The surge, known as a Kelvin wave, can lift coastal sea levels by several inches to more than a foot, scientists say. On its own, it may cause minor flooding. But if it coincides with high tides, powerful winter storms and the ongoing rise in sea level driven by climate change, the added water could inundate roads, homes and other low-lying areas.

Kelvin waves are a recurring feature of El Niño, the climate pattern associated with unusually warm Pacific Ocean surface waters. As the tropical Pacific warms, trade winds that typically push water westward can weaken or reverse. Water then shifts east across the ocean; when the pulse reaches South America, it splits and moves north and south along the coast.

The wave travels at about 6 miles per hour, or roughly 150 miles a day. It is not a visible wall of water like a tsunami, but a broad change in ocean conditions that can move up the coast and raise sea levels along the way.

“El Niño is a good opportunity to see where our vulnerabilities are throughout the state,” said Mark Merrifield, director of the Scripps Center for Climate Change Impacts and Adaptation. He called the conditions a sobering preview of risks associated with climate change.

The greatest danger comes when multiple sources of high water overlap. El Niño warms and expands ocean water, while storms can drive large waves and storm surge toward shore. King tides — the highest tides of the year — can add to the total, as can decades of rising seas.

Winter storms are especially concerning because their waves can strike the coast when sea levels are already elevated, said Michael Beck, a coastal climate resilience expert at UC Santa Cruz. The combined conditions could push water onto roads and into homes, he said.

UC climate scientist Daniel Swain said flooding in the San Francisco Bay region could be comparable to, or worse than, its most severe coastal flooding on record if several factors align. Such conditions do not all have to occur at once for flooding to happen, he said.

The threat could test a mix of formal and informal flood defenses. San Francisco has invested in a seawall, while some other communities rely on patchwork levees, said Kristina Hill, a UC Berkeley professor who studies sea-level rise. Those barriers may not hold everywhere, she warned.

Flooded streets can also become dangerous for drivers. The National Weather Service’s Bay Area office advises people not to drive through floodwater because its depth may be unknown.

Kelvin waves are a natural part of El Niño, but their effects are landing on a coast already facing higher seas. Scientists say the coming winter could reveal where existing protections and infrastructure are most exposed.