El Niño Could Bring ‘Disruptive Coastal Flooding’ to Bay Area This Winter

El Niño forms when tropical trade winds weaken or reverse, allowing warm ocean water near Asia to move toward the Pacific Coast. This process heats the eastern Pacific Ocean and can alter the jet stream. As a result, it can lead to a stormier winter in California. It can also disrupt the ocean’s upwelling — a natural process that drives cool, nutrient-rich water to the surface — raising local ocean temperatures and affecting marine life.

The Pacific Ocean has risen by about 8 inches since the 1880s. State scientists project an additional rise of over a foot by 2050, and in worst-case scenarios, 6 feet or more by the end of the century.

“El Niño is going to temporarily elevate that baseline even further,” Swain said. “There’s significant potential that the combination of accumulated global warming plus a very strong to maybe even historic El Niño event in its own right could cause big problems.”

Swain said El Niño could temporarily raise sea levels on average by “6 inches to 2 feet in elevation for the rest of the year.” Storms and onshore winds can also raise sea levels by a foot or two.

Though that increase may not seem like a lot, combined, Swain said they could add up to a “net increase in sea level during the largest coastal flood events that’s comparable to mid-chest height on the average person.”

This level of inundation could pose a major risk when natural high tides and storms occur in tandem. Swain said elevated sea levels will be a big deal for places that routinely flood across Northern California, including coastal cities like Pacifica, San Francisco, Santa Cruz and parts of Marin County along the bayshore.

“All of a sudden, we kind of get to the point where 2 to 3 plus feet of temporary sea elevation is possible near California later this year during a major storm event and at least a foot or two the rest of the time,” Swain said. “We may see all-time record high water levels during storm events or king tides this year.”

He adds that this advance notice should prompt local governments to prepare for the coming waves and high tides, especially agencies that run low-lying highways and communities that flood during extreme high tides and storm events.

“We’re going to have problems,” Swain said. “There is some time to do some mitigation. You have several months at least before the most disruptive coastal flooding is likely to arrive.”