The sea level along California’s coast is projected to rise by about a foot by mid‑October as a coastal Kelvin wave — driven by the developing El Niño — pushes a large volume of water northward from offshore Mexico and Central America. According to Dillon Amaya, an oceanographer at North Carolina State University, the wave’s arrival marks one of the first significant impacts of the powerful El Niño taking shape in the tropical Pacific.
“Typically, we think of El Niño and its impact on precipitation, but some of the first indications that El Niño is really occurring are the arrival of these coastally trapped waves,”
Amaya said.
What Is a Coastal Kelvin Wave?
A coastal Kelvin wave is not a breaking wave visible at the beach. It is barely perceptible to the human eye as it slithers northward from offshore of Mexico’s Baja Peninsula at roughly six miles per hour. Satellites that measure changes in ocean height detect these waves. While you cannot surf a Kelvin wave, it affects surfing by raising coastal sea levels, which increases the likelihood of damage from astronomical high tides and winter storms also influenced by El Niño. In essence, one El Niño‑related phenomenon — sea level rise — compounds the damage caused by others.
Kelvin waves in the open ocean are massive, slow‑moving undulations that slosh up and down like water in a bathtub. Once they reach land, they can shift direction, moving north and south parallel to the coast and raising ocean levels by 6 to 12 inches as they travel.
El Niño’s Broader Impact on Sea Levels
The ongoing El Niño, characterized by much hotter‑than‑average sea‑surface temperatures in the tropical Pacific, is forecast to be the strongest such event on record since at least 1950. El Niño temporarily increases water levels on top of the long‑term sea‑level rise driven by human‑caused climate change.
Water levels along the California coastline have already been rising due to earlier El Niño‑linked Kelvin waves, including one that reached California in May and June. The upcoming wave will further elevate water levels and put the state at greater risk of coastal flooding, Amaya said.
“It’s an epic transit,”
Amaya said of the Kelvin wave’s journey. In about 120 days, a Kelvin wave can travel from the western Pacific to the Pacific Northwest coastline of the U.S., or even as far as the Gulf of Alaska, raising ocean heights along the way.
These large, undulating Kelvin waves are part of what helps El Niño form, transporting heat from the western Pacific Ocean to its central and eastern regions near the equator. The waves push huge quantities of ocean heat under the sea surface in a process known as downwelling. When they reach the central and eastern tropical Pacific, that undersea heat rises, or upwells, increasing ocean temperatures there.
In addition, sea levels are rising along the West Coast because the hotter waters associated with El Niño expand to take up more space.
“The wave would add to what is already there,”
Amaya said. The total sea level height, including this latest wave, is anticipated to be 10 to 14 inches higher than normal along the California coast.
Expert Warnings on Coastal Flooding
Patrick Barnard, research director at the Center for Coastal Climate Resilience at the University of California, Santa Cruz, said the ocean height increase puts many coastal residents at risk of flooding.
“Because so many of our communities are built right at sea level on the West Coast, and our extreme water levels don’t vary widely, every 2 to 4 inches of sea level doubles our flood risk,”
Barnard said.
Amaya noted that background sea level rise since the preindustrial era along parts of the California coast has been about one foot, so these El Niño‑related changes will double that century‑long trend all at once, albeit temporarily.
“El Niño has arrived. This is some of the first evidence that El Niño is impacting the United States.”
