What Causes Waves in the Ocean? Energy Analysis and Wave Types

Ocean waves are a ubiquitous part of coastal landscapes and beach vacations. But have you ever paused to ponder where a wave originates, how far it travels, or why it forms in the first place?

A wave forms whenever energy passes through a body of water, thereby causing the water to move in a circular motion. While any number of events—including hurricanes, full moons, and earthquakes—can transfer kinetic, or motion-generated, energy onto water, it’s the wind that’s most often to blame. The type of wave that’s created depends on which of the above events initiates the wave action.

Anatomy of a Wave

When the wind blows across a smooth water surface, two things happen: Friction is created as air rubs against water, and this frictional force begins stretching the water’s surface. As the wind continually blows, the water's surface roughs up into choppy surf, then whitecaps, and then begins to stretch upward, building into a crest—a wave's highest point.

Wave Height

While the highest part of a wave is known as its crest, its bottom is called a trough. The vertical distance between the crest and the trough tells you the wave's height.

How tall a wave gets depends on the wind’s speed, duration (how long it blows), and fetch (how far it blows in a single direction). Slow wind speeds create small waves. Similarly, if winds only blow for a short while, or if they blow over a short fetch, smaller waves will result. In order for a large wave to form, all three of these factors must be great. For example, a steady 33 mph (30 knot) wind blowing for 24 hours over a fetch of 340 miles (547 km) stirs up average wave heights of 11 feet (3.3 m), according to NOAA and the book Oceanography and Seamanship.

As for how tall a wave is capable of growing, NOAA notes that while 65-foot-plus (19.8 m) "rogue" waves can occur in extreme storm conditions, such wave heights are extremely rare. During Hurricane Sandy, several ocean buoys measured individual wave heights of over 45 feet (13.7 m).

Wave Speed

How fast a wave moves depends on how deep the water its travelling in is, and what its wavelength (the distance between two successive waves) is. Waves with longer lengths generally move faster through the water.

Breakers

At the same time all of this is happening above the waterline, a turbulent column of water is also moving just beneath it. However, as a deep-ocean wave approaches shore and this shadow wave meets the shallower seafloor, its motion is interrupted. It slows down, compresses, and forces the wave's crest higher into the air. This causes the wave to become imbalanced, and the wave comes crashing down in what's referred to as a "breaking wave." As for the wave energy which started as wind energy, it dissipates into the surf.

Wave Types

Wind-driven surface waves are the most common wave types, but they aren't the only type of wave you'll find at sea.

Tidal Waves

When the moon, rather than the wind, tugs on the ocean's surface, tidal waves form. Yes, the moon's gravity actually pulls on our planet's surface. (This gravitational pull affects both land and water, but it's the more malleable water that's impacted the most.)

The type of tidal wave that forms depends on which side of Earth you're on. When your region is directly facing the moon, you'll experience rising water levels that creep inland up the beach (high tide) due to oceans bulging moonward. But when your region is farthest from the moon, sea levels will recede and shrink away from shore (low tide) because they're essentially being pulled inward toward the center of the earth.

Tsunamis

While tsunamis are sometimes called tidal waves, they're not the same thing. Although they act like tidal waves in that they run up the shore and inland, they're largely triggered by undersea earthquakes. An average of two tsunamis occur every year in the Pacific Ocean, which is the world's most seismically active ocean basin.

Storm Surge

When a hurricane's winds blow across a sea surface, gradually pushing water out ahead of it, it creates a series of long waves known as storm surge. By the time the storm nears the coast, water has “piled up” into a dome several hundred miles wide and tens of feet high. This ocean swell then travels onshore, inundating the coast and eroding beaches.