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 Ni¿o/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. ¿ 1999 American Geophysical Union