A Kelvin wave that struck the coast of South America in late August is moving north along the Pacific rim, and Live Science reports it could raise sea levels off California by up to 1 foot (30 centimeters) for months. The warning arrives as an El Niño that NOAA forecasters say may be historic in strength keeps building, and with California’s governor already under a state of emergency.
The number needs careful reading, because the physics behind it is less dramatic than the popular shorthand, and the people who study the wave say so.
A warm-water pulse born in the equatorial Pacific
Kelvin waves start at the equator when the trade winds weaken or reverse in westerly wind bursts. In May, NASA’s Sentinel-6 Michael Freilich satellite showed an earlier pulse of warm water heading east toward Peru, with sea levels more than 5.9 inches (15 centimeters) above long-term averages, as described in a NASA release featuring Josh Willis, a sea level researcher at the Jet Propulsion Laboratory and project scientist for the mission. The wave now in the news struck South America in late August and then turned poleward, trapped against the coast and travelling at roughly 6 miles per hour, according to Live Science.
The Live Science report quotes David Webb, a physical oceanographer at the National Oceanography Centre in the United Kingdom, who explained that El Niño-driven internal Kelvin waves “trigger the border between warmer, less-dense surface water and colder, denser bottom layers to move up and down within about 330 feet (100 m).” Warm water takes up more room than cold, so the sea surface at the coast sits higher while the pulse passes.
El Niño at record strength sets the backdrop
NOAA’s Climate Prediction Center put the underlying event in plain numbers in its September 10 ENSO discussion. An El Niño Advisory is in effect, sea surface temperature anomalies in the eastern equatorial Pacific have exceeded +3.0 degrees Celsius, and forecasters see a greater than 90 percent chance of a very strong event this fall and winter. For October through December they put a 75 percent chance on an event that exceeds the strength of any El Niño back to 1950. The next discussion is scheduled for October 8.
That heat is why this wave is being treated differently from the ones before it. A NOAA Fisheries feature published September 3 noted that Kelvin waves from the spring and summer reached California in May and June, and quoted Nate Mantua of NOAA’s Southwest Fisheries Science Center saying their West Coast effects would be minor compared with those expected this fall and winter.
One foot, and the months it could last
The size of the effect depends on what is being counted. NOAA Fisheries describes a single coastal Kelvin wave as producing sea level changes of about 15 centimeters, or 6 inches, which tide gauges and satellites can detect. Live Science’s figure of up to a foot is roughly double that, and the months-long duration comes from the way waves arrive in sequence during a strong El Niño rather than from one crest lingering. Dillon Amaya of North Carolina State University, quoted by Gizmodo, expects Bay Area sea levels to stay elevated until spring 2027, while UC Agriculture and Natural Resources climate scientist Daniel Swain said tides were already running a foot or more above average because of the marine heatwave, with more lift expected in October.
Webb told Live Science that observers will likely see a period, perhaps a few weeks, when the normal pattern of tides and tidal currents is distorted, with both high and low tides running higher than normal. The rise is a displacement of the whole water column, so it adds to every tide rather than replacing one.
A foot of extra water meets the highest tides
A foot of extra water at the coast is unremarkable on a calm day and consequential on a bad one. Kate Huckelbridge, executive director of the California Coastal Commission, put the risk directly: “The most damaging storms are often those that coincide with the highest tides.” A raised baseline means that the same storm surge reaches farther up beaches, cliffs and seawalls, and that high tide alone can push water into low-lying roads and parking areas.
Governor Gavin Newsom’s September 21 proclamation names Kelvin-wave sea level rise among the coastal hazards, together with flooding, erosion and large waves from Pacific hurricanes. Cal OES Director Caroline Thomas Jacobs said her agency is leading the statewide effort by “bringing state and local partners together, conducting advance planning,” and Natural Resources Secretary Wade Crowfoot said the state is fast-tracking flood-protection work.
The figure to watch is the one Amaya gave: elevated Bay Area water through spring 2027, a window that would take in every winter storm of the El Niño season. NOAA’s October 8 discussion is the next scheduled federal read on how strong the event has become.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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