Mesoscale Inhomogeneities and Turbulence in Ocean Acoustics

Abstract

Consideration of the nonlinear potential vorticity equation for quasi-geostrophic motion of a continuously stratified ocean on a β-plane shows the dynamics to be controlled by two specific horizontal wavenumbers; the first depends on β and marks the transition from turbulence to waves while the second depends on the modal Rossby deformation radius and marks the transition from uncoupled to vertically coupled motion. An examination of the dynamical processes involved, namely the cascade of energy to small wavenumber, the concomitant cascade of enstrophy to large wavenumber and the baroclinic instability-induced cascade to large wavenumber, shows that the most persistent structures which evolve are near-circular eddies having a vertical structure similar to a first baroclinic mode Rossby wave, a horizontal length scale equal to the corresponding internal deformation radius, and for which the linear and nonlinear effects are of equal magnitude. These eddies also have a unique non-dimensional number associated with them. The effects of these inhomogeneities on the propagation of an acoustic field are thus predicted in a fully deterministic manner.