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Ocean surface waves are generated by wind transferring energy to the water surface through friction. Wave height and energy depend on wind speed, the distance over which wind blows (fetch), and the duration of wind action. Waves are characterized by their wavelength, period, amplitude, and phase speed. Deep-water waves travel without feeling the seafloor, while shallow-water waves are affected by bottom friction and slow as they approach shore. Longer-period swells can travel thousands of kilometers from their generation region with little energy loss.
Distinguish between deep-water waves (period determines speed) and shallow-water waves (depth determines speed). Observe how wave energy spreads from storm centers as swell using wave period as a proxy for travel distance.
You already know from your study of wave properties that waves carry energy through a medium without permanently displacing the medium itself, and that waves are described by wavelength, frequency, amplitude, and speed. Ocean surface waves are a powerful application of these concepts — they are among the most energetic wave phenomena on Earth, and understanding them requires connecting your general wave knowledge to the specific physics of wind, water, and gravity.
Ocean surface waves begin when wind blows across water. The process starts small: turbulent eddies in the wind create tiny pressure variations on the water surface, producing capillary waves just millimeters long. Once these initial ripples exist, they present a rough surface for the wind to grip. Wind pushes harder on the windward face of each ripple and creates a partial vacuum on the lee side, transferring energy that makes the waves grow. As waves grow beyond a few centimeters, gravity becomes the dominant restoring force — the weight of water displaced above the equilibrium surface pulls it back down, and the resulting oscillation propagates outward. This is why ocean surface waves are classified as gravity waves.
How large waves grow depends on three factors: wind speed, fetch (the distance of open water over which wind blows uninterrupted), and duration (how long the wind has been blowing). Increase any of these and waves grow taller and longer. In a storm, the sea surface is a chaotic superposition of waves at many frequencies and directions — a state called wind sea or sea state. The total energy of this confused surface can be enormous, with significant wave heights exceeding 15 meters in extreme storms. But the waves are disorganized, with crests running in multiple directions and waves of different periods constantly interfering with one another.
The transformation from chaotic wind sea to clean swell is one of the most elegant phenomena in physical oceanography. In deep water, wave speed depends on wavelength — longer waves travel faster. This property, called dispersion, means that as waves leave the storm area, the longest-period waves outrun the shorter ones. After traveling hundreds or thousands of kilometers, the arriving wave field has been sorted by period into smooth, evenly spaced swell. A key relationship to remember is that in deep water, wave speed is proportional to wave period: a 10-second wave travels at about 15.6 m/s, while a 20-second wave travels at 31.2 m/s. This is why distant swells arrive with the longest periods first, followed by progressively shorter-period waves over the following days — a pattern that lets oceanographers trace swell back to its generating storm.
As waves approach shore and the water depth decreases below about half the wavelength, the seafloor begins to interfere with the circular orbital motion of water particles. The orbits flatten into ellipses, the wave slows down, and the wavelength shortens while energy is conserved — causing the wave to steepen. Eventually the wave becomes too steep to support itself and breaks. The depth at which breaking occurs is roughly 1.3 times the wave height. This transition from deep-water to shallow-water behavior explains why waves always appear to approach shore nearly head-on: as different parts of a wave crest encounter shallow water at different times, the inshore portions slow first, causing the entire wave front to bend — a process called refraction that aligns crests roughly parallel to the coastline.