Attribution of decadal variability in lower-stratospheric tropical ozone

The variability of ozone in the lower stratosphere in a climate-chemistry model is investigated by multiple regression analysis. The model includes forcing due to changes in solar irradiance and to anomalies in sea surface temperatures (SST). When ozone calculated for the period 1979-2003 is regressed against time and 10.7 cm radio flux (f10.7), the regression coefficient of f10.7 at 52 hPa is 2.8% per 100 flux units. This decreases to 1.8% if a lagged index of El Nino-Southern Oscillation (ENSO) is included in the regression model, and to 0.78% if the period of analysis is extended to 1950-2003. The last value is in good agreement with simulations of fixed solar maximum vs. solar minimum conditions that do not include SST variability. These results suggest that some of the decadal variability in tropical ozone previously attributed to solar variability may instead be related to the occurrence of ENSO events.