There are concerns the waves could worsen problems along the Southern California coast, said Alyssa Napuri, deputy chief of staff for Orange County Supervisor Katrina Foley. Already, waves are approaching an oceanfront railroad track and beachfront homes were damaged in last month’s swells linked to Hurricane Marie.

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

“We are already in trouble,” Napuri said.

California Gov. Gavin Newsom, a Democrat, declared 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 “act more quickly to protect communities, put critical resources in place and strengthen preparedness before severe weather conditions intensify,” the governor said in a statement.

Experts with the Washington State Department of Natural Resources are prepared to respond to any disaster that may occur with Kelvin waves, said spokeswoman Courtney James. They coordinate regularly through the state’s Division of Emergency Management, but they haven’t been asked to do any special preparation so far, she said.

The actual damage caused by Kelvin waves will depend on the storms and tides amplified by El Niño, and if they are stronger, the damage could be significant, Amaya said.

“It’s too early to know whether this particular winter season will be a record-breaker in terms of an El Niño, but this El Niño itself is certainly on track to become one of the largest, if not the largest, El Niños we’ve ever seen,” he said.

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