Ocean waves are disturbances that transfer energy through water, often over vast distances. These periodic motions are characterized by their crests, the highest points, and their troughs, the lowest points. Waves are fundamentally categorized by the primary force that generates them and the restoring force that attempts to flatten the water surface. Understanding the ocean’s dynamic surface involves recognizing the different mechanisms that create these disturbances. This article explores seven major classifications of ocean waves, each defined by unique characteristics and origins.
Waves Generated by Surface Interaction
The most commonly observed ocean waves are generated by direct physical interaction at the air-sea boundary. Capillary waves represent the smallest of these waves, arising from gentle breezes disturbing a calm surface. Their primary restoring force is the water’s surface tension, which attempts to minimize the surface area. They are identifiable by their very short wavelengths, typically less than 1.7 centimeters.
When wind energy increases, the waves transition into larger, dominant wind waves. The drag created by wind blowing across the surface transfers energy into the water, and gravity becomes the dominant restoring force for these larger disturbances. Wind waves can grow substantially in size, with height and speed increasing in proportion to the speed and duration of the wind, and the distance over which it blows.
Sea and Swell
It is helpful to distinguish between wind waves generated locally, known as “sea,” and those that have traveled out of their generation area, known as “swell.” Sea waves are typically steep and chaotic, still actively being formed by the local wind. Swell waves are smoother, more organized, and have a long, rounded profile, carrying kinetic energy across entire ocean basins long after the originating storm has passed.
Waves Driven by Gravitational and Atmospheric Forces
Large-scale external forces acting over immense distances generate waves with extremely long periods and wavelengths. Tides are the most predictable of these, resulting from the gravitational pull exerted by the Moon and, to a lesser extent, the Sun. The alignment of these celestial bodies generates a regular, cyclical rise and fall of sea level, with wavelengths that can span half the circumference of the Earth.
Storm Surge
Storm surge is a transient, powerful phenomenon primarily driven by severe weather systems like hurricanes. The storm’s high-speed winds push a massive dome of water toward the shore, an effect exacerbated by the low atmospheric pressure at the storm’s center. Storm surge is an abnormal rise in water level above the predicted astronomical tide, and its destructive potential is realized when it combines with normal high tide to form a storm tide.
The primary difference between these two wave types lies in their generating force and resulting nature. Tides are a nearly continuous, long-period oscillation of the entire ocean basin, which can be accurately forecast years in advance. Storm surge is a sudden, short-lived elevation of water that depends on immediate meteorological conditions and coastal topography.
Waves Resulting from Subsurface Displacement
Two distinct wave types are generated by the vertical movement or rapid displacement of large volumes of water beneath the surface. Tsunamis, often incorrectly called tidal waves, are generated by sudden, violent events like seismic activity, underwater landslides, or volcanic eruptions that vertically shift the seafloor. This displacement creates a series of waves that propagate through the entire water column.
Tsunamis travel at immense speeds in the deep ocean, yet they are barely noticeable, with an open-ocean wave height of less than one meter and extremely long wavelengths. As they approach the coast, the shallowing water causes them to slow dramatically, reducing their wavelength and forcing their height to amplify rapidly, resulting in a devastating rush of water inland.
Internal Waves
Internal waves are another form of subsurface disturbance, caused by differences in density between layers of water, typically occurring along the thermocline or halocline. These waves propagate along the interface between the stratified layers, much like wind waves move along the air-sea interface. Internal waves can be generated by tidal flow interacting with underwater topography, or by surface disturbances propagating energy downward.
Tsunamis affect the full depth of the ocean column and pose a threat to coastlines, while internal waves are localized to the density interfaces and play a significant role in mixing deep ocean waters.
Standing Waves in Confined Basins
Seiches are unique because they are not progressive waves that travel across the ocean, but rather standing waves that oscillate back and forth within a confined or partially enclosed body of water. These sloshing motions are commonly observed in lakes, harbors, and bays, where they are characterized by fixed points of minimal vertical movement, called nodes, and points of maximum movement, called anti-nodes.
The disturbance that initiates a seiche is often a sudden change in atmospheric pressure or strong winds pushing water to one side of the basin. When the initial force ceases, the water attempts to return to equilibrium, but the momentum causes it to oscillate rhythmically. Seismic activity can also trigger a seiche, causing the water level to fluctuate.