What causes ocean waves?

Most ocean waves are generated by wind dragging across the surface. Three factors determine how large they get: wind speed, duration, and fetch, meaning the uninterrupted distance over which the wind blows. Large storms with long fetch produce big waves, and those waves radiate outward and can travel thousands of miles across ocean basins, organizing into the smooth long-period swell that arrives at distant coasts days later under clear skies. That is why good surf frequently has no local weather associated with it.

The important counterintuitive point is that the water does not travel with the wave. Individual water particles move in roughly circular orbits, returning close to where they started as the wave passes, which is why a floating gull or buoy bobs up and down rather than being carried along. What moves is energy. As a wave approaches shore and the water shallows, the bottom of the orbit drags, the wave slows and steepens, and eventually the crest outruns the base and breaks.

Two other wave types work differently and are worth distinguishing. Tsunamis are generated by sudden displacement of the seafloor from earthquakes, landslides, or volcanic activity, and they involve the entire water column rather than just the surface, which is why they carry so much energy, travel at jet-aircraft speeds in deep water, and are nearly imperceptible at sea while becoming catastrophic in shallow water. They are unrelated to tides despite the outdated name tidal wave. Rogue waves, once dismissed as sailors' exaggeration, are now confirmed by instrument records and appear to arise when wave trains constructively interfere or interact with strong currents.