El Niño-driven waves and storm swells raise flood risk for Bay Area coasts

The National Weather Service warned that low-lying sections of the Bay Area could see water levels rise more than a foot and a half this week. Forecasters attribute the threat to a mix of high tides, swells from post-tropical cyclone Rachel, a deep-ocean Kelvin wave linked to El Niño, larger surface waves and already elevated sea levels caused by thermal expansion.

"Roads may be closed due to the low-lying areas being inundated with water. And if you do come around them, don’t drive around them because you don’t know the depths of the water," said Karleisa Rogacheski, a meteorologist with the weather service’s Bay Area office.

"When you have something like Rachel kicking up storm surge on top of the Kelvin wave, that’s when you get concerned," said a spokesperson identified only as Amaya. "In places like Long Beach, in Newport, some of that flooding almost certainly was exacerbated by the background conditions."

The NOAA Climate Prediction Center’s October outlook gave an 83 % chance that this year’s El Niño will become historic heading into winter. Michael Beck, professor and director of the Center for Coastal Climate Resilience at UC Santa Cruz, noted that the current Kelvin wave is likely peaking in San Francisco and that sea levels in Monterey Bay have risen about 15 inches above normal.

"We can only tell you how big the Kelvin wave itself was once it’s passed," Beck said. "But of all the El Niño components in Monterey Bay, we peaked at about 15 inches above normal."

Amaya cautioned that it is difficult to separate the Kelvin wave’s contribution from other factors in the observed 1-to-1.5-foot tide-gauge anomalies across Southern California and the Bay Area. She added that the region could face additional flooding when future storms combine storm surge with the existing Kelvin wave.

"The El Niño-driven storminess is still to come," Amaya said. "I don’t know if it’s going to reach the level of Rachel in terms of storm surge or wind-driven swells, but we expect stormier conditions later in the year."

Beck emphasized that winter storms can amplify the impact of elevated sea levels, producing coastal and inland flooding that threatens roads and homes.

"Our winter storms come straight at us with really huge waves. If they’re riding 15 inches to 20 inches when we’ve got full El Niño warming, that’s a lot of water and can hit in exactly the places where you don’t want to hit, like on roads or houses."

Scientists noted that up to six Kelvin waves can occur during an El Niño year; the current wave may have added roughly six inches to Bay Area sea levels, with another wave expected to arrive in about three weeks.