Relation between kinematic boundaries, stirring, and barriers for the Antarctic polar vortex

Maximum stretching lines in the lower stratosphere around the Antarctic polar vortex are diagnosed using a method based on finite-size Lyapunov exponents. By analogy with the mathematical results known for simple dynamical systems, these curves are identified as stable and unstable manifolds of the underlying hyperbolic structure of the flow. For the first time, the exchange mechanism associated with lobe dynamics is characterized using atmospheric analyzed winds. The tangling manifolds form a stochastic layer around the vortex. It is found that fluid is not only expelled from this layer toward the surf zone but also is injected inward from the surf zone, through a process similar to the turnstile mechanism in lobe dynamics. The vortex edge, defined as the location of the maximum gradient in potential vorticity or tracer, is found to be the southward (poleward) envelope of this stochastic layer. Exchanges with the inside of the vortex are therefore largely decoupled from those, possibly intense, exchanges between the stochastic layer and the surf zone. It is stressed that using the kinematic boundary defined by the hyperbolic points and the manifolds as an operational definition of vortex boundary is not only unpractical but also leads to spurious estimates of exchanges. The authors anticipate that more accurate dynamical systems tools are needed to analyze stratospheric transport in terms of lobe dynamics.