Impacts of centrifugal/inertial/symmetric instabilities on ocean fronts

Nicolas Grisouard and Varvara Zemskova, University of Toronto, Physics, Toronto, ON, Canada
Oceanic submesoscale density fronts are structures in geostrophic and hydrostatic balance and are ubiquitous features of the upper ocean. They are also prone to various instabilities, which oceanographers believe may be key to understanding the kinetic energy budget of the ocean, as well as their effects on gas and nutrients exchanges between the surface and the abyss. In this presentation, we focus on theoretical aspects of what is sometimes referred to as centrifugal, inertial and/or symmetric instability (CISI). We present a series of idealized numerical initial condition experiments and simple theoretical arguments to investigate impacts of CISI on fronts. We first argue that drainage of available potential energy from the geostrophic flow can be a leading-order source of their growth. We also illustrate how one of the unexpected by-product of CISI is radiation of internal waves, which we interpret as a near-resonance between the fastest growing modes of CISI and the polarization relations of near-internal waves. Finally, we argue that the choice of surface boundary conditions may play a previously-unacknowledged role, in that it can trigger a precursor instability that overpowers other instabilities.