Ryo Mizuta and Shigeo Yoden
( Department of Geophysics, Kyoto University, Japan )
Large-scale horizontal mixing and transport barriers around
the circumpolar vortex in the winter stratosphere is studied
using a two-dimensional barotropic model on a spherical domain,
A barotropically unstable jet is forced
in order to obtain a fluctuating polar vortex.
A flow with quasi-periodic time dependence and an aperiodic flow
with similar behavior are investigated using
several Lagrangian analyses.
Effective mixing inside and outside of the polar vortex is observed in the quasi-periodic flow. The process of the mixing is typical of chaotic mixing. The mixing regions are where the perturbations grow through barotropic instability. Poincare sections give accurate locations of chaotic mixing regions, and transport barriers are identified as the edges of invariant torus regimes. In addition to the transport barriers associated with strong potential vorticity gradients, another type of transport barrier exists, which is not related to the steep potential vorticity gradient.
The structure obtained in the quasi-periodic flow is relevant to the aperiodic flow. The evolution of the correlation function is more typical of a chaotic mixing. Chaotic mixing is also characterized by high average Lyapunov exponents. Isolated regions are found near the center of the polar vortex, corresponding to the invariant tori in the Poincare sections of the quasi-periodic flow.
|Received 27 June 2001; accepted 27 September 2001|