Kelvin Wave Raises SoCal Sea Levels As Tropical Storm Rachel Threatens Coast

A massive Kelvin wave appears to have reached Southern California on Friday, contributing to unusually high sea levels as powerful surf and elevated tides batter vulnerable coastal communities, according to a scientist tracking the phenomenon.

Dillon Amaya, an assistant professor of marine, earth and atmospheric sciences at North Carolina State University, said sea levels around San Diego are approximately 1.2 feet above normal, with the Kelvin wave — a surge of water that moves slowly beneath the ocean's surface — apparently accounting for about 6 inches of that increase.

"I do think the Kelvin wave has made it to San Diego/LA at this point, possibly even as far north as [San Francisco]," Amaya told Patch in an email Friday. "It's been harder to see because of noise introduced by the recent tropical storms."

The update comes as powerful surf and elevated tides batter Southern California's already vulnerable coastline. In Long Beach, several beachfront buildings have been declared unsafe or restricted, while Newport Beach officials were preparing a local emergency declaration after widespread flooding on the Balboa Peninsula.

Although the Kelvin wave may be contributing to elevated sea levels, there is no evidence establishing that it caused the damage in either community. Scientists warn, however, that higher background sea levels can amplify the effects of storms, large swells and high tides — the kind of conditions the region is bracing for this weekend as Tropical Storm Rachel approaches SoCal.

And more Kelvin waves are expected.

"I would expect another 3-4 Kelvin waves over the next several months," Amaya said. "There is already another wave along the Mexican coastline and it should reach U.S. coastal waters in the next few weeks."

What Is A Kelvin Wave?

Despite its name, a Kelvin wave is not a towering wall of water or a tsunami. It is a vast, largely invisible disturbance that travels along a coastline, temporarily raising background sea levels. According to the National Oceanic and Atmospheric Administration, these waves are associated with El Niño and can develop when changes in tropical Pacific winds push warm water eastward toward the Americas.

Once the disturbance reaches the coastline, it can travel north along the West Coast. Such waves can move at approximately 6 mph, covering about 150 miles a day. Unlike ordinary surf, Kelvin waves do not arrive as individual breaking waves. Instead, they elevate the water level over a broad area, potentially increasing the reach of tides and storm-driven swells.

"It's not that this Kelvin wave is going to come and flood anything on its own," Michael Jacox, a research oceanographer with NOAA Fisheries said.

The concern is what happens when additional water from storms, high tides or large swells arrives on top of that elevated baseline.

Has The Kelvin Wave Reached California?

Amaya believes the disturbance has reached as far north as San Francisco, although its precise location remains difficult to establish amid fluctuations caused by recent tropical storms.

His assessment marks a significant development since Patch's Oct. 2 report, when he estimated the wave was between Cabo San Lucas and San Diego and could reach Southern California within five to seven days.

"Right now sea levels around San Diego are about 1.2 ft higher than normal, which is likely partially the Kelvin wave and partially other influences such as the tropical storms and coastal warming," Amaya said Friday. "From my data, it looks like the Kelvin wave makes up something like 6 inches of that."

That does not mean the Kelvin wave alone has raised sea levels by more than a foot, or that identical increases are occurring along the entire California coast.

How Are Coastal Communities Being Affected?

The danger is especially apparent along the Long Beach Peninsula, where repeated episodes of hurricane-driven surf have stripped away protective sand and damaged oceanfront structures. According to reporting Friday by the Long Beach Post, inspectors red-tagged two buildings containing three residential units in the 6400 and 6500 blocks of Seaside Walk following damage from powerful surf and high tides.

Two other buildings have been yellow-tagged, including one damaged during Hurricane Marie in September. Waves also damaged a roughly 30-foot section of the protective bulkhead, allowing water to reach nearby homes and streets. Vehicle access was closed at 62nd Place as residents encountered flooding.

In an Oct. 8 coastal protection update, Long Beach officials said crews were reinforcing vulnerable areas with approximately 1,000 sandbags and carrying out repairs along the Peninsula. The damage follows September's destructive surf, when Hurricane Marie's swells eroded large amounts of sand, damaged the boardwalk and prompted the evacuation of approximately 20 homes.

Meanwhile, in Newport Beach, widespread coastal flooding inundated streets and properties on the Balboa Peninsula Friday, prompting officials to prepare a local emergency declaration. Six city crews were working to rebuild protective sand berms along the shoreline, while others operated pumps and opened tide valves to drain flooded streets. Officials urged residents and motorists to avoid the peninsula as high surf and elevated tides threatened further flooding.

The National Weather Service issued a coastal flood advisory for Orange County beaches through 2 p.m. Tuesday, with tides potentially reaching 6.5 to 7 feet and additional swells expected Sunday through Tuesday. Newport Beach officials were also distributing sandbags to help protect vulnerable neighborhoods.

Amaya previously told Patch that when beaches lose sand and become steeper, waves can interact with the shoreline more forcefully, allowing water to reach structures that would otherwise be protected by the beach. The flooding and erosion in Long Beach and Newport Beach have not been conclusively attributed to the Kelvin wave. Nevertheless, the damage illustrates why scientists are concerned about additional increases in background sea levels.

Could More Kelvin Waves Make Things Worse?

Amaya expects another three or four Kelvin waves over the coming months, with one already moving along Mexico's coastline and potentially reaching U.S. waters within weeks. Additional Kelvin waves could continue affecting the West Coast from October through February, overlapping with the winter storm season.

Sean Vitousek, a research oceanographer with the U.S. Geological Survey, said higher sea levels combined with large North Pacific swells and seasonal high tides could produce exceptional erosion.

Major El Niño events in 1982-83, 1997-98 and 2015-16 were associated with some of the worst beach erosion recorded along the West Coast, Vitousek said. "The loss of protective beaches also increases wave impacts and coastal flooding on property and infrastructure," he said.

The concern is not that each Kelvin wave will independently cause flooding, but that elevated sea levels could coincide with powerful storms and unusually high tides.

What Should Coastal Residents Know?

Coastal communities are already preparing for the latest round of dangerous conditions. In an Oct. 7 advisory, Long Beach warned of surf reaching 3 to 6 feet along south-facing beaches, with coastal flooding possible during high tides approaching 7 feet. The city activated sandbag distribution sites through Tuesday, Oct. 13, and urged residents in low-lying areas, particularly the Peninsula and Alamitos Bay, to prepare.

In Santa Monica, city spokesperson Cecile Oreste previously told Patch that officials were maintaining regular storm preparedness activities with heightened awareness of potentially increased coastal storm activity.

The city identified 85 properties along the northern section of Pacific Coast Highway within a mapped 100-year flood zone, meaning those properties face a 1 percent annual probability of flooding. That designation does not mean all 85 properties face an immediate threat from the Kelvin wave.

Residents can check flood-risk designations through FEMA's Flood Map Service Center, while Santa Monica provides sandbag locations and extreme-weather preparedness guidance. Residents should monitor National Weather Service coastal flood and high-surf alerts, avoid flooded roads and closed beach areas, and follow instructions from local emergency officials.

The Kelvin wave itself may pass almost unnoticed. Its effects become more consequential when other coastal hazards arrive.

As Amaya explained in an earlier interview with Patch, "Kelvin waves by themselves do not cause flooding. They raise background sea levels, which when combined with high tides, large swells, and storms can substantially increase flood risk."

The Associated Press contributed to this report.

Related:

• Kelvin Wave Closing In On CA, Scientist Warns
• Tropical Storm Rachel Threatens Southern California: What To Know
• Long Beach Ramps Up Coastal Defenses As Hurricane Polo Swells Approach: Reports