MODULATION OF MIDDLE AND UPPER-TROPOSPHERIC OZONE AT NORTHERN MIDLATITUDES BY THE EL-NINO SOUTHERN OSCILLATION

A five-year time series of lidar profiles from a site in Colorado is u sed to investigate the sources and scales of variability for ozone in the free troposphere. Analysis of 475 daily mean profiles derived from more than 25,000 individual profiles shows that similar to 66% of the total variance is due to fluctuations on time scales from I day to 1 month, similar to 30% on time scales from 1 month to 1 year, and simil ar to 4% on time scales greater than 1 year. The analysis also identif ies low-frequency fluctuations correlated with the El Nino/Southern Os cillation (ENSO) (following a similar to 6-month lag) that may account for as much as 5% of the total variance in the data. The ENSO-related changes can also lead to apparent ozone trends that range from -0.2 t o +0.5 ppbv/yr for different 10-year intervals between 1970 and 1990.