Ocean waves are disturbances that travel through water, transferring energy from one location to another. A wave is not a massive movement of water; instead, it is a pattern of motion that propagates, much like a ripple spreading from a dropped pebble. Water particles largely move in place, demonstrating that energy, rather than water mass, is transmitted across the ocean basin. This energy originates from varied sources, including wind friction, gravitational forces, and seismic events.
Formation by Wind: The Primary Driver
Wind is the origin of most ocean waves. The process begins when moving air applies friction to the smooth water, creating tiny wrinkles known as capillary waves. These initial ripples are small, with wavelengths less than 1.7 centimeters, and their motion is governed by surface tension. As the wind continues to blow, it transfers more energy, causing the waves to grow larger. Once the wavelength exceeds 1.7 centimeters, gravity overtakes surface tension as the restoring force, transforming the ripples into gravity waves.
The size of these gravity waves is determined by three factors: wind speed, duration, and fetch. Wind speed requires the wind to move faster than the wave crest for energy transfer to continue. Duration is the length of time the wind has consistently blown over the water surface. Fetch is the uninterrupted distance over the water that the wind blows in a single direction. Strong, consistent wind over a long distance is necessary for the largest waves to form, explaining why powerful storms generate the most energetic waves.
The Physics of Wave Movement
All ocean waves share a common structure. The highest point is the crest, and the lowest is the trough. The horizontal distance between successive crests is the wavelength, and the vertical distance between a crest and a trough is the wave height.
In deep water, where the depth is greater than half the wavelength, water particles move in a circular or orbital path as the wave energy passes through. A floating object traces this circle, moving up and forward on the crest and down and backward in the trough. This orbital motion decreases rapidly with depth, becoming negligible at a depth equal to half the wavelength, known as the wave base.
When waves travel into shallower water (depth less than half the wavelength), they begin to interact with the seabed. The circular orbits of the water particles flatten into ellipses because the bottom of the wave is dragged by friction against the sea floor. This interaction fundamentally alters the wave’s behavior before it reaches the shore.
Formation by Non-Atmospheric Forces
While wind generates most surface waves, other powerful forces create waves on a much larger scale. One such force is the gravitational pull exerted by the Moon and the Sun, which causes the tides. Tides are extremely long waves, with a wavelength that can span half the globe.
The gravitational difference across the Earth creates bulges of water on both the side facing the Moon and the side opposite the Moon, through which the Earth rotates. The resulting change in sea level behaves differently from wind-driven surface waves. Tsunamis, often mistakenly called tidal waves, are generated by sudden, large-scale displacement of the ocean floor.
Tsunamis are most often caused by large, shallow earthquakes at subduction zones, where vertical movement of the sea floor displaces the entire water column. These waves have extremely long wavelengths, sometimes hundreds of kilometers long, allowing them to travel across entire ocean basins at high speeds with minimal energy loss. Because their wavelength is so long, tsunamis behave as shallow-water waves even in the deepest parts of the ocean.
The End of the Journey: How Waves Break
The final stage of a wave’s journey occurs as it approaches the coast and transforms into a breaker. This process, known as shoaling, begins when the wave starts to “feel” the seabed in shallow water. Friction with the bottom slows the base of the wave, while the crest continues to move faster.
This difference in speed causes the wave to shorten its wavelength and simultaneously increase its height, making the wave face steeper. The wave crest eventually becomes unstable when the water particle velocity at the top exceeds the speed of the wave itself. When this occurs, the wave breaks, dissipating its energy in the turbulent surf zone.
The shape of the breaking wave is determined by the slope of the seabed. On gently sloping beaches, the crest tumbles down the face gradually, forming a spilling breaker. A steeper slope causes the wave to steepen more rapidly, resulting in a plunging breaker where the crest curls over and crashes into the trough.