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An infragravity wave is a long-period oceanic wave generated along continental coastlines by nonlinear wave interactions of storm-forced shoreward-propagating ocean swells. These differ from normal oceanic gravity waves, which are created by wind pushing 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 higher frequencies with periods about 30 seconds, but nonlinear processes convert some of this energy to sub-harmonics with periods ranging from 50 to 350 seconds (20 mHz to 3 mHz). Infragravity waves are these sub-harmonics of the impinging gravity waves.[1]

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 tides and oceanic Rossby waves, in the common literature their use is limited to gravity waves that are generated by the topography of the bottom.

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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.

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.[2] Similarly, coral reefs are effective in generating infragravity waves; in the case of coral reefs, the infragravity periods are established by resonances with the reef itself.[3][4]

Impact

Ice shelf processes.

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.[1][5]

References

  1. ^ a b Bromirski, Peter D.; Olga V. Sergienko and Douglas R. MacAyeal (2010). "Transoceanic infragravity waves impacting Antarctic ice shelves". Geophysical Research Letters (American Geophysical Union) 37 (L02502). doi:10.1029/2009GL041488. 
  2. ^ Leont’yev, I. O. (April 2009). "Generation mechanism of an alongshore bar on a sandy beach slope". Oceanology (MAIK Nauka/Interperiodica distributed exclusively by Springer Science+Business Media LLC.) 49 (2): 281-289. doi:10.1134/S000143700902012X. ISSN (Print) 1531-8508 (Online) 0001-4370 (Print) 1531-8508 (Online). 
  3. ^ Lugo-Fernández; H. H. Roberts, W. J. Wiseman Jr. and B. L. Carter (December 1998). "Water level and currents of tidal and infragravity periods at Tague Reef, St. Croix (USVI)". Coral Reefs (Springer Berlin / Heidelberg) 17 (4): 343-349. doi:10.1007/s003380050137. 
  4. ^ Péquignet, A. C.; J. M. Becker, M. A. Merrifield, and J. Aucan (2009). "Forcing of resonant modes on a fringing reef during tropical storm Man-Yi,". Geophys. Res. Lett (/ Heidelberg) 36 (L03607). doi:10.1029/2008GL036259.. 
  5. ^ Breaking waves: The coup de grace that shatters ice shelves is administered by ocean waves; The Economist (print edition); Feb 18, 2010. http://www.economist.com/sciencetechnology/displaystory.cfm?story_id=15543691

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