Kelvin Wave From a Strong El Niño Is Raising West Coast Sea Levels, With More Coming
The deep ocean wave added six to eight inches before Tropical Storm Rachel's surge flooded Southern California in early October, and scientists expect another by late October or early November.
One of the first major signs of this year's strong El Niño has reached the United States. NPR reports that a Kelvin wave, a deep ocean wave that travels east along the equator and then up the coast, is raising sea levels along the West Coast, and scientists say more are on the way.
Kelvin waves have been understood for well over a century and are named for the British physicist Lord Kelvin, who described them in the late 1800s. They form during El Niño years, when a vast region of the Pacific becomes much warmer than normal. Part of that warm water moves slowly east beneath the surface until it hits South America, then splits, running north along the West Coast and south as far as Chile. "It's like an enormous bathtub sloshing around," said Mark Cane, a Columbia University professor emeritus who built one of the first El Niño forecast models. "But the slosh is very peculiar, because it only goes west to east along the equator in a Kelvin wave."
This one is not a wall of water. "It's not a tsunami, it's not a wave you can surf," said Dillon Amaya, an assistant professor of atmospheric sciences at North Carolina State University. "It's a planetary-scale wave." Its effect is gradual: it lifts sea levels as it moves up the coast, which raises the odds of flooding, coastal erosion and unusually warm water.
By itself a Kelvin wave does not flood anything, Amaya said. It loads the dice. "It makes the background conditions more favorable towards coastal flooding, but what you need are triggering factors," such as a big storm or very high tides. NASA Jet Propulsion Laboratory climate scientist Josh Willis described a possible "triple whammy" on the California coast: El Niño, high tide and storm surge arriving together.
That is what happened in early October. Sea levels in Southern California were already six to eight inches above normal because of the Kelvin wave, Amaya said. Then Tropical Storm Rachel hit, and its storm surge flooded neighborhoods and left some oceanfront homes partly submerged. "That storm surge was almost certainly made more devastating because of the Kelvin wave," Amaya said.
The warm water has also upset marine life. Tropical fish have turned up where they are not normally seen, and a whale shark was spotted off San Diego, Amaya said. The wave can push down the cold, nutrient-rich water that feeds productive ecosystems, and researchers have already seen evidence of seabirds dying as coastal waters warm.
A strong El Niño year usually brings five to six Kelvin waves, Amaya said, and he expects the next to reach San Diego in late October or early November, with several more through fall and early winter. El Niño typically peaks late in the year or in January, and NOAA researchers predict it will stay strong through early 2027.
