The Lagrangian-mean transport and mixing properties associated with the life cycle of the stratospheric polar vortices are analyzed using nitrous oxide (N2O) as a tracer, both observed by the cryogenic limb array etalon spectrometer (CLAES) and simulated by the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model. Based on the modified Lagrangian-mean (MLM) diagnostic formalism, the area equivalent latitude-potential temperature cross sections are constructed for the N2O mixing ratio and the squared equivalent length, a Lagrangian equivalence of the eddy diffusion coefficient Kyy. The robustness of the analysis is tested and confirmed by subsampling the SKYHI tracer field at a CLAES-equivalent resolution and comparing the result with the full-resolution analysis. The seasonal and interhemispheric variabilities of the MLM cross sections are examined in detail. Both CLAES and SKYHI identify two major barriers to horizontal mixing (minimal equivalent length) in both hemispheres: a perennial subtropical barrier and an annual polar barrier. The polar barrier splits from the subtropical barrier in fall, migrates poleward in winter, and disappears at the demise of the polar vortex. The barriers are in general collocated with a tracer edge (concentrated gradients), but larger gradients do not necessarily translate to a greater barrier. For example, at the Arctic vortex edge, where the tracer gradients are greater than at the Antarctic vortex edge, the barrier is actually weaker. A significant difference is found in the structure of the barriers between the two northern hemisphere winters examined. ¿ 1997 American Geophysical Union