A combination of El Niño-driven waves, tropical storm swells and high tides could flood Bay Area shorelines this week.
Beachgoers wade in the waves at Natural Bridges State Beach in Santa Cruz, California. Temporary sea rises and powerful ocean forces threaten flooding in the Bay Area this week, offering a glimpse of what could lie ahead this winter. (Marcio José Sánchez/AP Photo)
Tides and waves of varying magnitude this week mean low-lying parts of the Bay Area could be flooded by more than a foot and a half, according to the National Weather Service.
Along San Pablo Bay, San Francisco Bay and the Pacific Coast, forecasters expect high tides, swells from post-tropical cyclone Rachel, a deep ocean wave or Kelvin wave caused by El Niño, larger surface waves and already above-average sea levels due to thermal expansion that would cause flooding.
El Niño is marked by warming of the surface layer of the Pacific Ocean, and Kelvin waves are the distribution system for warm water associated with El Niño. As the tropical trade winds slow or reverse, ocean water slowly moves eastward, eventually hitting South America and moving north and south. This deep ocean wave, moving beneath the cold and warm layers of the ocean, travels about 10 km/h and can cause up to 30 cm of temporary flooding for weeks or months.
“Roads may be closed due to flooding of low-lying areas. And if you go around them, don’t go around them because you don’t know the depth of the water,” said Karleisa Rogacheski, a meteorologist with the weather service’s Bay Area office.
“When you have something like Rachel triggering a storm surge on top of the Kelvin wave, that’s when you worry,” Amaya said. “In places like Long Beach, Newport, some of that flooding was almost certainly exacerbated by ambient conditions.”
This fall, there is an 83 percent chance that this year’s El Niño will become “historic” as winter approaches, according to an October forecast from the National Oceanic and Atmospheric Administration’s Climate Prediction Center.
Michael Beck, professor and director of the Center for Coastal Climate Resilience at UC Santa Cruz, said the current Kelvin wave will likely peak in San Francisco starting Monday.
“We can’t tell you how big the Kelvin wave itself was once it passed,” Beck said. “But among all the components of El Niño in Monterey Bay, we peaked at about 15 inches above normal.”
Beck said he expects the Bay Area to peak at just under 15 inches. But Amaya said it’s difficult to know exactly how much of the 1 to 1.5 feet of tide gauge anomalies in Southern California and the Bay Area “come from the Kelvin wave versus other factors.”
“This is a classic problem with the climate system,” Amaya said. “It’s a noisy system, and it can be very difficult to peel back the layers to see what’s going on, but we were lucky enough to be able to see that Kelvin wave several weeks ago, and then we had the misfortune of having two tropical storms in a row between Polo and Rachel.”
“Our winter storms are coming right at us with really huge waves,” Beck said. “If they roll between 15 and 20 inches when the El Niño fully warms up and depending on the size of the Kelvin wave, that’s a lot of water and can hit exactly where you don’t want to hit, like on roads or houses.”
Amaya said up to six Kelvin waves can occur in an El Niño year. He suspects the current Kelvin wave has raised sea levels by about six inches in the Bay Area. Amaya said another Kelvin wave is moving along the coast and is currently around the Gulf of California.
“It would be about three weeks before it reached the Bay Area,” Amaya said.
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