A massive pool of warm water moving toward the West Coast could raise sea levels and worsen flooding in California this winter as a powerful El Niño continues to strengthen, scientists say. The phenomenon, called a Kelvin wave, is not a breaking wave like those seen at a beach. It is a planetary-scale movement of warm water that travels east along the equator after the trade winds weaken, said Michael Wysession, a professor of geophysics at Washington University in St. Louis.
Warm water normally piles up in the western Pacific as the trade winds push from east to west, Wysession said. During El Niño, those winds weaken and the accumulated water begins moving toward South America. When it reaches the coast, it cannot continue east, so it turns north and south in what scientists call a coastal Kelvin wave.
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Wysession compared the motion to water from a hose striking a wall and spreading in both directions. The warm water moves only about 5 or 6 mph, he said, so its effects develop over weeks or months.
NASA’s Jet Propulsion Laboratory said the Sentinel-6 Michael Freilich satellite observed several Kelvin waves in 2026. One that reached waters off Peru by mid-May raised sea levels there by more than 5.9 inches, or 15 centimeters, above long-term averages. The National Oceanic and Atmospheric Administration said in September that El Niño was strengthening and had a greater than 90% chance of becoming very strong during fall and winter. The agency said coastally trapped Kelvin waves typically produce sea-level changes of about 6 inches, though local effects can vary as tides, storms and waves combine.
Even a modest rise can matter when it arrives with a high tide or strong storm, Wysession said. He compared the effect to raising a basketball court by a foot: The higher baseline makes it easier for storm surge and waves to reach roads, homes and other coastal infrastructure.
Gov. Gavin Newsom declared a state of emergency Sept. 21 to speed preparations for the 2026-27 wet season. The proclamation directed state agencies to stage flood-fighting supplies, prepare road-closure and snow-removal equipment and support local efforts to reduce flood, landslide and coastal risks.
El Niño does not guarantee a particular storm in a particular place, and its effects are not uniform across California. But NOAA said very strong events increase the chances of familiar El Niño impacts, including warmer coastal waters and wetter conditions in parts of the state.
The coastal threat comes on top of long-term sea-level rise. Warm water expands, and that thermal expansion has contributed substantially to rising seas, Wysession said. A temporary Kelvin-wave increase therefore begins from a higher baseline than similar events did decades ago.
Wysession said the warm water can also transfer heat to the atmosphere, alter circulation and influence weather far beyond the Pacific. As the water moves north, it cools and eventually loses its identity as a distinct wave.
How strong the El Niño becomes and how long its coastal effects persist remain uncertain. Forecast models allow for several outcomes, Wysession said, but the combination of elevated water, high tides and winter storms is enough to justify early preparation.
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