Potential vorticity as a barrier to exchange between the upper troposphere and lowermost stratosphere

High resolution measurements of ozone and other species in the Upper Troposphere Lower Stratosphere by HIRDLS have shown the existence of thin layers (laminae) of low or high ozone on potential temperature (PT) surfaces extending toward high or low latitudes respectively. To assess the contribution of these features to stratosphere-troposphere exchange, zonally averaged ozone mixing ratios on PT surfaces in the lowermost stratosphere (340-380K ) have been calculated for each day of the year. These are then compared to similarly averaged potential vorticity (PV) contours and tropopause locations based on Goddard Modeling and Assimilation Office (GMAO) version 5 data. During winter in each hemisphere, it is found that low ozone values at low latitudes are constrained equatorward of the PV = 4 PVU (1 PVU = 10-6 K m2/kg s) contour, while higher values from high latitudes are constrained poleward of the PV = 6 PVU contour. The thermal tropopause lies between these PV contours. Ozone mixing ratios on the PV = 4 PVU contour do not change during winter. Those on the PV = 6 contour increase due to mixing from high latitudes of higher values resulting from descent in the Brewer Dobson circulation. The results indicate that these PV contours act as elastic membranes that experience considerable latitudinal deformation under the influence of baroclinic waves, but that permit very little cross contour transport or mixing. Distributions of CFC's, which have larger values in the tropics, show variations that are opposite those of ozone, while water vapor shows a more complex behavior. Low gradients are seen over large latitude ranges in other seasons, especially late summer and early autumn. Calculation of a pseudo-effective diffusivity leads to distributions very similar to those shown by Haynes and Shuckburgh (2000).