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1. Introduction

The degrees of isolation of the polar vortex of the stratosphere have been extensively discussed in relation to the ozone depletion problem by isentropic advection in which air parcels or material contours are advected by large-scale winds from objective analysis data sets (e.g., Haynes and Shuckburgh (2000) and references therein). Haynes and Shuckburgh (2000) has recently applied the effective diffusivity diagnostic introduced by Nakamura (1996) to an artificial "test tracer" field in order to identify barriers to transport and mixing regions not only for the "winter polar vortex-edge barrier" but also for the "sub-tropical barrier" in the stratosphere.

In this paper, we describe a method of objectively identifying barriers of the polar vortex edge to quasi-isentropic transport and of quantifying the permeability of the barriers on the basis of the time threshold diagnostics (TTD) developed by Sugata (2000) as a method of quasi-isentropic advection of air parcels. The method is applied to the polar vortex in the 1996/1997 Northern Hemisphere winter. The winter is well-known in that the polar vortex was maintained until the beginning of May, i.e., the Arctic polar vortex was stable and maintained abnormally long to bring about anomalously low ozone over the Arctic in March (e.g., Newman et al., 1997).

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