+1.0 ft
Southern California tide gauges are running about a foot above the predicted tide this week (NOAA, 1–5 Oct)
6–12 in
additional sea level that a strong El Niño can hold against the California coast for months (NOAA Ocean Service, NOAA Fisheries)
>90%
chance of a very strong El Niño this fall and winter (NOAA Climate Prediction Center, 5 Oct)

Sources are linked in full at the end of this article. Nothing here is from a social-media post.

1The short version

A "massive Kelvin wave" has been in the headlines for three weeks. The phrase is real oceanography, but it has been illustrated with photographs of breaking surf, and that is not what a Kelvin wave is. A Kelvin wave is not a wave you can see, not a wall of water, and not a tsunami. It is a slow bulge of warm water, thousands of miles long, that raises the level the ocean sits at. It arrives over weeks, not in a moment, and it leaves the same way.

What it does is simple and worth taking seriously: it lifts the baseline that every tide and every swell rides on top of. A foot of extra ocean under a 7-foot high tide and a long-period south swell is the difference between a wet parking lot and a flooded one, and between a beach that holds and a beach that goes. That combination is already on the forecast for Ventura and Los Angeles County beaches this week.

2What a Kelvin wave actually is

In a normal year the trade winds blow from east to west along the equator and pile warm water up in the western Pacific. During El Niño those winds weaken and periodically reverse in what forecasters call westerly wind bursts. Each burst releases some of that piled-up warm water, which slides back east along the equator as an oceanic Kelvin wave. NOAA Fisheries describes the next step: after reaching Central and South America, the energy turns and travels north as a coastally trapped wave along the North American coast, past Baja, Southern California, Central California and on toward the Pacific Northwest.

Michelle L'Heureux of NOAA's Climate Prediction Center put it this way years ago, and it has not been improved on: unlike the waves at the beach, Kelvin waves do not curl over and crash. They are more like the water in a bathtub, which slowly sloshes. They have broad peaks and valleys that change the depth of the water, and the ocean's word for that depth is sea surface height.

Dillon Amaya, a marine scientist at North Carolina State University who has studied these events, described the coastal version to ABC7 as a bulging of the nearest 20 to 30 kilometres of ocean that propagates up the coastline. There is no mass of water travelling with it. What travels is a signal, and the signal is an elevated sea state.

NASA satellite map of the Pacific showing a band of higher-than-normal sea surface height (red) along the equator reaching the coast of South America, 8 June 2026
Figure 1. Sea surface height, 8 June 2026. Red is higher-than-normal sea level, which in the tropics means warmer, expanded water. The band along the equator and the warm water banked against Ecuador and Peru are the Kelvin waves that NASA's Sentinel-6 Michael Freilich satellite tracked crossing the Pacific from March onward. By May the sea near Peru was running close to six inches above normal. Image: NASA Earth Observatory / Lauren Dauphin, using Copernicus Sentinel-6 data processed by NASA JPL (public domain).

3Where it is right now

Two sets of government instruments track this, and both are public. The first is NOAA's array of equatorial buoys and satellites, which the Climate Prediction Center summarises every week. Its briefing of 5 October 2026 lists four warm (downwelling) Kelvin waves launched this cycle, in December 2025, January, March and June 2026, and shows subsurface water in the eastern Pacific running 6 to 8 °C above normal at depth, and the Center's September discussion noted anomalies in excess of +10 °C. That is the reservoir the coastal waves draw on.

NOAA chart of equatorial Pacific upper-ocean heat anomalies by longitude and month, November 2025 to September 2026, with dashed lines marking four warm Kelvin waves travelling east
Figure 2. The Kelvin waves themselves. Each row is a week, each column a longitude along the equator, and colour is upper-ocean heat relative to normal. The dashed lines sloping down to the right are the warm Kelvin waves moving east toward South America (the dotted lines are cool, upwelling waves from last autumn). The whole eastern basin has been deep red since spring. Source: NOAA Climate Prediction Center, ENSO status update, 5 October 2026 (public domain).

The second set of instruments is the one that matters on our beaches: NOAA's tide gauges. Every gauge publishes the water level it observes alongside the level the tide tables predicted from the sun and moon alone. The difference is everything the astronomy did not account for: the warm, expanded water of this summer's marine heatwave, weather, and now the coastal Kelvin wave. We pulled that difference for four Southern California stations since 1 June.

Line chart of observed minus predicted water level at Santa Barbara, Santa Monica, Los Angeles and La Jolla tide gauges from June to October 2026, rising from about 0.4 feet to about 1.0 to 1.1 feet
Figure 3. How much higher the ocean is sitting than the tide tables say. Daily mean of observed minus predicted water level, 7-day running mean, at NOAA stations Santa Barbara (9411340), Santa Monica (9410840), Los Angeles (9410660) and La Jolla (9410230). The four gauges averaged about +0.4 ft in June, climbed through September, and from 1 to 5 October have sat between +1.0 and +1.1 ft. The gauges record the rise; they do not say what caused it, and the marine heatwave is part of the answer. Data: NOAA Tides & Currents, retrieved 5 October 2026. Chart: Pacific Safety Group.

That lines up with what the people who study this have been saying. Daniel Swain of UC Agriculture and Natural Resources noted on 29 September that tides were already running a foot or more above average because of the marine heatwave, with more elevation likely as the largest coastally trapped Kelvin wave of the sequence so far arrives in October. The National Weather Service in San Diego noted in an early-September coastal flood advisory that actual tide levels had been running roughly six inches higher than predicted. Amaya expects three or four more coastal waves through the fall and winter and does not expect levels to settle until spring 2027.

4What it means on the sand

A foot of water does not sound like much until you remember that coastal flooding is a game of margins stacked on margins. Three things stack on top of the Kelvin wave this season:

What this means if you are going in the water

The raised baseline does not make the surf bigger. It makes the surf reach further. Rip currents form in places they did not form at this tide last year. Rock jetties and low walls that are usually dry get overtopped. Sand that was safe to stand on gets undercut. Check the posted hazard before you go in, swim near an occupied lifeguard tower, and if the Weather Service has a Beach Hazards Statement or Coastal Flood Advisory up, treat that as the final word. The live board on our home page carries both.

5What it is not

Three claims are circulating that the data do not support.

It is not a surge that hits on a date

There will be no moment when the Kelvin wave arrives. It quietly raises the baseline for weeks or months. The sharp rise at the right of Figure 3 took about ten days, and it is the baseline that rose, not any individual wave.

It does not flood anything on its own

Twelve inches of additional ocean on a low October tide with no swell floods nothing. Flooding needs the stack: the raised baseline, a high tide and a swell or a storm, together. That is why the Weather Service advisories matter more than the Kelvin wave itself.

It is not the storm forecast

The Kelvin wave is about the ocean's level. Whether this winter brings the rain and wind that many people expect from a very strong El Niño is a separate question, and the historical record on that is more mixed than the headlines. We wrote that up in our El Niño analysis in September, and nothing since has changed the conclusion.

6What to watch, and where

7Sources