QUANTIFICATION OF THE TRANSPORT OF CHEMICALLY ACTIVATED AIR FROM THE NORTHERN-HEMISPHERE POLAR VORTEX

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, 19 91-1992, 1992-1993, and 1993-1994. The structure of the polar vortex a nd area of temperatures below 195 K on the 475 K isentrope are compare d with 500 hPa geopotential height fields, These show how meridional e xcursions of the tropospheric jets can have a strong influence on the polar vortex. In contrast, approximately westerly flow in the troposph ere can result in a relatively undisturbed vortex with a large area of temperatures below 195 K, Two tropospheric flow patterns are identifi ed which frequently produce transport from the polar vortex into the m idlatitudes. These are ridges over northern Europe and troughs over th e North Pacific. By using a vortex-following coordinate system, the am ount of PSC-activated air transported into midlatitudes is quantified for the periods of this study at isentropic levels 400 K, 475 K and 55 0 K, This calculation indicates that there is considerable interannual variation in the amount. On the 475 K isentrope the amount ranges fro m nearly 50% of the total vortex mass in 1991-1992 to around 10% in 19 93-1994 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 w as largely contained within the vortex, with only around 20% on the 47 5 K isentrope and less than 10% on the 550 K isentrope of the total vo rtex mass transported into midlatitudes. However in 1992-1993, unlike the other two years, there was much stronger transport of PSC-activate d air into midlatitudes on the 400 K isentrope (this transport increas ed 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 t he observed ozone decrease in midlatitudes during winter. In particula r, the results indicate that in situ destruction of ozone in midlatitu des by PSC-activated air transported from the vortex cannot account fo r the large ozone decrease observed during the winter of 1992-1993.