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 waves, 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 subharmonics with periods ranging from 50 seconds (20 mHz) to 350 seconds (3 mHz). Infragravity waves are these subharmonics 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 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.
|This section's factual accuracy is disputed. (October 2011)|
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 reefs 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.
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