Sack Lunch Seminar (SLS)

Vortex Stability in a Large-Scale Internal Wave Shear


Lateral mixing and stirring processes are important for the ecosystem since they influence the nutrient or pollutant distribution and phytoplankton patchiness. Lateral mixing, which is often parameterized by lateral diffusivity, is increased by the geostrophic adjustment of multiple and sporadically occurring patches of well mixed water. Geostrophic adjustment transforms the mixed patches into vortices. Vortices are subject to internal wave shear and strain, both of which can coexist in the ocean. Once the shear exceeds a certain threshold the vortex is torn apart and ceases to exist.


In this study, the effect of a large-scale internal wave shear field on a single vortex was numerically simulated using a 3D Boussinesq pseudospectral method. In a suite of simulations, different magnitudes of background internal wave shear were tested, including a no shear run. If there is no background shear, the vortex remained stable for many hundreds of inertial periods but then split into two dipoles. With the introduction
of the background shear, the dipole splitting occurred earlier and became less symmetric in space with increasing shear. The observed kinetic energy spectrum indicated that the internal wave supplied the energy for the pole splitting. Theoretical considerations suggest that the vortex alone is subject to a barotropic instability, while the wave induces a baroclinic instability. For a given scenario, the two instability growth times compete, and the faster one determines which mechanism governs the breakup process.