Propagation of the internal tide from a continental shelf: laboratory and numerical experiments
Seminar Room 1, Newton Institute
Internal gravity waves in the deep ocean have been given much importance for the last ten years, since oceanographers realized the important role of fluid mixing in raising the cold abyssal water masses. Mixing in the deep ocean, below 2 or 3 kms, is indeed thought to be mostly due to nonlinear internal gravity waves. In the abyss, internal waves result mainly from the interaction of the surface tide with the variable bottom topography and are referred to as the internal tide.
The thermal equilibrium of the deep ocean thus depends upon local and strongly intermittent processes -the breaking internal tide- which occur on temporal scales of the order of one minute and on spatial scales of the order of ten meters. This gigantic difference in spatial and temporal scales requires the parameterization of the mixing processes in large scale circulation models. A direct study of the breaking internal tide is therefore needed.
The present study focuses upon the academic situation of a uniformly stratified ocean, when the surface tide interacts with an idealized two-dimensional continental shelf. Numerical experiments have been conducted in close correspondance with the laboratory experiments, the geometrical and physical parameters being the same. The laboratory experiments have been performed on the rotating Coriolis platform in Grenoble. The nonlinear non-hydrostatic finite-volume numerical code developed at MIT was used to closely model the experiments.
In this simplified situation, the internal wave field organizes as a rectilinear wave beam with tidal frequency. We shall discuss the generation mechanism and structure of the wave beam, relying on theoretical modelling of internal wave emission by an oscillating body source. We shall also investigate the nonlinear dynamics of the tide, when harmonics and parametric sub-harmonics are generated. The localization and quantification of mixing induced by the internal tide will eventually be considered.