AILSA CHANG, HOST:
OK, you probably have heard of El Nino, but maybe you haven't yet heard of a Kelvin wave. It's a deep ocean wave that's driving up sea levels on the West Coast here, and it's increasing the risk of coastal flooding. Ezra David Romero from KQED reports from Santa Cruz, California.
(SOUNDBITE OF WAVES CRASHING)
EZRA DAVID ROMERO, BYLINE: Michael Beck stands on a bluff along the Santa Cruz coast, looking down at what's usually a beach and one of his favorite surf spots.
MICHAEL BECK: I'm not sure I would even be out surfing on a day like today.
ROMERO: Today, the beach is underwater, and waves are crashing into rocks and nearly into homes nearby.
That one almost got us (laughter). We're just above getting wet here, maybe.
BECK: (Laughter).
ROMERO: Beck is the director of the Center for Coastal Climate Resilience at UC Santa Cruz. He's tracking the impacts of El Nino, which has already raised sea levels along the California coast. Now a related phenomenon is raising water levels even higher.
BECK: Right now, we also have a Kelvin wave creeping up the coast. That raised tide levels up to 17 inches above normal down in La Jolla over in San Diego.
ROMERO: So what is a Kelvin wave? It's a massive ocean wave driven by El Nino, where the surface of the Pacific ocean along the equator warms up. As the water warms, it slowly sloshes east. When it reaches South America, the wave splits, sending pulses north and south.
BECK: Think of it like a bath tub, where you're pushing water from one end of the bathtub to the other.
ROMERO: The wave isn't visible like a tsunami. It moves slowly under the ocean's surface.
BECK: The Kelvin wave moves at about 5 to 6 miles an hour, so you can imagine it as a brisk walk or a jog.
ROMERO: The pulse of energy can raise sea levels from a few inches to around a foot. That comes on top of temporary sea level rise caused by warm water expanding during El Nino and sea level rise driven by climate change. That high water from the Kelvin wave can last weeks to months, and Beck says it's increasing the risk of flooding and erosion along the west coast.
BECK: Once you've got that elevated sea level, then that wave energy gets to come in further and really smack right on our shorelines, right on our roads, right on the houses that we're looking at.
ROMERO: So far, the flooding from this Kelvin wave has been the worst in southern California. Further north, some beaches were entirely covered in Santa Cruz, and some roads were inundated in places like Marin County north of San Francisco. Dillon Amaya is also tracking Kelvin waves. He studies marine extremes at North Carolina State University. He says another Kelvin wave is already moving up the Mexico coastline.
DILLON AMAYA: This is not totally unexpected, but we expect, you know, five, maybe six of these coastally trapped waves over the life of the El Nino event into the winter.
ROMERO: Amaya expects the next wave to start creeping up the California coast later this month. Back in Santa Cruz, Beck says he's worried about flooding in low-lying areas this winter, especially if a big storm lines up with extra high tides on top of these elevated sea levels.
BECK: In our major storm events, these areas get completely flooded and the waves are breaking on the roads themselves.
ROMERO: This year's highest tides are set for the weeks of Thanksgiving and Christmas, which could make for a wet holiday season. For NPR news, I'm Ezra David Romero in Santa Cruz.
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