Internal wave
If the density changes over a small vertical distance (as in the case of the thermocline in lakes and oceans or an atmospheric inversion), the waves propagate horizontally like surface waves, but do so at slower speeds as determined by the density difference of the fluid below and above the interface.
If propagating horizontally along an interface where the density rapidly decreases with height, they are specifically called interfacial (internal) waves.
If moving vertically through the atmosphere where substantial changes in air density influences their dynamics, they are called anelastic (internal) waves.
If generated in the ocean by tidal flow over submarine ridges or the continental shelf, they are called internal tides.
The outflow of cold air from a thunderstorm can launch large amplitude internal solitary waves at an atmospheric inversion.
In northern Australia, these result in Morning Glory clouds, used by some daredevils to glide along like a surfer riding an ocean wave.
Undulations of the oceanic thermocline can be visualized by satellite because the waves increase the surface roughness where the horizontal flow converges, and this increases the scattering of sunlight (as in the image at the top of this page showing of waves generated by tidal flow through the Strait of Gibraltar).
The displacement of the thermocline of a lake, which separates warmer surface from cooler deep water, feels the buoyancy force expressed through the reduced gravity.
to the vertical: This is one way to write the dispersion relation for internal waves whose lines of constant phase lie at an angle
In deriving this structure, matching conditions have been used at the interface requiring continuity of mass and pressure.
The structure and dispersion relation of internal waves in a uniformly stratified fluid is found through the solution of the linearized conservation of mass, momentum, and internal energy equations assuming the fluid is incompressible and the background density varies by a small amount (the Boussinesq approximation).
Most people think of waves as a surface phenomenon, which acts between water (as in lakes or oceans) and the air.
Internal waves are the source of a curious phenomenon called dead water, first reported in 1893 by the Norwegian oceanographer Fridtjof Nansen, in which a boat may experience strong resistance to forward motion in apparently calm conditions.
The atmosphere and ocean are continuously stratified: potential density generally increases steadily downward.
An internal wave may also become confined to a finite region of altitude or depth, as a result of varying stratification or wind.
At large scales, internal waves are influenced both by the rotation of the Earth as well as by the stratification of the medium.
Similarly, atmospheric tides arise from, for example, non-uniform solar heating associated with diurnal motion.
The prevalence of each type of event depends on a variety of factors including bottom topography, stratification of the water body, and tidal influences.
[8] The largest of these waves are generated during springtides and those of sufficient magnitude break and progress across the shelf as bores.
[12] Additionally, while both surface waters and those at depth tend to have relatively low primary productivity, thermoclines are often associated with a chlorophyll maximum layer.
These layers in turn attract large aggregations of mobile zooplankton[13] that internal bores subsequently push inshore.
[15][16] Waters above an internal wave converge and sink in its trough and upwell and diverge over its crest.
[15] The convergence zones associated with internal wave troughs often accumulate oils and flotsam that occasionally progress shoreward with the slicks.
[17][18] These rafts of flotsam can also harbor high concentrations of larvae of invertebrates and fish an order of magnitude higher than the surrounding waters.
[13] Internal waves represent oscillations of these thermoclines and therefore have the potential to transfer these phytoplankton rich waters downward, coupling benthic and pelagic systems.
[19][20] Areas affected by these events show higher growth rates of suspension feeding ascidians and bryozoans, likely due to the periodic influx of high phytoplankton concentrations.
[21] Periodic depression of the thermocline and associated downwelling may also play an important role in the vertical transport of planktonic larvae.
Large steep internal waves containing trapped, reverse-oscillating cores can also transport parcels of water shoreward.
[22] These non-linear waves with trapped cores had previously been observed in the laboratory[23] and predicted theoretically.
[22] The conditions favorable to the generation of these waves are also likely to suspend sediment along the bottom as well as plankton and nutrients found along the benthos in deeper water.
