Growing alarm surrounds a massive oceanic phenomenon known as a Kelvin wave that could elevate sea levels along the California coastline by up to a foot, with scientists warning that a strengthening El Niño is amplifying the risk of storm surges and flooding in the months ahead.
As an exceptionally powerful El Niño takes shape across the Pacific, it triggers a fundamental shift in ocean dynamics. Kelvin waves form when the trade winds that typically blow from South America toward Asia weaken or reverse during El Niño years, unleashing a colossal, slow-moving oscillation of water across the basin.
Unlike typical surf, these waves operate on a planetary scale, stretching thousands of miles. When a Kelvin wave initiates, it transports an immense volume of warm water from the western Pacific eastward along the equator toward the Americas.
“You can track their progression … we observe elevated sea levels along the equator, and when the wave encounters the South American coast, it can no longer travel eastward,” said Severine Fournier, a research scientist specializing in ocean circulation at NASA’s Jet Propulsion Laboratory. “It deflects north and south instead.”
One such wave struck the northern tip of South America in late August and has since begun propagating up the western coast of the United States. The disturbance could reach California within days, and oceanographers project it may sustain sea levels up to a foot above normal for months as El Niño persists and traps warm water along the shoreline.
Nate Mantua, a research scientist at NOAA’s Southwest Fisheries Science Center, explained that in typical years, a deep pool of warm water accumulates in the western Pacific.
“When the winds slacken or cease, the force pinning that water mass in place disappears, allowing it to surge or slosh back toward California,” he said, adding that researchers anticipate the Kelvin wave’s influence will persist through winter and into early next year.
Mantua emphasized that Kelvin waves are a natural phenomenon occurring for millennia, tracked by agencies like NOAA since record-keeping began. However, with sea levels already rising roughly eight inches over the past century, these waves now stack additional water atop an already elevated baseline.
“We can forecast many of the impacts,” he noted, “but not all.”
This year’s El Niño is projected to rank among the strongest in at least 1,000 years. Climate metrics have already surpassed the highest temperatures ever recorded for the phenomenon, months before its expected peak.
The climate pattern also ushers in more severe storms, storm surges, flooding, and coastal erosion. For California, these effects will compound the sea-level rise driven by the Kelvin wave, significantly escalating the overall threat.
“It’s a wave unlike the ones you see at the beach. They don’t break, and you can’t really see them pass by,” said Mike Jacox, a research oceanographer at the Southwest Fisheries Science Center. “This winter, we’re expecting exceptionally high tides around Thanksgiving and Christmas. If storms coincide with those peaks, the storm surge rides on top of that elevated baseline. Effectively, you’re raising the floor for everything that follows.”
California is already mobilizing for a potentially catastrophic season.
This week, Governor Gavin Newsom declared a state of emergency, pre-positioning resources ahead of anticipated winter storms, coastal flooding, and other impacts. Road-closure and snow-removal equipment is being staged along highways; the state National Guard is preparing for potential search-and-rescue operations; and agencies are streamlining procedures to reduce bureaucratic delays during disasters.
The California Coastal Commission, which oversees land and water use along the shoreline, has urged residents and property owners to prepare immediately. Recommendations include assessing flood and erosion vulnerabilities, improving drainage, and addressing structural weaknesses before severe weather arrives.
“A key lesson from past El Niño events is that the most destructive storms often coincide with the highest tides,” wrote Kate Huckelbridge, the commission’s executive director, in a memo earlier this month. “Storms peaking during high tide are far more likely to cause damage than identical storms at low tide.”
Mantua noted that previous extreme El Niño years—1983, 1998, and 2016—have improved predictive capabilities. Yet with rising sea levels and the “truly exceptional intensity of this event,” he cautioned, “it may ultimately inflict more damage than we’ve witnessed in prior major El Niño winters.”
“Other factors are in play,” he added. “During El Niño winters in the northern Pacific, we typically see increased storminess—more frequent storms and larger ocean swells. Layer that atop elevated sea levels, and you expose more areas to the ocean’s energy and straightforward inundation.”
