Previous: Geometric properties Next: References Up: Ext.Abst.

7. Conclusions

Some Lagrangian techniques have been applied to isentropic flow near the tropopause to help identify some large scale flow features as well as the seasonal and interannual variability of the tropopause as a transport barrier. Both contour stretching rates and effective diffusivity diagnostics show seasonal and interhemispheric variability that is consistent with our current perception of the upper troposphere/lower stratosphere. In addition, they suggest considerable seasonal variability of the PV value best suited to defining the tropopause. Finally, although the time period of 5 years is not enough to show any clear pattern, the results hint at interannual variability that may be linked with the phase of ENSO.

Geometric aspects of the isentropic flow have also been considered. Consistent with Koh & Plumb (2000) and Joseph & Legras (2000), the uniform hyperbolic trajectories, and the associated kinematic barrier defined by the local manifolds, lie in a stochastic layer equatorward of the dynamical barrier associated with the tropopause. Thus, transport deduced from lobe dynamical considerations will not necessarily be the most relevant to cross-tropopause transport. It is anticipated that a more detailed investigation of the persistence time of the geometric structures and of the width of the stochastic layer will help clarify the problem of transport across such dynamical barriers.

Previous: Geometric properties Next: References Up: Ext.Abst.