What Is a Kelvin Wave? Scientists Watch California Flood and Storm Surge Risks as El Niño Strengthens

Scientists say a massive pulse of warm water racing beneath the Pacific could raise California's sea levels by up to 10 inches within weeks, increasing the risk of winter flooding and storm surges as El Niño strengthens.

The so‑called Kelvin wave reached the northern tip of South America in late August before moving northwards. It is expected to arrive in Southern California by early October, temporarily lifting sea levels by about 5 to 10 inches.

How a Kelvin Wave Travels

The phenomenon begins when Pacific trade winds weaken or reverse, releasing warm water that usually builds up in the western Pacific. These winds typically blow from South America towards Asia. When they ease, the water travels eastwards along the equator towards the Americas in a wave spanning thousands of miles.

Severine Fournier, a research scientist at Nasa's Jet Propulsion Laboratory, explained that the wave changes direction when it reaches South America because it cannot continue eastwards. It then travels north and south along the coastline.

That movement is not a tsunami. Swimmers and boaters may not notice it passing, although the increased water level can become more consequential when storms or high tides arrive.

Nate Mantua, a research scientist at the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center, told the Guardian that winds normally hold the warm water in place.

'When the wind stops blowing or weakens, the force that's holding that pile of water in place goes away, so that water surges or sloshes back towards California,' he said.

Estimates suggest the Kelvin wave could temporarily raise sea levels along parts of the California coast by roughly 5 to 10 inches. Researchers expect the wave's influence to persist through winter and into early next year as El Niño holds warm water along the coast.

Kelvin Wave Flood Risks

Those increases would sit on top of sea levels that Mantua said had already risen about 8 inches over the past century. The additional water would leave coastal areas more exposed when storms push seawater ashore.

Coastal erosion is another concern, as storm waves reach areas exposed by elevated water levels. Mantua said El Niño winters in the northern Pacific tend to bring more storms and big ocean waves than usual.

Mike Jacox, a research oceanographer at the same centre, said unusually high tides were expected around Thanksgiving and Christmas. Storm surges occurring at those times would add to the elevated water level.

Mantua said strong El Niño winters in 1982–83, 1997–98 and 2015–16 had helped researchers anticipate what could happen and where. He warned that rising seas and the developing event's intensity could cause greater damage than in previous major El Niño winters.

California Governor Gavin Newsom has declared a state of emergency to prepare resources for winter storms and coastal flooding. Road clearance and snow removal equipment is being positioned along highways, while the state's National Guard will be ready for search and rescue operations.

In a memo, California Coastal Commission Executive Director Kate Huckelbridge emphasised the importance of timing. 'Storms that peak during high tides are far more likely to cause damage than if the same storm peaked during low tide.'

The commission has urged homeowners and residents to inspect property vulnerable to flooding or erosion, improve drainage and address existing causes of damage before severe weather arrives.