Chaotic mixing and transport barriers in an idealized stratospheric polar vortex

Chaotic mixing processes and transport barriers around the wintertime strat ospheric polar vortex are investigated with an idealized barotropic model, previously used by Ishioka and Yoden. A barotropically unstable jet is forc ed in order to obtain a fluctuating polar vortex. A flow with quasiperiodic time dependence and an aperiodic flow with similar behavior are investigat ed using several Lagrangian methods. A typical chaotic mixing process is observed in the quasiperiodic flow, res ulting in effective mixing inside and outside of the polar vortex. The mixi ng regions are on the critical latitudes of several planetary waves that gr ow through barotropic instability. Poincare sections give accurate location s of chaotic mixing regions, and transport barriers are identified as the e dges of invariant torus regimes. In addition to the transport barriers asso ciated with strong potential vorticity gradients, another type of transport barrier exists, which is not related to the steep potential vorticity grad ient. Chaotic mixing is dominant also in the aperiodic flow. Comparing with the q uasiperiodic flow, an aperiodic flow with the same wave energy has a higher average Lyapunov exponent. This arises because the area involved in chaoti c zones increases. The evolution of the correlation function is also more t ypical of a chaotic zone. Isolated regions are found near the center of the polar vortex, which can be explained by the invariant tori in the Poincare sections of the quasiperiodic flow. Implications of the results for the ob served "4-day wave'' in the upper stratosphere are discussed.