Ocean Waves

Waves continually shape and reshape beaches, and wave energy can be harnessed to generate power. Waves generated by the force of wind pose great dangers to life and human-made structures, and waves generated by seismic activity have killed thousands of people.

Causes of Waves

The waves that agitate a lake or ocean are rhythmic, vertical disturbances of the water’s surface. Their appearance may vary from a confused seascape of individual hillocks of water, each with a rounded or peaked top, to long, orderly swell waves with parallel, rounded crests. Waves involve transferring energy from place to place on the ocean’s surface. An earthquake that jolts the California coast one evening may generate a tsunami that races across the Pacific and destroys a pier in Japan the next morning. The water itself, however, does not move—the waveform, or the energy impulse, travels. The water stays where it is but oscillates as the waveform goes past.

Waves can originate in many ways. Tsunamis are shock waves resulting from a sudden water displacement by a submarine earthquake, landslide, or volcanic eruption. Shock waves are also generated when a pebble is tossed into a pond or a moving ship creates a wake. A second wave type is produced by the sun's and moon's gravitational pull, the tides that raise and lower the ocean’s surface. Tidal waves are the largest ocean waves, stretching halfway around the world as they travel up to 1,600 kilometers per hour along the equator.

Most waves originate through the action of the wind. Ordinary waves on the ocean or a lake form in this way. If there is no wind, the water surface is calm and smooth. If a slight breeze arises, the water surface becomes roughened by patches of tiny capillary waves—the smallest waves. As the wind continues to blow steadily and in the same direction, ripples appear because the surface roughness created by the capillary waves provides a vertical surface for the wind to push against. Soon, the crests of adjacent ripples are pushed together to create increasingly larger crests. This process continues as the intensity of the wind increases, with small waves steadily giving way to larger and larger ones.

Three factors determine the size of the waves ultimately produced—the wind speed. The duration, or the length of time that the wind blows in a constant direction; and the fetch, or the extent of open water over which the wind blows. A thirty-seven-kilometer-per-hour wind blowing for ten hours along a fetch of 120 kilometers will generate waves three meters high, but a ninety-two-kilometer-per-hour wind blowing for three days along a 2,400-kilometer fetch will generate waves thirty meters high, but such waves are rare.

Rather than measuring the height of individual waves, scientists use a measure known as "significant wave height," which takes the average of the highest third of waves that occur during a particular storm. The largest recorded significant wave height is nineteen meters (sixty-two feet), about the height of a six-story building, and was recorded in the North Atlantic between Iceland and the United Kingdom on February 4, 2013. It was certified as the largest significant wave height on record by the World Meteorological Organization (WMO) three years later. A record for the largest individual wave in the Southern Hemisphere was set in 2018 by a 23.8-meter (78-foot) wave in the Southern Ocean off the coast of New Zealand. The significant wave height, 14.9 meters (48.8 feet), was a record for the Southern Ocean but did not exceed the 2013 world record. The South Pacific's submarine volcano, Hunga Tonga–Hunga Haʻapai, erupted in January 2022, producing the most intense atmospheric blast ever recorded with twenty-first-century instruments. Though the resulting Tsunami was relatively small, its force extended to the Atlantic Ocean and the Medetaranian Sea.

One way to understand the motion of water particles within a wave is to analyze the direction of water movement at various places in the wave. One can do so by sitting in a boat beyond the breakers. As the forward slope of a wave crest approaches, a lifting motion is experienced, followed by a forward push as the crest passes beneath the boat. This forward push is seen when waves break at the beach, and their crests are thrown forward in a violent rush of water. Once the crest has passed, the boat is on the back slope of the wave, and now a downward motion is experienced. Next comes a backward motion as the trough passes beneath the boat. This backward motion is also in a beach’s breaker zone; after the crest has crashed forward on the beach, there follows a strong outward surge of water. This outward surge represents the backward water motion in the wave’s trough. When the preceding observations are combined, it can be seen that when a wave passes, the water particles move first up, then forward, then down, and finally back. This circular path is known as the wave orbit.

