On the Attribution of Tropospheric Ozone Variability

Abstract:

The attribution of tropospheric interannual ozone variability to specific processes has proved difficult despite the need to understand the impact of emission changes and climate on tropospheric ozone. Using four ensemble simulations of the Free Running (FR) Whole Atmosphere Community Climate Model (WACCM) we find the first empirical orthogonal function of interannual detrended ozone variability explains from 40% (at the surface) to over 80% (at 150 hPa) of the simulated ozone variability from 30–90 N. This identified mode of ozone variability shows strong stratosphere–troposphere coupling, demonstrating the importance of the stratosphere in an attribution of tropospheric ozone variability. The simulations, with no change in emissions, capture almost 50% of the measured ozone change during the 1990s at a variety of locations. This suggests that a large portion of the measured ozone change is not due to changes in emissions, but can be traced to changes in large-scale modes of ozone variability. This emphasizes the difficulty in the attribution of ozone changes, and the importance of natural variability in understanding the trends and variability of ozone.