Super El Niño's Kelvin wave could lift California sea levels by up to 30 centimetres
Researchers warn an internal Kelvin wave from this year's super El Niño could lift California sea levels by up to 30 cm once it reaches the coast. The raised water would add to winter storm surges, though the oceanographer quoted expects it to distort tides for weeks.
The Scientist · Science desk
What happened
- The wave began when westerly wind bursts in the far-western Pacific weakened the trade winds, letting warm water and raised seas move east along the equator.
- Blocked by South America, the wave split into two coastal trapped waves that travel north and south for thousands of miles at about 10 km/h.
- California Coastal Commission director Kate Huckelbridge warned coastal residents in a Sept. 11 memo that higher tides magnify storm impacts and told them to prepare for flooding.
- Governor Gavin Newsom declared a state of emergency so the state could move equipment and resources into place before storms hit, The Guardian reported on Sept. 28.
- Webb doubts this El Niño will produce more than one full wave, although a single event can generate several, with the largest usually recorded in winter.
Why it matters
- What doesn't add up contradiction The report says months while Webb says a few weeks, and the shorter window lowers the odds that a winter storm lands on the raised water.
- Who's exposed exposure Central American coasts meet the wave before California does, and Webb counts every eastern Pacific shoreline as vulnerable, while California's emergency declaration covers only one stretch of that coast.
- What it limits constraint Low-lying shoreline has less clearance even in calm weather, because the rise lifts low tides as well as high ones whether or not a storm arrives.
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The "internal" in the name is what separates this wave from the textbook kind. A Kelvin wave in general is a swell spanning thousands of miles that balances Earth's rotation, David Webb, a physical oceanographer at the UK's National Oceanography Centre, told Live Science [3]. The El Niño variety also moves the boundary between warm, less dense surface water and the colder, denser water beneath it. That layer rises and falls within about 100 metres while the surface lifts [4]. The coastal branches travel at roughly 10 km/h [7], or about 240 km a day [16].
On effect size, Webb's own figure is "tens of centimeters," about a foot [9]. Storms alone can push coastal sea level up by more than a metre, he noted, and climate change has already added at least 20 cm [14][15]. Against a one-metre surge, the wave's 30 cm ceiling [1] is an extra 30% at most. Against a larger surge the share is smaller [18].
The Coastal Commission's concern is about timing. "A key observation from past El Niño events is that the most damaging storms are often those that coincide with the highest tides," Kate Huckelbridge, the commission's executive director, wrote [11]. "Storms that peak during high tides are far more likely to cause damage than if the same storm peaked during low tide," she wrote [12]. The wave raises high tides along with low ones [19], so while it lasts, every high tide starts from a higher level.
The report and its main source differ on how long that is. Live Science's report says the wave could lift sea levels for months [1]. Webb described a shorter window. He said the most likely outcome is a period, or maybe a few weeks, when the normal pattern of tides and tidal currents is distorted, with both high and low tides higher than normal [19]. The wave may also be irregular. Its generation "will have involved changes in the winds at different locations and at different times, so it may not be a particularly clean single wave," Webb said [6].
The thing this doesn't tell you is whether those weeks will overlap the storms. The report says only that the wave will reach California "soon" [1], and the storms this El Niño is expected to bring are forecast for winter [2]. I'd treat 30 cm as the upper limit for a few weeks of raised water. Any storm that arrives inside that window starts from a higher level. A storm that peaks at high tide during those weeks is the case the commission's memo singles out [12].
What to watch
- Tide-gauge readings on the Central American coast as the wave passes, giving its measured height before it reaches California.
- The arrival date of the wave in California compared with the timing of the first major winter storms, especially any that peak at high tide.
- Whether a second full internal Kelvin wave forms this winter, an outcome Webb currently doubts.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence72
- Adoption
- Not scored
- Hype gap+15
- Incentives
- Not scored
- Confidence65
Perspective Coverage
3 publishers- Builder
- Builder 18%
- Operator
- Operator 69%
- Investor
- Investor 13%
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
A powerful Kelvin wave triggered by this year's super El Niño is moving up the Pacific coast of Central and North America and will soon reach California's shores, where it could raise sea levels by up to 1 foot (30 cm) for months, researchers warn.
- [2]
Experts are worried because the wave will likely amplify damage from severe storms, storm surges, flooding and coastal erosion that the record-shattering El Niño is expected to bring this winter.
- [3]
A Kelvin wave is a swell wave that spans thousands of miles and balances Earth's rotation, according to David Webb, a physical oceanographer at the National Oceanography Centre in the U.K.
- [4]
El Niño-generated Kelvin waves cause the boundary between warmer, less-dense surface water and colder, denser bottom layers to move up and down within about 100 m, as well as a height increase at the surface; they are therefore called internal Kelvin waves.
- [5]
Internal Kelvin waves are triggered when westerly wind bursts in the far-western Pacific weaken the trade winds, allowing warm water and elevated sea states to slosh eastward along the equator.
- [6]
"the generation process will have involved changes in the winds at different locations and at different times, so it may not be a particularly clean single wave"
- [7]
The wave cannot go farther east than the South American coast, so it splits into two coastal trapped waves that continue north and south for thousands of miles at around 10 km/h.
- [8]
The wave's influence will be in addition to the tides, regardless of whether El Niño sparks storm activity.
- [9]
Webb said tides could rise by "tens of centimeters," or about a foot.
- [10]
Kate Huckelbridge, executive director of the California Coastal Commission, wrote in a Sept. 11 memo that higher tides magnify the impacts of storms; the memo warned coastal homeowners and residents to prepare for extreme conditions such as flooding.
- [11]
"A key observation from past El Niño events is that the most damaging storms are often those that coincide with the highest tides,"
- [12]
"Storms that peak during high tides are far more likely to cause damage than if the same storm peaked during low tide."
- [13]
California is not uniquely exposed; all countries and regions bordering the eastern Pacific Ocean are vulnerable to the wave, Webb said.
- [14]
Storms alone can raise coastal sea levels by more than 1 m, Webb noted.
- [15]
Sea levels have already increased by at least 20 cm due to climate change.
- [16]
At about 10 km/h, the coastal trapped waves cover roughly 240 km per day.
- [17]
California Gov. Gavin Newsom declared a state of emergency in response to weather predictions so the state could mobilize equipment and resources before storms strike, The Guardian reported on Monday, Sept. 28.
- [18]
At its 30 cm ceiling, the wave adds at most about 30% to a 1 m storm surge; for surges larger than 1 m the share is smaller.
- [19]
"What is most likely to be seen is a period, or maybe a few weeks, when the normal [pattern] of tides and tidal currents is distorted, with both high and low tides being higher than normal,"
- [20]
Several internal Kelvin waves can occur within a single El Niño event, with the largest typically recorded in winter, but Webb said he doubts there will be more than one full wave this time.
Sources
3 independent publishers whose own reporting we read for this story.
- livescience.comPowerful 'Kelvin wave' triggered by super El Niño threatens to raise sea levels off the California coast for months
1 article · September 30, 2026
- phys.orgSea levels to rise in California as a coastal wave surges up the West Coast
1 article · October 3, 2026
- scientificamerican.comThe ‘Kelvin wave’ hitting California is a potent warning about sea-level rise
1 article · October 8, 2026
Topics and entities
Entities
- National Oceanography Centre
- Daniel Swain
- California Coastal Commission
- Séverine Fournier
- NASA Jet Propulsion Laboratory
- Gavin Newsom
- Scripps Institution of Oceanography
- Kate Huckelbridge