Ocean waves are disturbances that transfer energy across the water’s surface, not the water itself traveling great distances. This energy is visible as the rhythmic rising and falling of the water, characterized by a crest (the highest point) and a trough (the lowest point). The forces that create these disturbances vary, ranging from the friction of wind to the gravitational pull of celestial bodies.
Generating Waves Through Wind Stress
Wind is the most common and easily observed force that generates ocean waves. The process begins with the frictional drag, or stress, created as air moves across the water’s surface. This friction transfers momentum and energy from the atmosphere to the ocean.
Initial wave formation starts with tiny, almost imperceptible disturbances called capillary waves, or ripples, which have wavelengths less than 1.7 centimeters. These ripples give the wind something to “grip” onto, creating a positive feedback loop that allows the waves to grow. As more energy is transferred, the waves transition into larger wind waves, which are classified as surface gravity waves.
The size and energy of a wind-generated wave depend on three factors: wind speed, duration, and fetch. Wind speed must exceed the wave crest’s speed for energy transfer to continue. Duration is the time the wind blows continuously, and fetch is the uninterrupted distance over open water the wind travels. When these factors are maximized, the waves absorb maximum energy, leading to a fully developed sea. Waves that move out of the generation area continue to travel as swell, which can cross entire ocean basins.
The Pull of Celestial Gravity
Tides are the ocean’s largest waves in terms of wavelength, generated by the gravitational pull of the Moon and the Sun. Although the Sun is more massive, the Moon exerts a greater tidal force because tides result from the difference in gravitational attraction across the Earth’s diameter, and the Moon is much closer. This differential gravity causes the Earth’s oceans to bulge.
The differential pull creates two tidal bulges of water: one on the side of the Earth facing the Moon, where the pull is strongest, and another on the opposite side. The far-side bulge occurs because the gravitational force is weakest there, allowing the water to be left behind as the solid Earth is pulled toward the Moon. As the Earth rotates underneath these fixed bulges, coastal locations experience the cycle of high and low tides.
The Sun’s gravitational pull contributes to the tides, either amplifying or dampening the Moon’s effect. When the Earth, Moon, and Sun align (during new and full moon phases), their combined forces produce the highest high tides and lowest low tides, known as spring tides. Conversely, when the Sun and Moon are at a 90-degree angle, their pulls partially cancel, resulting in the smallest tidal range, called neap tides.
Rapid Displacement and Tsunamis
Tsunamis are a unique class of ocean wave generated not by atmospheric or celestial forces, but by rapid, massive vertical displacement of the water column. The primary mechanism involves tectonic energy forcing a large section of the seafloor to move abruptly up or down. Submarine earthquakes, particularly those occurring in subduction zones where one tectonic plate slides beneath another, are the most frequent cause.
When the seafloor snaps upward during a large-magnitude earthquake, it acts like a giant paddle, pushing the entire column of water from the ocean floor to the surface. Underwater landslides, volcanic eruptions, and meteorite impacts can also displace enough water to trigger a tsunami. The resulting waves are not “tidal waves” and behave differently from wind waves.
In the open ocean, a tsunami is characterized by an immense wavelength, which can be hundreds of kilometers long, and a relatively small wave height, often less than a meter. This characteristic allows them to travel across ocean basins at speeds comparable to a jet airliner, often exceeding 500 miles per hour, without losing much energy. Only when a tsunami approaches shallow water does its speed decrease and its height increase dramatically.
The Restoring Forces
While generating forces initiate waves, restoring forces return the disturbed water surface to its flat, equilibrium state, allowing the wave to propagate. Without a restoring force, the initial disturbance would simply be a momentary splash. The dominant force for most common, large waves is gravity.
For larger wind waves, tsunamis, and tides, Earth’s gravity pulls the water in the elevated wave crests downward. This downward pull overshoots the equilibrium point, which causes the water to rise again, perpetuating the wave motion. These waves, due to the role of gravity as the restoring force, are generally termed gravity waves.
For very small disturbances, such as initial ripples, the dominant restoring force is surface tension. This force is the cohesion between water molecules that creates a thin, elastic “skin” on the water’s surface. These tiny, short-wavelength disturbances are known as capillary waves.