Internal wave

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Internal waves are gravity waves that oscillate due to the buoyancy force. The simplest example are the waves generated on the interface between two fluids of different densities. A good way to visualize internal waves of this sort is to observe the waves on an oil-water interface. Internal waves typically have much lower frequencies and higher amplitudes than surface waves. This is because the density difference between two fluids is typically much less than the density difference between water and air.

The atmosphere and ocean are continuously stratified: potential density generally increases steadily downward. Internal waves in a continuously stratified medium may propagate vertically as well as horizontally. Internal wave motions are ubiquitous in the ocean and atmosphere. The frequencies of these geophysical wave motions vary from a lower limit of the Coriolis frequency (inertial motions) up to the Brunt-Väisälä, or buoyancy frequency (buoyance oscillations). Internal waves at tidal frequencies are produced by tidal flow over topography/bathymetry, and are known as internal tides. Nonlinear solitary internal waves are called solitons.

Some gravity waves have scales on the order of a few hundred kilometers or more. These waves typically obey the hydrostatic approximation and have periods of several hours or more. Unlike pure gravity waves, gravity waves of this magnitude are subject to the Coriolis effect. Due to the Coriolis effect, parcel oscillation is characterized by elliptical lines, whereas pure gravity waves can be characterized by straight lines. Qualitatively, one can imagine elliptical oscillations as a resistance to horizontal motion due to the Coriolis force, present in all rotating frames and fluids. With the Coriolis effect, this resistance is always at right angles to the horizontal parcel velocity.