Output from a three-dimensional off-line chemical transport model and the technique of contour advection are used to give a high-resolution view of the transport of PSC-activated air in the northern hemisphere lower stratosphere. Three northern hemisphere winters are examined, 1991--1992, 1992--1993, and 1993--1994. The structure of the polar vortex and area of temperatures below 195 K on the 475 K isentrope are compared with 500 hPa geopotential height fields. These show how meridional excursions of the tropospheric jets can have a strong influence on the polar vortex. In contrast, approximately westerly flow in the troposphere can result in a relatively undisturbed vortex with a large area of temperatures below 195 K. Two tropospheric flow patterns are identified which frequently produce transport from the polar vortex into the midlatitudes. These are ridges over northern Europe and troughs over the North Pacific. By using a vortex-following coordinate system, the amount of PSC-activated air transported into midlatitudes is quantified for the periods of this study at isentropic levels 400 K, 475 K, and 550 K.

This calculation indicates that there is considerable interannual variation in the amount. On the 475 K isentrope the amount ranges from nearly 50% of the total vortex mass in 1991--1992 to around 10% in 1993--1994 or, when expressed as a percentage of the mass at midlatitudes, 25% and 5%, respectively. In 1992--1993, even though there was a long period with temperatures below the PSC threshold, PSC-activated air was largely contained within the vortex, with only around 20% on the 475 K isentrope and less than 10% on the 550 K isentrope of the total vortex mass transported into midlatitudes. However in 1992--1993, unlike the other two years, there was much stronger transport of PSC-activated air into midlatitudes on the 400 K isentrope (this transport increased right down to the 350 K isentrope), the figure being around 25% of the total vortex mass. These results are discussed in the context of the observed ozone decrease in midlatitudes during winter. In particular, the results indicate that in situ destruction of ozone in midlatitudes by PSC-activated air transported from the vortex cannot account for the large ozone decrease observed during the winter of 1992--1993. ¿ American Geophysical Union 1995