Infragravity waves are surface gravity waves with

frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

lower than the

wind wave In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction of t ...

s – consisting of both wind sea and swell – thus corresponding with the part of the

wave spectrum In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction of t ...

lower than the frequencies directly generated by forcing through the wind. Infragravity waves are ocean surface

gravity wave In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere ...

s 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, which is close to the typical periods of

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...

s, with which they share similar propagation properties including very fast celerities in deep water. This distinguishes infragravity waves from normal oceanic

s, 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. Munk ICCE 1950 Fig1

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

tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...

s 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 Walter Heinrich Munk (October 19, 1917 – February 8, 2019) was an American physical oceanographer. He was one of the first scientists to bring statistical methods to the analysis of oceanographic data. His work won awards including the Nation ...

in 1950.

Generation

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 In music, the undertone series or subharmonic series is a sequence of notes that results from inverting the intervals of the overtone series. While overtones naturally occur with the physical production of music on instruments, undertones must ...

interaction of trains of wind waves which was first observed by Munk and Tucker and explained by Longuet-Higgins and Stewart. Because wind waves are not

monochromatic A monochrome or monochromatic image, object or color scheme, palette is composed of one color (or lightness, values of one color). Images using only Tint, shade and tone, shades of grey are called grayscale (typically digital) or Black and wh ...

they form groups. The

Stokes drift For a pure wave motion in fluid dynamics, the Stokes drift velocity is the average velocity when following a specific fluid parcel as it travels with the fluid flow. For instance, a particle floating at the free surface of water waves, experien ...

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 The group velocity of a wave is the velocity with which the overall envelope shape of the wave's amplitudes—known as the ''modulation'' or ''envelope'' of the wave—propagates through space. For example, if a stone is thrown into the middl ...

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. 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. Glacier-ice shelf interactions

Impact

Infragravity waves are thought to be a generating mechanism behind sneaker waves, unusually large and long-duration waves that cause water to surge far onshore and that have killed a number of people in the US

Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though ...

. Infragravity waves generated along the Pacific coast of North America have been observed to propagate transoceanically to

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

and there to impinge on the

Ross Ice Shelf The Ross Ice Shelf is the largest ice shelf of Antarctica (, an area of roughly and about across: about the size of France). It is several hundred metres thick. The nearly vertical ice front to the open sea is more than long, and between hi ...

. 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.

References

External links

{{physical oceanography Fluid dynamics Water waves