Kelvin Wave and El Niño Raise California Flood Risk

A massive underwater band of warm water is moving toward California, expected to raise sea levels significantly by early October. This Kelvin wave, driven by El Niño, could lift coastal sea levels by 5 to 10 inches, with some scientists warning of up to a foot of rise in the San Francisco Bay Area by the end of next month. The wave is nearing southern California and will reach the Bay Area by early October, setting the stage for a severe winter flood season.

How the wave affects sea levels

The Kelvin wave does not transport water like a tsunami but creates conditions for higher sea levels to linger. It does not crash like a typical wave but swells tides from beneath the surface. This phenomenon changes direction when it reaches South America, traveling north and south along the coastline after moving east across the Pacific.

Amplified risks during high tides

When the Kelvin wave coincides with very high tides or storm surges, its effects are amplified, increasing flood potential. High tides, called spring tides, will push above 6 feet around Oct. 11 and peak on Oct. 27 at 6.68 feet along San Francisco. Climate scientist Daniel Swain said coastal flooding is practically certain. If rising waters coincide with rainstorms, the phenomenon could swallow beaches, erode bluffs, and submerge streets, homes, and businesses. Storms peaking during high tides are far more likely to cause damage. Research oceanographer Mike Jacox noted, "But when there are very high tides or storm surges, their effects can be amplified."

Broader El Niño context

The current El Niño is strengthening and linked to historic patterns that brought severe winter storms and flooding to California. It is marked by warming in the Pacific Ocean's surface layer. When tropical trade winds weaken, warm water sloshes toward the Pacific Coast. This pattern is fueling destructive weather globally. A 'super' El Niño is brewing and could become the most severe ever recorded, increasing the likelihood that higher sea levels will overlap with severe storms. The highest tides of the year will occur in November and December, further increasing water levels during California's typically wet months. Daniel Swain pointed to the winter of 1997-1998 as having similar hazards.

That season saw heavy storms cause massive coastal erosion, landslides that buried highways, and huge waves sweeping into harbors. Levees failed north of Sacramento and seawalls were overwhelmed. Relentless rains damaged thousands of homes and businesses, leading to disaster declarations in twenty-one counties. Surface temperatures in the Pacific are warmer now than before the 1997 El Niño onset.

Vulnerability and preparation

The San Francisco Bay Area faces heightened risk due to dense population and critical infrastructure near current sea levels, prompting emergency preparations. A huge amount of people and infrastructure sits within 6 feet of sea level. The first few inches of elevation in southern California have already caused enough erosion to close the Amtrak line along the coast. Governor Gavin Newsom has declared a state of emergency to prepare for winter storms and coastal flooding. Road clearance equipment is being positioned, and the National Guard will be ready for search and rescue. Across the Bay Area, sandbags are being supplied as emergency managers secure infrastructure. The Kelvin wave's influence is expected to persist through winter and into early next year. High tides around Thanksgiving and Christmas are expected to be particularly high this winter.