Other Wave Types

The term “heavy sea” is often encountered in descriptions of the ocean. A heavy sea results from the prolonged action of strong winds over the open ocean. The waves are large, peaked, and confused, totally lacking in orderly arrangement by size. Frequently, there is much spray in the air due to the tops being blown off the waves. A heavy sea is what one would expect to encounter in a hurricane or a violent storm. The term “swell,” in contrast, refers to waves that have moved out of the wind-generating area. As these waves approach the beach, they appear as long rows of smoothly rounded wave crests, evenly spaced at wide intervals and of uniform height. These swells have been produced by a distant storm at sea and have then moved out of the wind-generating area. As they travel outward, their original irregularities are diminished. Very little energy is lost, however, because a wave traveling at the ocean’s surface encounters very little friction.

Groups of larger swell waves will be interspersed with groups of smaller ones. Oceanographers believe that such variation is caused by two or more wave patterns traveling together across the ocean’s surface. When the crest of one wave pattern is superimposed on the crests of the other wave pattern, larger swell waves will result. This is known as “constructive interference.” When the trough of one pattern is superimposed on the crests of the other pattern, the swell waves will be smaller. This is known as “destructive interference.”

Oceanographers also recognize two major categories of wind-generated waves: deep-water and shallow-water waves. Deep-water waves travel in water with depths greater than one-half of the wavelength, and wind waves in the open ocean are generally in this category. Shallow-water waves, in contrast, travel in water so shallow that their wave orbits are affected by friction with the bottom. The shallower the water becomes, the more slowly the wave moves forward. This reduction in speed as waves approach the shoreline results in a process known as wave refraction, in which apparently straight wave crests approaching a shoreline from an angle are seen to be bent when viewed from above.

Close to the beach is the surf zone. There, the forward speed of waves is progressively slowed as the water depth decreases, and their crests become bunched more closely together. The shape of the crests changes from nearly flat to broadly arched, and there is a conspicuous increase in the height of the wave. In addition, the water in the crest of the wave begins moving faster than the water in the trough because of the friction created by the bottom, and this friction soon causes the crest to collapse in a torrent of water. The wave has “broken.” Oceanographers recognize two types of breakers. The first is a plunging breaker, in which the wave crest curls smoothly forward and over, enclosing a tubular pocket of air below. The other type is known as a spilling breaker, in which foaming water spills down the forward slope of the crest as the breaker advances. This type of breaker has no air-filled tube.

The final zone, found between the breakers and the beach, is a narrow strip characterized by the rhythmic alteration of water rushing shoreward on the beach and water sliding back out to sea. The inward rush of water is known as the swash; it is a miniature wall of foaming water filled with air bubbles. The backward flow is a thin, glistening film of water termed the backwash.

Tsunamis and Storm Surges

Two additional wave types require special mention. The first is a giant ocean wave known as a tsunami, which can be caused by submarine earthquakes, volcanic eruptions, or a landslide dumping massive amounts of debris into a body of water. In the open ocean, tsunamis behave just as any other ocean wave. They have crests and troughs that vary in height by one meter or so while the wave is still at sea. The tsunami wavelength is enormously long, however, averaging perhaps 240 kilometers between crests. Tsunamis also have astonishingly high speeds, sometimes 650 kilometers per hour or more. A tsunami does not come ashore as a plunging breaker; rather, the crest rushes in as a surge of foaming water whose motion is more indicative of a sudden rise in water level than of a typical wave.

The second special wave type is known as a storm surge. Storm surges are drastic rises in sea level accompanying hurricanes or other severe coastal storms. Several factors combine to create such a surge. One factor is the reduced atmospheric pressure that occurs in the eye of a hurricane. This reduction may allow the ocean’s surface to rise one meter or more. If, in addition, the sun and moon are aligned in such a way as to produce unusually high tides, the storm surge may rise one meter higher. A third contributing factor is the presence of strong onshore winds. In a major hurricane, these winds will push an accumulating mass of water toward the coast by preventing it from washing completely back into the sea as it normally would, increasing the impact of the storm surge. Finally, the nature of the offshore bottom plays a role. Shallow offshore bottoms permit wind to get a better “grip” on the water, raising its level higher. As a result, a hurricane that creates a four-meter storm surge along Florida’s east coast, with its deep offshore waters, would be able to raise a ten-meter storm surge on Florida’s west coast, where the bottom is flat and shallow for a distance of 160 kilometers offshore.

