Sea levels off Southern California are anticipated to rise by approximately half a foot or more in the coming days due to a significant wave of ocean energy generated by the El Niño weather pattern, experts said. This elevated water level is expected to persist for months, increasing risks of coastal flooding, erosion, and saltwater intrusion.
The phenomenon driving this rise is known as a Kelvin wave, a large, subsurface pulse of warm water that travels along the coast, elevating sea levels over a swath extending 12 to 18 miles offshore. The arrival of this wave signals one of the initial clear impacts of the ongoing El Niño event, according to Dillon Amaya, an assistant professor of marine, earth and atmospheric sciences at North Carolina State University.
This development compounds existing factors that have kept sea levels off California unusually high in recent months. Warmer ocean temperatures—partly attributed to human-induced climate change—cause seawater to expand, contributing to a gradual rise in sea level globally. The timing also coincides with a naturally high tidal cycle in the region, which can amplify the impacts of elevated waters.
In addition, storms fueled by El Niño are expected to increase wave activity through fall and winter. Hurricane Polo, one of the strongest hurricanes recorded in the eastern Pacific, recently made landfall in Mexico, exemplifying the storm intensification linked to the phenomenon.
Mark Merrifield, director of the Center for Climate Change Impacts and Adaptation at Scripps Institution of Oceanography, noted that coastal flooding and overtopping generally occur when large waves arrive during high tide, a scenario increasingly likely under current conditions. Merrifield emphasized that the convergence of multiple factors makes it "highly probable" California will soon record some of its highest sea levels on record.
In response, California Governor Gavin Newsom declared a state of emergency last week to bolster preparedness ahead of the anticipated impacts. State agencies have been directed to mobilize resources such as sandbags and pumps and assist local governments with plans for coastal flooding and landslides. The California National Guard has also been placed on alert to support search and rescue operations if necessary.
El Niño, a periodic climate pattern in the Pacific, affects global weather by shifting ocean temperatures and atmospheric conditions. This year's event is forecasted to be the strongest on record, likely intensifying its influence on regional and worldwide temperature, precipitation, and storm activity.
The elevated sea levels along California’s coast are linked to changes originating in the equatorial Pacific. Under typical conditions, trade winds push warm surface waters westward toward Asia, creating a higher ocean level near Indonesia compared to South America. During El Niño, weakened trade winds allow this warm water to move eastward in the form of Kelvin waves, named after the 19th-century physicist Lord Kelvin. Unlike surface waves, these are large subsurface pulses that can extend hundreds of feet deep.
In August, a series of sustained westerly winds produced a strong Kelvin wave that reached the coast of Peru and Ecuador around early September. Restricted from moving further east by South American landmass, the wave’s energy split and traveled along the coastlines north and south. The northward pulse is projected to reach San Diego by early October and could sustain elevated sea levels for months.
Further bursts of westerly winds might generate additional Kelvin waves, perpetuating elevated sea levels along the Pacific coast into early 2024, according to Amaya.
The warm pulse also poses potential risks to marine ecosystems by increasing heat stress and suppressing the usual upwelling of cold, nutrient-rich water off the coast. This suppression may reduce food availability for marine species, potentially driving whales closer to shore and raising the likelihood of collisions with vessels or beaching incidents.