Spontaneous inertia-gravity-wave generation by surface-intensified turbulence

Type Article
Date 2012-05
Language English
Author(s) Danioux Eric1, 2, Vanneste J.1, 2, Klein Patrice3, Sasaki H.4
Affiliation(s) 1 : Univ Edinburgh, Sch Math, Edinburgh EH9 3JZ, Midlothian, Scotland.
2 : Univ Edinburgh, Maxwell Inst Math Sci, Edinburgh EH9 3JZ, Midlothian, Scotland.
3 : CNRS, Ifremer UBO IRD, Lab Phys Oceans, F-29280 Plouzane, France.
4 : JAMSTEC, Earth Simulator Ctr, Kanazawa Ku, Yokohama, Kanagawa 2360001, Japan.
Source Journal Of Fluid Mechanics (0022-1120) (Cambridge Univ Press), 2012-05 , Vol. 699 , P. 153-173
DOI 10.1017/jfm.2012.90
WOS© Times Cited 36
Keyword(s) internal waves, quasi-geostrophic flows, wave-turbulence interactions
Abstract The spontaneous generation of inertia-gravity waves (IGWs) by surface-intensified, nearly balanced motion is examined using a high-resolution simulation of the primitive equations in an idealized oceanic configuration. At large scale and mesoscale, the dynamics, which is driven by baroclinic instability near the surface, is balanced and qualitatively well described by the surface quasi-geostrophic model. This however predicts an increase of the Rossby number with decreasing spatial scales and, hence, a breakdown of balance at small scale; the generation of IGWs is a consequence of this breakdown. The wave field is analysed away from the surface, at depths where the associated vertical velocities are of the same order as those associated with the balanced motion. Quasi-geostrophic relations, the omega equation in particular, prove sufficient to separate the IGWs from the balanced contribution to the motion. A spectral analysis indicates that the wave energy is localized around dispersion relation for free IGWs, and decays only slowly as the frequency and horizontal wavenumber increase. The IGW generation is highly intermittent in time and space: localized wavepackets are emitted when thin filaments in the surface density are formed by straining, leading to large vertical vorticity and correspondingly large Rossby numbers. At depth, the IGW field is the result of a number of generation events; away from the generation sites it takes the form of a relatively homogeneous, apparently random wave field. The energy of the IGW field generated spontaneously is estimated and found to be several orders of magnitude smaller than the typical IGW energy in the ocean.
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