FILE: Waves depicted over a rough sea.

El Niño has sent a deep-sea wave from the tropics into California waters.

Unlike waves that crash over the beach, this so-called Kelvin wave traveled underwater, lifting the sea surface by up to a foot. 

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The Kelvin wave arrived in San Francisco Bay in recent days, contributing to local flooding. 

“I do think it made it to San Francisco,” Dillon Amaya, an assistant professor of marine, earth and atmospheric sciences at North Carolina State University, told SFGATE. “It originated in the western equatorial Pacific in August, propagated along the equatorial strip, ran into South America, moved up the Central American and Mexican coastlines, entered the Gulf of California and moved up the U.S. West Coast.” 

This year’s El Niño, which the National Oceanic and Atmospheric Administration already ranks as “very strong” and forecasts will likely surpass the strength of all previous recorded events, is driving the wave. During El Niño conditions, winds that usually blow westward........

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