Coherent flows with near zero potential vorticity

Abstract

The dynamics of small, intense inviscid fluid vortices rotating near the local Coriolis are examined through numerical experiments. These vortices have near zero potential vorticity (which is conserved) and are inherently nonlinear. Solutions for a quasigeostrophic vortex with no external forcing are contrasted with solutions where this balance was violated in specific ways. It was found that departure from quasigeostrophy can produce a rich variety of superinertial oscillations in the hydrodynamic fields. The effects of time-dependent external forcing are also considered. An initially circular vortex with zero potential vorticity was forced at its natural frequency (the inertial frequency). The vortex developed an elliptical structure and was stable, even when the magnitude of the forcing deformation was comparable to that of the unforced vortex. Of course, as the forcing amplitude was increased, the solution became unstable. These results are discussed in light of recent observations of small, coherent structures. It is suggested that these structures may contribute to the superinertial variability previously observed (and not fully explained) in several internal wave observations.