Critical reflection of internal waves in submesoscale flows

[This is a modified version of this article published about me in our April 2015 departmental newletter]

Fronts criss-cross the surface of the ocean. They can trap internal waves, which are then squeezed towards the surface and reflect on it. When they do, and this is where the internal wave physics is unlike any other, they do not reflect against it in a specular manner, like light against a mirror or sound against a wall. Instead, the angle of propagation of their energy is modified after reflection, and if the conditions are right, this angle can even be parallel to the ocean surface, effectively trapping the waves in a thin and very energetic layer just below the surface:

[For caption, go directly to the YouTube page]

As it turns out, and this is one of the many small miracles happening in this problem, the “right conditions” are often met in the ocean: the waves simply have to oscillate at the same frequency as the rotation rate of the Earth, and nature finds many ways to create such waves.

When focused in this thin, energized layer, the waves become very non-linear, break and dissipate. They don’t do it with a whimper but a bang: some of the energy associated with the front also dissipates with it, one of the practical consequences this seemingly abstract problem. Why does it matter? Fronts occupy a special place in oceanography; for example, they host intense ecosystems. They are also cracks in which the large ocean vortices that store most of the ocean’s kinetic energy dissipate, although in ways that are still mysterious. Our study provides the oceanographic community with a new process, which could partially explain how internal waves make this dissipation happen.

[More videos about this project on YouTube]

References:
  1. Grisouard, N. and L. N. Thomas. Energy exchanges between density fronts and near-inertial waves reflecting off the ocean surface. Journal of Physical Oceanography, 46:501–516, 2016. (doi | | poster).
  2. Grisouard, N. and L. N. Thomas. Critical and near-critical reflections of near-inertial waves off the sea surface at ocean fronts. Journal of Fluid Mechanics, 765:273-302, 2015. (pdf* | doi | poster)