The wave has lifted sea levels by about 6 to 8 inches, according to Dillon Amaya, an assistant professor of atmospheric sciences at North Carolina State University. It is not a tsunami or a surfable wall of water, he said, but a broad ocean disturbance that moves gradually along the coast.

The added height can make flooding more likely when a storm surge or unusually high tide pushes water farther inland. “It makes the background conditions more favorable towards coastal flooding,” Amaya said, though another factor — such as a major storm or high tide — is needed to trigger flooding.

Southern California saw the risks converge in early October, when Tropical Storm Rachel brought a storm surge to communities already experiencing higher-than-normal sea levels. Some oceanfront homes were partly submerged. Amaya said the Kelvin wave almost certainly made the storm surge more damaging.

Kelvin waves form during El Niño when unusually warm water spreads across a large area of the equatorial Pacific. The warm water travels east beneath the ocean surface toward South America, then divides, with part moving north along the West Coast. Columbia University professor emeritus Mark Cane compared the process to “an enormous bathtub sloshing around.”

The waves also carry warmer water up the coast, which can disrupt marine ecosystems by pushing down cold, nutrient-rich water. Amaya said researchers have seen tropical fish in unusual locations, including a whale shark off San Diego, and cited evidence of seabird deaths associated with warmer coastal waters.

Amaya expects the next Kelvin wave to reach San Diego in late October or early November, with several more possible through fall and early winter. Strong El Niño years typically bring five or six waves, he said. The National Oceanic and Atmospheric Administration forecasts that this El Niño will remain strong into early 2027.