SAN FRANCISCO - Sea levels along the California coast are expected to rise by a foot as soon as late September, as El Niño-warmed tropical water pushes a slow-moving wave up the coast of North America.

The swell isn't something seen crashing at a beach but a planetary-scale process known as a Kelvin wave that is triggered by changes in wind at the equator.

"It's not a wave you can surf," said Dillon Amaya, an assistant professor of marine science at North Carolina State University. "You're not gonna be able to see it from your boat, but our instruments will see it as it passes by, like tide gages."

The coastally-trapped Kelvin wave, as it's called, originated along the equator and hit the South America coast before turning north at about 6 mph. It raised water levels about nine to 14 inches (relative to 1993-2025 averages) along the coast of Central America in late August. That surge will hit Cabo in late September and eventually reach San Francisco the first or second week of October.

The Kelvin waves won't build on each other. Rather, sea level heights and temperatures will remain elevated for weeks, gradually subsiding before the next wave hits.

Water temperatures along the California coast are expected to increase as the wave arrives, because it will weaken coastal upwelling - the process that brings cold, deep water to the ocean surface near California. That will further warm near-shore water that is already as much as 6 degrees above normal in Southern California, according to the Scripps Institution of Oceanography.

Here's how the Kelvin wave works: During El Niño, tropical trade winds frequently reverse direction and blow out of the west instead of the east, pushing warm water toward the coast of South America. When this water - which is also a foot or so higher than normal - hits the coast, its energy slithers up the Mexico coastline through the Gulf of California, along the U.S. West Coast and all the way to Alaska. This whole process takes months.

El Niño usually induces a series of coastally-trapped waves that keep water warmer and higher than normal the entire fall and winter along the California coast.

September and October tides are among the lowest of the year, so the sudden rise in sea levels probably won't cause immediate coastal flooding, but it is an ominous sign for winter.

California's highest king tides are predicted on Thanksgiving Day and Christmas Day, with another in late January. Water levels will probably rival the highest in recorded history, even without a nearby storm.

"If we get big waves coming at the same time, (flooding) is gonna be worse," said Gary Griggs, a professor of earth sciences at UC Santa Cruz.

A double whammy of king tides and storm surge would be particularly problematic.

Sea levels along the U.S. West Coast are predicted to rise 4 to 8 inches in the next three decades due to human-caused warming, according to a federal interagency report.

Already, West Coast seas have risen 9 inches to 1 foot since the late 19th century. The upcoming Kelvin wave could double that amount. (Source: The San Franscisco Chronicle)

Story Date: September 2, 2026