Southern California's coastal communities are currently assessing damage from recent waves and flooding, and now face an additional threat: a "coastally trapped Kelvin wave." This phenomenon is expected to bring elevated sea levels to an already impacted region in early October.
Homes from south Orange County to Malibu were flooded and severely damaged in recent weeks, with erosion chewing away at the coast from Carlsbad to Long Beach. Laguna Beach, Dana Point, and Carlsbad each declared local states of emergency, citing coastal erosion and sand loss attributed to a south swell from Hurricane Marie. El Niño conditions typically lead to an active hurricane season in the eastern Pacific, where Marie formed.
Laguna Beach City Manager Dave Kiff noted the widespread concern among city managers along the coast. "I think Laguna's was among the greatest, or most significant, but everybody talked about concern about sand and sand replenishment," Kiff said. Orange County officials have activated their emergency operations center, a required step when two cities in the county proclaim a local emergency. Orange County Board of Supervisors Vice Chair Katrina Foley affirmed, "Our coastal cities are experiencing serious damage, and the county is mobilizing our emergency response."
Evidence of the damage is visible, with transportation officials posting video of severe erosion jeopardizing Metrolink train tracks in San Clemente. The post described the situation as "that serious," showing strong ocean waves crashing onto the tracks and sediment crumbling below. In San Diego County, Carlsbad's local emergency declaration specifically aims to save Carlsbad Boulevard, also known as Old Highway 101, from erosion.
While Kelvin waves have been known to scientists for decades, their real-time tracking has only recently become possible due to computational advancements. Michael Jacox, a research oceanographer with the National Oceanic and Atmospheric Administration, explained that "coastally trapped" waves must follow a coastal boundary, with the shoreline to their right in the Northern Hemisphere.
Dillon Amaya, an assistant professor at North Carolina State University and an expert on these waves, noted, "we're experiencing one of the largest El Niños on record, which is going to correspond with some of the largest coastally trapped waves on record." He explained that as El Niño develops, weakening trade winds allow hot water to move east across the equatorial Pacific, pushed by equatorially trapped Kelvin waves. Upon hitting South America, these waves split, with one branch heading north toward California and becoming coastally trapped.
These waves elevate sea levels, increase ocean temperatures, and push colder, nutrient-rich waters deeper below the surface, according to Jacox. They can raise sea levels by 6 to 12 inches for potentially months. Amaya warned that these elevated sea levels, when combined with passing weather or storms, "can substantially increase flood risk and risk for things like beach erosion."
The current coastally trapped Kelvin wave is presently off the west coast of Mexico. Amaya anticipates its entry into the Gulf of California "on or around Saturday," followed by an 11-day journey along the Gulf's U-shaped coast. From the southernmost tip of Baja California, it will take an estimated seven days to reach San Diego, then another two to three days to reach Los Angeles. This timeline places the wave's arrival in Southern California in early October, though the precise timing remains uncertain.
Amaya described these waves as "pretty epic," traveling approximately 15,500 miles from the western equatorial Pacific to the Gulf of Alaska over about 120 days. Despite their long journey, this is not a surfable wave and would not be overtly noticeable to swimmers or boaters. However, it is expected to increase the sea level off the California coast by an additional 6 inches or so, with effects that could last for months.
Jonathan Warrick, a research geologist with the U.S. Geological Survey, emphasized the significance of even a 6-inch rise. It "could further bludgeon the already battered California coast, especially if combined with the effects from a storm," he stated. Warrick added that "when the water level is extra high, a foot or more higher than we expect it to be, then it can really wreak havoc." This comes as California has already experienced about a foot of sea level rise over the last 125 years due to global warming, alongside recent abnormally high tides.
