How Kelvin Waves Reach California: El Niño’s Hidden Ocean Motion

An oceanic Kelvin wave carries a change in water level and the structure of water below the surface. It can cross the Pacific and continue along a coast without forming a breaking crest.

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A wave you will not see breaking

The video begins with a broad ocean disturbance, rather than a tall wall of water. Its horizontal extent can reach thousands of kilometres. Much of its structure lies beneath the surface.

The opening comparison gives San Diego a September sea-level anomaly near 21 centimetres. An anomaly means a difference from a stated reference value. It does not mean a wave with that crest height reaches the beach.

The later gauge section explains the reference period and preliminary data behind that comparison. It also separates a total sea-level anomaly from the contribution of one Kelvin wave.

The trade winds and the warm pile

Trade winds usually push warm surface water west across the equatorial Pacific. Warm water accumulates toward Indonesia. The warm layer becomes thicker in the western ocean than in the eastern ocean.

The animation shows a tilted boundary beneath the warm layer. This represents the ocean's temperature structure, rather than a rigid underwater surface. Wind maintains part of that difference across the basin.

When the trade winds weaken, the ocean can adjust. Bursts of wind toward the east can help generate an eastward disturbance. This process connects a change in atmospheric winds with a change far below the sea surface.

What happens under the surface

The thermocline is a depth range where water temperature changes rapidly. It separates relatively warm upper water from colder deep water. Real temperature changes occur through a layer, rather than across the drawing's sharp line.

A downwelling Kelvin wave deepens the thermocline as it passes. Downwelling means a downward movement or displacement of upper water. The surface can rise slightly while the warm layer thickens much more below it.

NOAA's October 8, 2026 discussion reports subsurface temperature anomalies above 10°C at some depths. That comparison concerns temperature relative to the usual value at those depths. It does not describe the entire ocean warming by 10°C.

The connection to El Niño involves both the ocean and atmosphere. Changes in winds move heat and alter the thermocline. The changed ocean can then affect atmospheric circulation.

How fast it travels

NOAA gives a typical speed near 6 miles per hour, or about 10 kilometres per hour. A crossing of the tropical Pacific takes roughly 2–3 months in this explanation.

This is the speed of the disturbance. It does not mean one parcel of water travels from Indonesia to California at that speed. Waves transfer a changing pattern through water while individual parcels follow their own motions.

The basin's great width explains the long travel time. A slow change at one coast can therefore connect to a wind event far away and much earlier.

Why it stays near the equator

Earth's rotation produces the Coriolis effect. This effect deflects motion to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

For eastward motion north of the equator, the deflection points south. For eastward motion south of the equator, it points north. Both directions point toward the equator.

The diagram uses these arrows to explain equatorial confinement. Pressure differences and Earth's rotation form a balanced wave structure. The equator is not a physical channel, and the Coriolis effect itself reaches zero exactly on it.

Why this equatorial wave travels east

An equatorial Kelvin wave has an eastward direction in this ocean model. A westward version does not have the same confined structure on both sides of the equator.

This rule applies to the Kelvin-wave solution. Other kinds of ocean waves can travel west. The video's opposing arrows explain a particular wave's structure, rather than a ban on westward ocean motion.

Turning along the coast

The eastward disturbance eventually reaches the American boundary of the Pacific. Part of the response continues north along Central America and Mexico. Another branch continues south along South America.

The coastline now supplies a boundary for coastal Kelvin waves. In the Northern Hemisphere, the coast lies on the right of the wave's direction. In the Southern Hemisphere, it lies on the left.

For the northern branch, this geometry permits travel toward California and farther north. The water-level change generally decreases with distance offshore. Its width depends on the ocean structure and location.

The video's narrow strip is a scale illustration, rather than a universal width. The cited NOAA account does not establish one fixed width for every coastal branch.

Reading NOAA tide-gauge anomalies

A tide gauge records water level at a fixed coastal station. To calculate a monthly anomaly, compare that month's mean with an appropriate reference mean.

The video's calculation uses the mean for the same calendar month during 1990–2025. This compares September with previous Septembers, rather than with a winter month. The calculation therefore accounts for the usual seasonal pattern.

Recalculation from the saved NOAA records gives these values. September uses preliminary measurements at six-minute intervals, rather than a final monthly product.

These values describe total departures from the chosen reference. They include effects beyond a Kelvin wave, such as winds and other ocean changes. The gauge alone cannot assign the full anomaly to one cause.

Comparing sea-level records

Long-term sea-level rise changes the starting point for a modern event. A direct comparison with an older high-water period mixes that trend with shorter changes.

The video removes a straight-line trend fitted to the reference years. The saved records give trends near 3 millimetres per year at San Diego and 2 at San Francisco. These fitted rates describe this calculation and time range.

After that adjustment, San Diego's September 2026 anomaly is about 15.9 centimetres. The comparison gives about 17.9 centimetres for November 1997 and 16.9 for October 2015.

Thus the selected modern value approaches those older peaks but does not exceed them after this adjustment. Different reference periods or later quality checks can change the comparison. The preliminary September value must retain its preliminary label.

How elevated sea level changes risk

A higher background sea level gives tides and storm effects a higher starting point. Water can reach a coastal threshold more easily when that starting point rises. Local elevation and timing determine the actual flooding.

The change does not require a breaking Kelvin-wave crest. It can persist over weeks or months and alter the conditions in which ordinary tides occur.

California's September 11, 2026 memo discusses possible El Niño winter impacts. It connects high water, storms, and coastal exposure. El Niño can favour wetter conditions in southern and central California, but does not determine each storm.

The video's statement that elevated water alone cannot flood is too broad. Sufficiently high water can cross a local threshold even without a major storm. A regional ocean explanation is still not a local flood forecast.

Recap

The process follows a connected sequence from wind to coastal water level.

1. Changing winds disturb the equatorial Pacific's warm-water layer.
2. An eastward Kelvin wave carries a change in the thermocline and sea surface.
3. Earth's rotation helps keep the disturbance near the equator.
4. Coastal branches continue along the American boundary.
5. Higher background water can increase the effects of tides and storms.

What this means

A Kelvin wave can have important coastal effects without looking like a surf wave. Gauge data help measure the water-level change, but cannot identify a single cause without other evidence.

FAQ

Does the whole ocean move from Indonesia to California?

No. A wave transfers a disturbance. Individual water parcels do not follow the entire wave path.

Is the September anomaly a measured Kelvin-wave height?

No. It is a total monthly departure from the stated reference, with several possible influences.

Do all ocean waves travel east?

No. The eastward rule here concerns equatorial Kelvin waves.

Does this article predict flooding at a particular beach?

No. Local forecasts, tides, weather, and coastal height determine local risk.