Hemispheric asymmetries and seasonal variations of the lowermost stratospheric water vapor and ozone derived from SAGE II data

The distributions of water vapor and ozone in the lowermost stratosphere, the part of the stratosphere between the tropopause and the 380-K potential temperature surface, are examined using the Stratospheric Aerosol and Gas Experiment II measurements for the period of March 1989 to February 1990. The monthly mean values of water vapor and ozone are computed for the stratospheric portions of the 320-K and 350-K potential temperature surfaces for both hemispheres. We find a strong seasonal cycle in the water vapor mixing ratio on the 320-K potential temperature surface for both hemispheres, with maximum values in the summer season and minimum values in early spring. The seasonal cycle at the 350-K isentrope is similar to that at the 320 K but with a reduced amplitude. Comparisons with their distributions on the 420-K isentrope are made in order to explore information on stratosphere-troposphere exchange via the extratropical tropopause. Both water vapor and ozone data suggest that troposphere to stratosphere exchange across the extratropical tropopause has a significant influence on the lowermost stratosphere, especially for the northern hemisphere in the summer season.