Ocean waves are a disturbance that moves through a medium, representing the propagation of energy across the water surface. The water itself does not travel great horizontal distances with the wave. Instead, individual water particles move in a circular or orbital motion as the energy passes by. These disturbances are generated by a diverse set of forces, ranging from gentle breezes to catastrophic underwater shifts.
The Primary Driver Wind
The most common source of ocean waves is the wind, which transfers energy to the water surface through friction. As air moves across the water, the drag creates small ripples, which provide a rougher surface for the wind to push against, intensifying the wave. The size of a wind-generated wave is controlled by three interconnected factors: wind speed, duration, and fetch.
The wind must move faster than the wave crests for energy to be continuously transferred, meaning stronger winds generate larger waves. Duration is the length of time the wind blows; a short gust will not produce the same size waves as a sustained gale. Fetch refers to the uninterrupted distance over the water that the wind blows in a single direction.
Waves created directly by the local wind are known as a “sea” and are characterized by a chaotic, choppy appearance. Once these waves travel outside their generation area, they organize into smoother, more regular patterns called “swell.” Swell waves can travel thousands of miles from their source, with longer-wavelength waves traveling faster and arriving first at distant shorelines. This organization of waves by speed is known as dispersion.
Gravitational and Atmospheric Influences
Gravitational forces create the ocean’s longest and most predictable wave systems: tides. Tides are extremely long-period waves caused primarily by the gravitational pull of the Moon and the Sun. The Moon is the dominant factor because its closer proximity results in a stronger tidal force. This interaction creates bulges of water on both the side of Earth facing the Moon and the opposite side, causing the rhythmic rise and fall of high and low tides as the Earth rotates.
Atmospheric pressure changes also influence the sea surface, generating slow-moving, long waves. Low-pressure systems, such as those found in powerful storms, cause a slight lifting or doming of the water surface. This effect contributes to a “storm surge,” which is a rise in sea level above the predicted astronomical tide. The severity of a storm surge is compounded by strong winds that push the elevated water toward the coast.
Waves Caused by Seismic Activity
A different, more sudden type of wave is the tsunami, caused by the vertical displacement of a large volume of water. The most frequent cause is a powerful underwater earthquake, typically over magnitude 7.0, involving the sudden uplifting or sinking of the seafloor. This movement abruptly pushes the overlying water column out of equilibrium, initiating a series of waves. Other triggers include large submarine landslides and volcanic eruptions that collapse into the ocean.
Unlike wind waves, which only affect the surface, a tsunami encompasses the entire water column from the ocean floor to the surface. In the deep ocean, tsunamis have extremely long wavelengths and travel at speeds exceeding 500 miles per hour. Their immense speed allows them to cross entire ocean basins with minimal energy loss, making them a significant hazard upon reaching coastal areas.
The Transformation of Waves Near Shore
As any wave approaches the coast, it undergoes a transformation known as shoaling. Shoaling begins when the wave starts to feel the friction of the seabed, typically when the water depth is about half of the wave’s wavelength. This friction causes the wave’s speed to slow down and the wavelength to shorten, while the wave period remains constant. To maintain the flow of energy, the wave height must increase, causing the wave to become steeper and taller.
If the wave approaches the shore at an angle, the portion in shallower water slows down first, causing the wave crest to bend, a process called refraction. The wave finally breaks when its crest outruns the slower-moving base, causing the wave form to become unstable. This instability occurs when the ratio of wave height to water depth reaches a certain threshold. The type of breaking wave is determined by the steepness of the seabed slope.
Gently sloping beaches typically produce “spilling” breakers, where the crest gently tumbles down the face of the wave. Steeper slopes result in “plunging” breakers, where the crest curls over to form a hollow tube before collapsing.