The ocean wave that a surfer rides is a concentrated packet of energy that has often traveled thousands of miles from its birthplace. These organized, surfable waves are fundamentally different from the choppy water stirred up by a local breeze. The journey from a distant storm to a breaking wave involves a methodical transfer of energy, starting with the wind and culminating in a dramatic interaction with the seabed near the coast.
The Role of Wind in Wave Generation
The initial transfer of energy from the atmosphere to the ocean surface is driven by wind blowing over the water. Wind creates friction, known as drag, which slightly stretches the water surface, forming small capillary waves that grow into larger gravity waves. This mechanism allows the wind to push against the rougher surface, continually transferring energy to the water.
The ultimate size and power of the waves generated in a storm area, known as the “sea state,” are determined by three main factors. The first is wind speed; the wind must be moving faster than the wave crests for effective energy transfer. The second factor is the duration, the length of time the wind blows consistently over the water.
The third factor, called “fetch,” is the uninterrupted distance over which the wind blows without a significant change in direction. A strong wind blowing for a long time over a vast, open ocean area—a large fetch—will generate the largest waves. If any one of these three factors is limited, such as a strong wind over a small bay, the resulting waves will remain relatively small.
How Swell Travels Across the Ocean
Once waves leave the storm area, they are no longer dependent on the local wind and begin organizing themselves into “swell.” This transition occurs because deep-water waves of different wavelengths and periods travel at different speeds, a phenomenon called wave dispersion.
In the deep ocean, the speed of a wave crest is directly related to its period, the time interval between successive crests passing a fixed point. Longer period waves, which have longer wavelengths, travel significantly faster than shorter period waves. Consequently, as the waves propagate away from the storm center, the faster, longer-period waves outrun the slower, shorter-period waves.
This sorting process results in the arrival of orderly, smooth, and uniform wave trains at distant coastlines. These organized, long-period swells carry energy efficiently across vast oceanic distances, often traveling thousands of miles with minimal energy loss. A typical deep-ocean swell can have a period ranging from 10 to 20 seconds, representing a substantial energy packet.
The Final Transformation: Why Waves Break Near Shore
The final step in the wave’s journey occurs as the deep-ocean swell encounters the shallower water near the coast. This process, known as shoaling, begins when the water depth decreases to less than half of the wave’s wavelength, causing the wave to “feel bottom.”
As the wave travels into progressively shallower water, friction with the seabed causes the wave’s speed and wavelength to decrease. However, the wave’s period remains constant, meaning the energy flux must be conserved, forcing the wave to increase in height. The combination of slowing the wave base and increasing the wave height causes the wave to steepen, becoming unstable as its crest moves faster than its trough.
The wave eventually breaks when it reaches a limiting steepness, which typically happens when the wave height is about three-quarters of the water depth. The precise way the wave breaks is determined by the bathymetry—the shape and slope of the seafloor—of the nearshore area.
A steeply sloping seabed causes the wave to slow down rapidly, making the crest pitch forward violently in a “plunging breaker.” This is the classic hollow tube or barrel that surfers seek, often associated with long-period swell and a sudden change in water depth.
Conversely, a gently sloping seabed causes the wave to lose energy more slowly, resulting in a “spilling breaker,” where the crest tumbles down the face of the wave. These waves are less intense and break over a wider area, making them ideal for beginner surfers. A very steep beach or seawall can lead to a “surging breaker,” where the wave crest rushes up the beach face without fully breaking.