Stratosphere-troposphere exchange at the subtropical jet: contribution to the tropospheric ozone budget at midlatitudes

The interannual variability of the springtime maximum in free tropospheric ozone above Colorado is examined using 5-1/2 years (1993-1998) of Differential Absorption Lidar (DIAL) data. Year-to-year differences of up to 20% in the monthly mean mixing ratios are shown to be correlated with changes in the 200 hPa wind speed over the eastern Pacific that are associated with the El Nino/Southern Oscillation (ENSO). This correlation is attributed to variations in the intensity of stratosphere-troposphere exchange (STE) associated with the mean transverse circulation at the subtropical jet exit over the eastern Pacific.