Ocean waves are dynamic disturbances that constantly reshape the relationship between the sea and the atmosphere. These visible undulations represent a fundamental transfer of energy across the water’s surface, not a mass movement of the water itself. As a wave form travels, the water particles primarily move in a circular or orbital motion, returning nearly to their original position. Waves manifest in many forms, ranging from minor ripples to immense, slow-moving bulges, each originating from a distinct physical process.
The Dominant Force: Wind-Generated Waves
The most familiar waves are created by the kinetic energy transferred from the wind to the water’s surface. This energy transfer begins through friction and pressure differences, as the moving air creates drag on the water. Initially, tiny ripples, known as capillary waves, form. These ripples provide a rougher surface for the wind to push against, accelerating energy transfer and wave growth.
The ultimate size and power of a wind-generated wave system are governed by three primary factors. The first is wind speed, where stronger winds transfer more energy to the water. The second is wind duration, which is the total time the wind consistently blows over the surface, allowing waves to accumulate energy.
The third factor is fetch, defined as the uninterrupted distance over open water that the wind blows in a single direction. For large waves to develop, all three factors—speed, duration, and fetch—must be sufficient.
As wind waves travel away from their origin, they begin to sort themselves out through a process called dispersion. Shorter, choppier waves, often termed “chop,” are irregular and steep, generated by local winds. Conversely, the longer, more uniform waves that travel great distances are known as swell. Swell is characterized by a long period—a greater time interval between successive wave crests—giving them a smooth, rolling appearance even when local winds are calm.
Gravitational Forces and Tidal Movement
Tides are essentially giant, extremely long-period waves caused by the gravitational interaction between the Earth, the Moon, and the Sun. Tides cause the regular rise and fall of sea level along coastlines. The Moon is the dominant force because of its relative proximity to Earth, despite being much smaller than the Sun.
The Moon’s gravity pulls the water toward it, creating a bulge on the side of the Earth facing the Moon. A second bulge forms simultaneously on the side opposite the Moon. This occurs because the inertia of the water exceeds the gravitational pull, leaving the water behind as the Earth’s crust is pulled toward the Moon. These two bulges represent the high tides.
As the Earth rotates, a specific location passes through these two bulges and the two low-water areas in between, resulting in the typical pattern of two high tides and two low tides each day. The Sun also exerts a gravitational force that modifies the tidal bulges. When the Sun, Moon, and Earth align during a new or full moon, their combined gravity produces the most extreme high and low tides, known as spring tides.
Sudden Displacement: Seismic and Geological Events
A tsunami is a highly destructive wave type not caused by wind or astronomical forces. It is generated by the sudden, massive vertical displacement of a column of water throughout the ocean depth. The most common trigger is a large underwater earthquake, particularly those occurring in subduction zones where one tectonic plate abruptly slips beneath another.
This rapid vertical movement of the seafloor acts like a giant paddle, pushing the entire water column above it. Submarine landslides, volcanic eruptions, and large meteorite impacts can also cause this displacement and generate tsunamis. Unlike wind waves that only affect the surface layer, a tsunami involves the full depth of the ocean.
In the deep ocean, tsunamis travel incredibly fast, often exceeding 500 miles per hour, and their wavelength can be hundreds of miles long. This immense wavelength means that in the open sea, the wave height is often barely noticeable. As the tsunami approaches a coastline and the water depth decreases, the front of the wave slows down, but the trailing waves catch up. This causes the wave height to increase dramatically in a process called shoaling.