Infragravity waves are surface gravity waves with frequencies lower than the wind waves – consisting of both wind sea and swell – thus corresponding with the part of the wave spectrum lower than the frequencies directly generated by forcing through the wind.
Infragravity waves are ocean surface gravity waves generated by ocean waves of shorter periods. The amplitude of infragravity waves is most relevant in shallow water, in particular along coastlines hit by high amplitude and long period wind waves and ocean swells. Wind waves and ocean swells are shorter, with typical dominant periods of 1 to 25 s. In contrast, the dominant period of infragravity waves is typically 80 to 300 s,[1] which is close to the typical periods of tsunamis, with which they share similar propagation properties including very fast celerities in deep water. This distinguishes infragravity waves from normal oceanic gravity waves, which are created by wind acting on the surface of the sea, and are slower than the generating wind.
Whatever the details of their generation mechanism, discussed below, infragravity waves are these subharmonics of the impinging gravity waves.[2]
Classification of the spectrum of ocean waves according to wave period.[3]
Technically infragravity waves are simply a subcategory of gravity waves and refer to all gravity waves with periods greater than 30 s. This could include phenomena such as tides and oceanic Rossby waves, but the common scientific usage is limited to gravity waves that are generated by groups of wind waves.
The term "infragravity wave" appears to have been coined by Walter Munk in 1950.[3][4]
Generation
Surf can be seen breaking as it crosses the sand bar offshore. Sandbars aid in generating infragravity waves and in turn are shaped by them.
Two main processes can explain the transfer of energy from the short wind waves to the long infragravity waves, and both are important in shallow water and for steep wind waves. The most common process is the subharmonic interaction of trains of wind waves which was first observed by Munk and Tucker and explained by Longuet-Higgins and Stewart.[5] Because wind waves are not monochromatic they form groups. The Stokes drift induced by these groupy waves transports more water where the waves are highest. The waves also push the water around in a way that can be interpreted as a force: the divergence of the radiation stresses. Combining mass and momentum conservation, Longuet-Higgins and Stewart give, with three different methods, the now well-known result. Namely, the mean sea level oscillates with a wavelength that is equal to the length of the group, with a low level where the wind waves are highest and a high level where these waves are lowest. This oscillation of the sea surface is proportional to the square of the short wave amplitude and becomes very large when the group speed approaches the speed of shallow water waves. The details of this process are modified when the bottom is sloping, which is generally the case near the shore, but the theory captures the important effect, observed in most conditions, that the high water of this 'surf beat' arrives with the waves of lowest amplitude.
Another process was proposed later by Graham Symonds and his collaborators.[6] To explain some cases in which this phase of long and short waves were not opposed, they proposed that the position of the breaker line in the surf, moving towards deep water when waves are higher, could act like a wave maker. It appears that this is probably a good explanation for infragravity wave generation on a reef.
In the case of coral reefs, the infragravity periods are established by resonances with the reef itself.[7][8]
Ice shelf processes.
Impact
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.[2][9]
References
Ardhuin, Fabrice; Arshad Rawat; Jerome Aucan (2014), "A numerical model for free infragravity waves: Definition and validation at regional and global scales", Ocean Modelling, 77, Elsevier, pp. 20–32
Bromirski, Peter D.; Olga V. Sergienko; Douglas R. MacAyeal (2010). "Transoceanic infragravity waves impacting Antarctic ice shelves". Geophysical Research Letters. 37 (L02502): n/a. Bibcode:2010GeoRL..37.2502B. doi:10.1029/2009GL041488.
Munk, Walter H. (1950), "Origin and generation of waves", Proceedings 1st International Conference on Coastal Engineering, Long Beach, California: ASCE, pp. 1–4, ISSN 2156-1028
Kinsman, Blair (1965). Wind Waves: Their Generation and Propagation on the Ocean Surface. Englewood Cliffs, N.J.: Prentice-Hall. pp. 22–23. OCLC 489729.