Study of Ocean Waves

Until the early 1940s, the principal method for studying waves was observing the sea’s surface and recording individual waves' length, height, speed, and period. Based on an analysis of wave periods, scientists determined that various ocean wave types could be arranged in an increasing size spectrum. Capillary waves were found to be the smallest ocean waves, with periods of less than 0.1 seconds. Ripples came next, with periods of 0.1 to one second. Ordinary wind waves followed, with periods ranging from one second to 1.4 minutes. Larger still were tsunamis, with periods averaging seventeen minutes, and finally, the tides, with periods of twelve or twenty-four hours.

One basic wave-measuring instrument is the tide gauge, used to study tsunamis. The tide gauge is usually mounted on a pier in the quiet waters of a harbor, where it will not be exposed to damaging surf. It consists of a float inside a vertical, hollow pipe. The float is free to rise and fall with the water level, and a continuous record is made of the float’s movement. The pipe is sealed at the bottom so that only the long-period waves associated with a tsunami can force the float to rise. In this way, the tide gauge can record the preliminary waves of a tsunami and serve as a warning of the larger waves that follow. After the destruction of Hilo, Hawaii, by a tsunami on April 1, 1946, a seismic sea wave warning system was set up for the Pacific Ocean utilizing seismograph records and the type of tide gauge just described. Present-day tsunami warning systems linked to seismic observatories can provide warnings of tsunami activity up to several hours in advance, depending on the location of the causal event.

Other instruments measure the impact of storm waves against pilings, piers, and deep-water structures. The measurements obtained from these instruments have enabled engineers to design structures that can better withstand the impact of storm waves. Before the 1950s, lighthouses and breakwaters were the structures most vulnerable to wave attack, but since that time, large oil drilling and production platforms have been built in the open ocean many kilometers from shore. During severe storms, many of these platforms have been capsized by the impact or damage caused by oncoming waves, resulting in tragic losses of life.

In laboratory experiments, ocean waves can be simulated in wave tanks. These range from tabletop models with glass sides that look like aquariums to outdoor tanks that can hold large boats and generate breakers several meters in height. The mathematical treatment of waves is facilitated by the regularity of their pattern and technologymodern computers, advances in machine learning, artificial neural networks, and support vector machines. These can predict wave heights and other wave characteristics with a high degree of accuracy.

Stimulated by the energy crisis in the late 1970s, intensive consideration has been given to the possibility of harnessing wave energy to generate power. Many systems have been designed and tested, and some have been constructed and now produce millions of watt-hours of energy each year. The fundamental principle on which most wave generators work is that the motion of a passing wave exerts a force similar to a piston's rise and fall. This can then be mechanically harnessed and used to drive a generating system.

Significance

Beaches owe their origin to wave action. The waves carry in the sand from which the beach is formed over time and then smooth it daily, erasing imprints with their in-and-out motion. New sand is also formed as stones, and rocks are made to grind against each other, chipping and eroding them slowly into sand grains. Storm waves, however, can do much damage in a much shorter period. The height of such waves in the open ocean can be dramatic. The USS Ramapo measured waves thirty-four meters high in 1933, for example. When waves reach shore at this height, destruction can be enormous due to the energy and momentum of the moving mass of water. Such waves have torn concrete blocks weighing sixty-five tons or more from breakwaters. Sandy coastlines are also susceptible to attack. A longshore current develops when the waves encroach upon the shoreline at an angle, and this current can transport vast quantities of sand along a coast. Where this transported sand is trapped by an obstacle, such as a harbor jetty, excessive deposition will occur, perhaps requiring expensive dredging. At other points along the coastline, beach erosion may occur, also causing problems.

One of the most feared wave types is the tsunami, which is most frequently encountered on the shores of the Pacific due to the seismic activity associated with the Pacific Rim’s so-called “Ring of Fire” but can occur in any body of water. Although tsunamis are almost imperceptible in the open ocean, their nature changes dramatically when they reach shore. Video records of incoming tsunamis demonstrate the destructive force of such waves on ships, buildings, and people. There are records of tsunamis rushing up mountainsides to elevations over thirty meters. For example, the tsunami generated by the Krakatau explosion is known to have transported an ocean-going vessel several kilometers inland, depositing it in the jungle far up a mountainside. An added danger is that tsunamis have a series of crests that arrive approximately twenty minutes apart, and the third or fourth to arrive is often the largest.

