Internal wave breaking
Internal wave breaking is a process during which internal gravity waves attain a large amplitude compared to their length scale, become nonlinearly unstable and finally break.
Consequently, internal wave breaking influences even the large scale flows and composition in both the ocean and the atmosphere.
[2] In the deep ocean, mixing induced by internal wave breaking is an important driver of the meridional overturning circulation.
[3] On smaller scales, breaking-induced mixing is important for sediment transport and for nutrient supply to the photic zone.
The contribution of breaking internal waves to many atmospheric and ocean processes makes it important to parametrize their effects in weather and climate models.
Similar to what happens to surface gravity waves near a coastline, when internal waves enter shallow waters and encounter steep topography, they steepen and grow in amplitude in a nonlinear process known as shoaling.
[6] Due to the relatively small density differences (and thus small restoring forces) over the ocean depth, ocean internal waves may reach amplitudes up to around 100 m.[5] Analogous to surface wave breaking in the region known as the surf zone, internal breaking waves dissipate energy in what is known as the internal surf zone.
[5] Low mode internal waves, with wavelengths exceeding 100 km, generated by either tides or winds acting on the sea surface, can travel thousands of kilometers from their regions of generation, where they will eventually encounter sloping topography and break.
This then leads to an unstable density profile that eventually overturns and breaks.
The ratio of the topographic slope to the wave steepness can be characterized by the internal Iribarren number:
For a gentle slope, as is typical for the continental shelf and nearshore areas, the internal Iribarren number is low (
Canonical bores are generally accompanied by an intense drop in temperature as the wavefront passes by, followed by a gradual increase over time.
[5] Breaking internal waves are regarded to play an important role on mixing of the ocean based on lab experiments and remote sensing.
The effect of internal waves on mixing is also studied extensively in direct numerical simulations.
Even though research indicates that internal wave breaking is important for local turbulence, there remains uncertainty in global estimates.
[13][14][15][16] Breaking internal tidal waves can result in turbulent water columns of several hundred meters high and the turbulent kinetic energy may reach levels up to 10.000 times higher than in the open ocean.
That means that a small internal Iribarren number predicts that a lot of the wave energy will be transferred to mixing and turbulence, while a large internal Iribarren number predicts that the wave energy will reflect offshore.
are not directly observable, studies use different definitions to determine the mixing efficiency.
Besides measurements of ocean dynamics, the mixing efficiency can also be obtained from lab experiments and numerical simulations, but they also have their limitations.
In theory these three approaches should give the same estimates for the mixing efficiency, but there remain discrepancies between them.
Therefore, there are varying estimates and disagreements on mixing efficiency and comparisons are difficult due to the different definitions.
[17] Studies that quantify the mixing properties of breaking internal solitary waves have split estimates of the mixing efficiency range, with values between 5% and 25% for laboratory experiments[18] or between 13% and 21% for numerical simulations[14] depending on the Internal Iribarren number.
Wave breaking causes mass and sediment transport that is important for the ocean biology and shaping of the continental shelves due to erosion.
[19] The erosion caused by internal wave breaking can result in sediment to be suspended and transported off-shore.
This off-shore sediment transport may give rise to the emergence of nepheloid layers, which are in turn important for the ocean biology.
[20] Although many studies show that internal wave breaking leads to sediment transport, their traces in the geologic record remain uncertain.
[19] The mixing and transport of nutrients in the ocean is affected largely by internal wave breaking.
The arrival of internal tidal bores has been shown to cause a 10 to 40 fold increase of nutrients on Conch Reef.
[21] Here it has been shown that the appearance of internal bores provide a predictable and periodic source of transport that can be important for a diversity of marine life.
[24] Internal wave breaking may also cause ecological hazards, such as red tides[4] and low dissolved oxygen levels.
