Infragravity wave
Encyclopedia
Infragravity waves are surface gravity waves with frequencies
lower than the wind waves – consisting of both wind sea and swell
– so corresponding with the part of the wave spectrum lower than the frequencies directly generated by forcing
through the wind.
Infragravity waves consist, among others, of long-period oceanic waves generated along continental coastlines by nonlinear wave interactions of storm-forced shoreward-propagating ocean swells
. These differ from normal oceanic gravity wave
s, which are created by wind acting on the surface of the sea. Normal gravity waves typically have a frequency on the order of 50 millihertz
(i.e., a period of 20 seconds). Interactions of these waves with coastlines filters out the frequencies with periods about 30 seconds, but nonlinear processes convert some of this energy to sub-harmonics with periods ranging from 50 seconds (20 mHz) to 350 seconds (3 mHz). Infragravity waves are these sub-harmonics of the impinging gravity waves.
Technically infragravity waves are simply a subcategory of gravity waves and refer to all gravity waves with periods greater than 30 s. Although they include phenomena such as tide
s and oceanic Rossby waves
, in the common literature their use is limited to gravity waves that are generated by the topography of the bottom.
The term "infragravity wave" appears to have been coined by Walter Munk
in 1950.
s are effective in generating infragravity waves; in the case of coral reefs, the infragravity periods are established by resonances with the reef itself.
Infragravity waves generated along the Pacific coast of North America have been observed to propagate transoceanically to Antarctica and there to impinge on the Ross Ice Shelf
. Their frequencies more closely couple with the ice shelf natural frequencies and they produce a larger amplitude ice shelf movement than the normal ocean swell of gravity waves. Further, they are not damped by sea ice as normal ocean swell is. As a result they flex floating ice shelves such as the Ross Ice Shelf; this flexure contributes significantly to the breakup on the ice shelf.
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
lower than the wind waves – consisting of both wind sea and swell
Swell (ocean)
A swell, in the context of an ocean, sea or lake, is a series surface gravity waves that is not generated by the local wind. Swell waves often have a long wavelength but this varies with the size of the water body, e.g. rarely more than 150 m in the Mediterranean, and from event to event, with...
– so corresponding with the part of the wave spectrum lower than the frequencies directly generated by forcing
Forcing
Forcing may refer to:*Forcing , a technique for obtaining proofs in set theory*Forcing *Radiative forcing, the difference between the incoming radiation energy and the outgoing radiation energy in a given climate system...
through the wind.
Infragravity waves consist, among others, of long-period oceanic waves generated along continental coastlines by nonlinear wave interactions of storm-forced shoreward-propagating ocean swells
Swell (ocean)
A swell, in the context of an ocean, sea or lake, is a series surface gravity waves that is not generated by the local wind. Swell waves often have a long wavelength but this varies with the size of the water body, e.g. rarely more than 150 m in the Mediterranean, and from event to event, with...
. These differ from normal oceanic gravity wave
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
s, which are created by wind acting on the surface of the sea. Normal gravity waves typically have a frequency on the order of 50 millihertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
(i.e., a period of 20 seconds). Interactions of these waves with coastlines filters out the frequencies with periods about 30 seconds, but nonlinear processes convert some of this energy to sub-harmonics with periods ranging from 50 seconds (20 mHz) to 350 seconds (3 mHz). Infragravity waves are these sub-harmonics of the impinging gravity waves.
Technically infragravity waves are simply a subcategory of gravity waves and refer to all gravity waves with periods greater than 30 s. Although they include phenomena such as tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
s and oceanic Rossby waves
Rossby wave
Atmospheric Rossby waves are giant meanders in high-altitude winds that are a major influence on weather.They are not to be confused with oceanic Rossby waves, which move along the thermocline: that is, the boundary between the warm upper layer of the ocean and the cold deeper part of the...
, in the common literature their use is limited to gravity waves that are generated by the topography of the bottom.
The term "infragravity wave" appears to have been coined by Walter Munk
Walter Munk
Walter Heinrich Munk is an American physical oceanographer. He is professor of geophysics emeritus and holds the Secretary of the Navy/Chief of Naval Operations Oceanography Chair at Scripps Institution of Oceanography in La Jolla, California.-Early life:Born in 1917 in Vienna, Austria-Hungary,...
in 1950.
Generation
As a result of geology, infragravity-wave-induced large-scale bedforms (e.g., bars), and biologic process (e.g., reefs) the shoreline and the near-shore features of the sea floor often has a periodic character. Coastal sand bars are a significant contributor to the generation of infragravity waves and are shaped by them. On the inner side of a sand bar, the size of the bar is determined by the length of short wavelength wind-generated gravity waves. On the outer side of the bar the bar shape is dictated by the length of the infragravity waves which correlate to and are driven by the groups of short waves. Similarly, coral reefCoral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
s are effective in generating infragravity waves; in the case of coral reefs, the infragravity periods are established by resonances with the reef itself.
Impact
Ross Ice Shelf
The Ross Ice Shelf is the largest ice shelf of Antarctica . It is several hundred metres thick. The nearly vertical ice front to the open sea is more than 600 km long, and between 15 and 50 metres high above the water surface...
. Their frequencies more closely couple with the ice shelf natural frequencies and they produce a larger amplitude ice shelf movement than the normal ocean swell of gravity waves. Further, they are not damped by sea ice as normal ocean swell is. As a result they flex floating ice shelves such as the Ross Ice Shelf; this flexure contributes significantly to the breakup on the ice shelf.