NONLINEAR OSCILLATORY EVOLUTION OF A BAROCLINICALLY UNSTABLE GEOSTROPHIC VORTEX

In a two-layer quasi-geostrophic model, we numerically investigate the nonlinear evolution of a baroclinically unstable geostrophic vortex. For perturbations with a moderate growth rate and a weak amplitude and weak viscosity, a regime of regular oscillation is attained. This reg ime has a quasi-periodic reversal of the vertical phase shift between the flow patterns in each layer, with a corresponding back-and-forth p otential energy transfer between the mean vortex state and the fluctua tion pattern. This oscillating vortex, averaged in a frame of referenc e co-rotating with the angular velocity of each layer, has an elliptic al horizontal configuration with a simple relation between potential v orticity and co-rotating streamfunction. Simulations with smaller Burg er or Reynolds number or larger initial perturbation amplitude show le ss regularity in the oscillation and eventually result in vortex break ing.