Internal wave

An internal wave is a gravity wave that propagates within a density-stratified fluid, displacing boundaries between layers rather than the fluid’s outer surface.

Density stratification: The arrangement of a fluid into layers of differing density, often increasing with depth. Density differences provide the restoring force that lets displaced fluid layers oscillate.
Internal solitary wave: A localized, nonlinear internal wave that travels without spreading rapidly. Large solitary waves can carry substantial energy across oceanic density layers.
Tidal forcing: The periodic gravitational forcing that produces ocean tides and associated currents. Tidal currents generate internal tides when they cross sloping seafloor features.
Surface gravity wave: A wave restored primarily by gravity that propagates along the boundary between a fluid and another medium. Unlike internal waves, it displaces the fluid’s outer surface rather than an internal density interface.
Brunt–Väisälä frequency: The natural oscillation frequency of a vertically displaced parcel in a stably stratified fluid. It sets the upper frequency limit for freely propagating internal waves.
Ocean mixing: The transport and redistribution of heat, salt, nutrients, and other properties within the ocean. Internal-wave breaking supplies energy for mixing that helps shape ocean circulation.
Lee wave: A stationary or propagating wave formed as a flow passes over an obstacle. Flow over underwater ridges can create internal waves analogous to atmospheric lee waves.
Acoustic wave: A mechanical pressure disturbance that propagates through a medium by compressions and expansions. Internal waves are restored mainly by buoyancy, not compressibility.
Internal-wave dispersion relation: The mathematical relation linking an internal wave’s frequency, wavelength, and propagation direction in a stratified fluid. Its unusual direction-dependent form explains why wave energy and phase travel differently.
Turbulence: Irregular fluid motion characterized by interacting vortices and transport across a range of scales. Wave breaking often transfers internal-wave energy into turbulent motion.

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