Ocean waves are a familiar and powerful display of energy transfer. A wave is fundamentally a disturbance that moves through the water, transferring energy from one location to another. While it may appear that water travels across the ocean, the water itself only moves in a localized, circular pattern. This energy transmission is driven by various forces, but most everyday waves are created by the interaction between the atmosphere and the sea surface.
Wind: The Main Driver of Surface Waves
The vast majority of waves are generated by the friction between wind and the ocean surface. As air moves across the water, it exerts pressure and drag, creating small ripples known as capillary waves. If the wind is sufficient and sustained, the surface becomes rougher, allowing the wind to push against a larger area.
These ripples grow into larger gravity waves as energy transfers from the wind to the water. Wave size is determined by three interconnected factors: wind speed, duration, and fetch. Wind speed is the most influential factor, as faster winds transfer more energy.
Fetch refers to the uninterrupted distance over open water that the wind can blow. A long fetch allows for prolonged energy transfer, resulting in larger waves and swells. When these three factors are maximized, the sea reaches a fully developed state where the waves have absorbed the maximum possible energy.
How Wave Energy Travels
Once generated, a wave propagates across the ocean as an organized energy form. The wave structure is defined by the crest (highest point) and the trough (lowest point); the distance between successive crests is the wavelength. In deep water, water particles move in nearly circular orbits rather than traveling horizontally with the wave.
This orbital motion passes energy from one water molecule to the next. A floating object, like a buoy, demonstrates this by bobbing up and down and returning to its original position as the wave passes. The diameter of these circular paths is greatest at the surface and rapidly decreases with depth.
The influence of the surface wave becomes negligible at the wave base, which is approximately half the wavelength. This means submarines or deep-sea creatures are unaffected by surface waves. The energy travels forward, but the water mass remains essentially in place.
Waves Caused by Seismic and Underwater Events
While wind creates most surface waves, some destructive waves are caused by massive, rapid displacement of water. Tsunamis are the most well-known, generated primarily by large underwater earthquakes that cause a sudden vertical movement of the seafloor. This abrupt displacement starts the tsunami wave train.
Underwater landslides and volcanic eruptions can also displace large volumes of water, creating these long-wavelength waves. Unlike wind-driven waves, tsunamis possess extremely long wavelengths, often spanning hundreds of kilometers, and travel at speeds sometimes exceeding 800 kilometers per hour in the deep ocean. In the open ocean, their small height makes them virtually unnoticeable.
Tides are also long-period waves, caused by the gravitational pull of the Moon and Sun, resulting in the periodic rise and fall of sea level.
What Happens When Waves Reach the Coast
The life cycle of an ocean wave culminates as it approaches the shoreline, entering the shallower water zone. This process, known as shoaling, begins when the water depth decreases to about half the wave’s wavelength. Interaction with the seabed causes friction, which slows the wave down and shortens the wavelength.
As the wave slows, its energy is compressed into a smaller space, forcing the wave height to increase to conserve energy. The wave becomes progressively steeper until it reaches a point of instability. Breaking occurs when the ratio of wave height to wavelength becomes too large, or when the crest’s orbital velocity exceeds the wave speed.
The type of breaking wave is determined by the slope of the seafloor and the wave’s steepness. Spilling breakers occur on gently sloping beaches, where the crest gently cascades down the face. Plunging breakers, which form the classic “tube” shape, occur on steeper beaches where the crest curls over and crashes violently into the trough.