A Kelvin wave is moving up the North American West Coast during El Niño conditions, shifting warm water and temporarily raising sea level. It is an underwater wave, not a breaker headed for California’s beaches, and its passage is not a forecast for any one community.
The added height can matter when high tides, storm surges or large waves arrive. But the wave alone does not pose much danger, the National Oceanic and Atmospheric Administration says; coastal risk depends on how it coincides with other conditions.
What is a Kelvin wave, and where is it now?
Kelvin waves are long waves that travel through the ocean rather than breaking at the shore. They often form in the western equatorial Pacific and move east along the equator, carrying warm water. After reaching South America, they can continue north and south along the coasts of the Americas as coastal-trapped waves.
They move about 6 miles per hour, or 150 miles a day, NOAA says. As they pass, they can shift warm water eastward and raise sea level and ocean temperatures along the West Coast. Paul Roundy, a professor of atmospheric science at the University at Albany, explained the process to TIME: “It pushes the warm water down deeper and produces a ridge in the ocean itself.”
On Tuesday, NOAA said one wave was moving up the North American West Coast, with one or two more in the tropics. That describes a broad ocean pattern, not the conditions at a particular California shoreline.
What coastal risks can a Kelvin wave add to?
NOAA estimates that Kelvin waves can temporarily add 6 to 12 inches to sea level. The effect can be small from shore but consequential when water is already high. Roundy told TIME that the waterline may look much the same even as sea level rises by a few centimeters.
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The wave can affect the coast and ocean in three ways:
- Higher water: A temporary rise in sea level can raise the starting point for high tides and storm surges.
- Coastal hazards: When that higher baseline coincides with tides, surges or large waves, erosion and flooding risks can increase. A Kelvin wave by itself does not mean a community will flood.
- Marine life: The waves can suppress the upwelling of cold, nutrient-rich water. Less upwelling can reduce phytoplankton growth, affecting the base of the marine food web.
NOAA also says factors beyond Kelvin waves, including marine heatwaves, contribute to elevated sea levels and ocean temperatures along the West Coast. Its outlook calls for weaker coastal upwelling and warmer-than-normal water from fall through winter, likely continuing into spring.
What does this mean for California during El Niño?
El Niño can raise the risk of strong storms, but it does not guarantee a wet winter or any particular storm, California’s Governor’s Office says. The office also cautions that local effects are unpredictable. A regional climate pattern does not specify what a particular town will face.
California is preparing for possible impacts. On Sept. 21, Gov. Gavin Newsom proclaimed a state of emergency and directed agencies to prepare communities, roads and infrastructure and to stage emergency supplies. The proclamation covers potential El Niño effects including flooding, landslides and coastal risks.
For coastal communities, the concern is the overlap of conditions: temporarily higher sea levels can compound tides, storm surges and waves. The Kelvin wave is one part of that risk picture, while the state’s preparations are aimed at potential impacts that can vary from place to place.
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