Longuet-Higgins, Michael; R.W. Stewart (1962), "Radiation stress and mass transport in gravity waves, with application to 'surf beats", Journal of Fluid Mechanics, 13, Cambridge University Press, pp. 481–504, doi:10.1017/S0022112062000877
Symonds, Graham; D. A. Huntley; A. J. Bowent (1982), "Two-dimensional surf beat: Long wavegeneration by a time-varying breakpoint", Journal of Geophysical Research, 87 (C1): 492–498, Bibcode:1982JGR....87..492S, CiteSeerX 10.1.1.474.7148, doi:10.1029/JC087iC01p00492
Lugo-Fernández, A.; H. H. Roberts; W. J. Wiseman Jr.; B. L. Carter (December 1998). "Water level and currents of tidal and infragravity periods at Tague Reef, St. Croix (USVI)". Coral Reefs. 17 (4): 343–349. doi:10.1007/s003380050137. S2CID 24665450.
Péquignet, A. C.; J. M. Becker; M. A. Merrifield; J. Aucan (2009). "Forcing of resonant modes on a fringing reef during tropical storm Man-Yi" (PDF). Geophys. Res. Lett. 36 (L03607): n/a. Bibcode:2009GeoRL..36.3607P. doi:10.1029/2008GL036259.
"Breaking waves: The coup de grace that shatters ice shelves is administered by ocean waves". The Economist. February 18, 2010. Retrieved 2010-11-25.
External links
vte
Physical oceanography
Waves
Airy wave theory Ballantine scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Edge wave Equatorial waves Fetch Gravity wave Green's law Infragravity wave Internal wave Iribarren number Kelvin wave Kinematic wave Longshore drift Luke's variational principle Mild-slope equation Radiation stress Rogue wave Rossby wave Rossby-gravity waves Sea state Seiche Significant wave height Soliton Stokes boundary layer Stokes drift Stokes wave Swell Trochoidal wave Tsunami
megatsunami Undertow Ursell number Wave action Wave base Wave height Wave power Wave radar Wave setup Wave shoaling Wave turbulence Wave–current interaction Waves and shallow water
one-dimensional Saint-Venant equations shallow water equations Wind wave
model
Upwelling
Antarctic bottom water
Circulation
Atmospheric circulation Baroclinity Boundary current Coriolis force Coriolis–Stokes force Craik–Leibovich vortex force Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño–Southern Oscillation General circulation model Geochemical Ocean Sections Study Geostrophic current Global Ocean Data Analysis Project Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Modular Ocean Model Ocean current Ocean dynamics Ocean gyre Princeton ocean model Rip current Subsurface currents Sverdrup balance Thermohaline circulation
shutdown Upwelling Wind generated current Whirlpool World Ocean Circulation Experiment
Tides
Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rip tide Rule of twelfths Slack water Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide gauge Tideline Theory of tides
Landforms
Abyssal fan Abyssal plain Atoll Bathymetric chart Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Submarine volcano
Plate
tectonics
Convergent boundary Divergent boundary Fracture zone Hydrothermal vent Marine geology Mid-ocean ridge Mohorovičić discontinuity Vine–Matthews–Morley hypothesis Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault Volcanic arc
Ocean zones
Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic Pelagic Photic Surf Swash
Sea level
Deep-ocean Assessment and Reporting of Tsunamis Future sea level Global Sea Level Observing System North West Shelf Operational Oceanographic System Sea-level curve Sea level rise World Geodetic System
Acoustics
Deep scattering layer Hydroacoustics Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics
Satellites
Jason-1 Jason-2 (Ocean Surface Topography Mission) Jason-3
Related
Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Ocean exploration Ocean observations Ocean reanalysis Ocean surface topography Ocean thermal energy conversion Oceanography Pelagic sediment Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean Atlas
Hellenica World - Scientific Library
Retrieved from "http://en.wikipedia.org/"
All text is available under the terms of the GNU Free Documentation License
