Internal gravity waves propagate in density-stratified fluids owing to the restoring effect of
buoyancy. They are ubiquitous in the ocean and their amplitude may be large enough for these waves to
be observed in satellite images as striking band-like features that travel for thousands of kilometers across
the ocean. This is only the surface signature of large internal motions. In terms of global budgets, the
most energetic part of the wave activity is indeed located in the deep ocean where measurements and
associated modelling still lack confidence. This is a key problem for oceanography, as the internal wave
field of the ocean involves an available amount of energy that can be used for mixing (and then influence
the global circulation) and transport (sediment, nutriments and plancton) and dissipated through
turbulent processes. The global circulation, in turn, may profoundly influence the Earth’s climate.
Many scientific programs are currently developed in order to better estimate these transfers in the
ocean. However, focusing on oceanic perspectives should go with advances in the theoretical
background needed for a complete understanding of these different processes. From the physicist’s point
of view, internal gravity waves are indeed particularly interesting. They are transverse waves (group and
phase velocity being perpendicular), that do not respond to our classical perception of wave phenomena.
Linear solutions are often also solutions of the nonlinear equations, reflection laws are completely
different from the usual Snell-Descartes laws, the Huyghens-Fresnel diffraction laws are not valid
anymore. All these reasons lead to paradoxes that are of high interest from the ‘fundamental physics’
point of view. Moreover, part of the above properties, which are consequences of an anisotropic
dispersion relation due to gravity, are also encountered for inertial waves (in presence of rotation) or
plasma waves (in presence of a magnetic field), but also in the astrophysical context, a domain where
interest for internal waves is developing very fast. Generalization to physical processes different from
those directly considered within this project may thus be expected.
The focus of this proposal will therefore be on physical mechanisms of internal waves, a
necessary stage aside oceanographic projects. Regarding the complexity of these phenomena, we also
think that progress in these fundamental studies would be of primary interest to the understanding of
oceanic internal waves, and to the interpretation of field measurements. Thus, important tasks in our
project will consist in estimating mixing associated with different mechanisms, investigating the origin
and properties of solitons and solibores (two striking nonlinear wave features of the ocean interior)
and studying the origin of the statistical equilibrium of large scale oceanic internal waves.