Storm surges are another dangerous wave type. They are commonly encountered along the Atlantic and Gulf coasts of the United States and have been known to carry oceangoing ships one kilometer or more inland. Six thousand people lost their lives in one such storm surge in Galveston, Texas, in 1900, and hundreds of thousands have drowned in a single storm surge on the shores of India’s Bay of Bengal. Expensive storm surge barriers now guard Providence, Rhode Island, London, and the Netherlands.

Two other wave types that may present hazards for humans are rogue waves and seiches. Rogue waves are huge, solitary waves occasionally encountered in the ocean. They are particularly associated with the southward-flowing Agulhas Current off South Africa and have been credited with sinking or severely damaging several large cargo ships. Seiche waves are oscillations in an enclosed water body such as a lake or bay. One such seiche, created by a hurricane, overflowed the dike surrounding Florida’s Lake Okeechobee in 1928 and drowned two thousand people.

Principal Terms

deep-water wave: a wave traveling in water with a depth greater than one-half of its wavelength

fetch: the area or length of the sea surface over which waves are generated by wind having a constant direction and speed

storm surge: a general rise above normal water level resulting from a hurricane or other severe coastal storm

swell: ocean waves that have traveled out of their wind-generating area

tsunami: a long-period sea wave produced by a displacement of crustal material due to submarine earthquake, volcanic eruption, or landslide

wave height: the vertical distance between a wave crest and the adjacent wave trough

wavelength: the horizontal distance between two successive wave crests or wave troughs

wave orbit: the path followed by a water particle affected by wave motion; in deep water, the orbit is nearly circular

wave period: the time (usually measured in seconds) required for two adjacent wave crests to pass a point

wave refraction: the process by which a wave crest is bent as it moves toward shore

Bibliography

Charlier, R. H., and C. W. Finkl. Ocean Energy: Tide and Tidal Power. Springer-Verlag, 2009.

Clarke, Allan J. An Introduction to the Dynamics of El Niño and the Southern Oscillation. Academic Press, 2008.

Cruz, Joan. Ocean Wave Energy: Current Status and Future Perspectives. Springer-Verlag, 2010.

Geggel, Laura. "78-Foot Wave Is the Largest Ever Recorded in Southern Hemisphere." LiveScience, 11 May 2018, www.livescience.com/62543-largest-wave-hits-southern-hemisphere.html. Accessed 20 July 2024.

Grossman, David. "Scientist Confirm the Largest Wave Ever Recorded." Popular Mechanics, 15 Dec. 2016, www.popularmechanics.com/science/environment/a24326/worlds-largest-wave-recorded-near-scotland. Accessed 6 Jan. 2017.

"Why Does the Ocean Have Waves?" National Oceanic and Atmospheric Administration, 16 June 2024, oceanservice.noaa.gov/facts/wavesinocean.html. Accessed 20 July 2024.

Holthujsen, Leo H. Waves in Oceanic and Coastal Waters. Cambridge University Press, 2009.

Joseph, Antony. Tsunamis: Detection, Monitoring and Early Warning Technologies. Academic Press/Elsevier, 2011.

Jackson, Derek W. T., and Andrew D. Short. Sandy Beach Morphodynamics. Elsevier, 2020.

Kennish, Michael J. Practical Handbook of Marine Science. 4th ed., CRC Press, 2022.

Shamshirband, Shahaboddin, et al. “Prediction of Significant Wave Height; Comparison between Nested Grid Numerical Model, and Machine Learning Models of Artificial Neural Networks, Extreme Learning and Support Vector Machines.” Engineering Applications of Computational Fluid Mechanics, vol. 14, no. 1, 2020, pp. 805–17. https://doi:10.1080/19942060.2020.1773932.

Talley, Lynne D., et al. Descriptive Physical Oceanography: An Introduction. 6th ed. Elsevier, 2016.

Trujillo, Alan P. and Harold V. Thurman. Essentials of Oceanography. Prentice Hall, 2010.

Young, Ian, and Alexander Babanin. Ocean Wave Dynamics. World Scientific Publishing, 2020.