Internal gravity waves
From Glossary of Meteorology
internal gravity wave
(Also called internal waves, gravity waves.) A wave that propagates in density-stratified fluid under the influence of buoyancy forces.
The dispersion relation is given by frequency
in which N is the buoyancy frequency and kh is the horizontal component of the wavenumber vector k. For all wavenumbers, internal gravity waves have frequency smaller than N. Their group velocity is perpendicular to the phase velocity such that the vertical component of the group velocity is opposite in sign to the vertical component of the phase velocity.