Ensemble simulations of the role of the stratosphere in the attribution of tropospheric ozone variability
Authors: Hess P. , Kinnison D. , Tang Q. 
Published: August 2014

Abstract. Despite the need to understand the impact of changes in emissions and climate on tropospheric ozone, attribution of tropospheric interannual ozone variability to specific processes has proved difficult. Here we analyze the stratospheric contribution to tropospheric ozone variability and trends from 1953–2005 in the Northern Hemisphere (N.~H.) mid-latitudes using four ensemble simulations of the Free Running (FR) Whole Atmosphere Community Climate Model (WACCM). The simulations are forced with observed time varying: (1) sea surface temperatures (SSTs), (2) greenhouse gases (GHGs), (3) ozone depleting substances (ODS), (4) Quasi-Biennial Oscillation (QBO); (5) solar variability (SV) and (6) stratospheric sulfate surface area density (SAD). Detailed representation of stratospheric chemistry is simulated including the ozone loss processes due to volcanic eruptions and polar stratospheric clouds. In the troposphere ozone production is represented by CH4-NOx smog chemistry, where surface chemical emissions remain interannually constant. Despite the simplicity of the tropospheric chemistry, the FR WACCM simulations capture the measured N. H. background interannual tropospheric ozone variability in many locations to a surprising extent, suggesting the importance of external forcing in driving interannual ozone variability. The variability and trend in the simulated 1953–2005 tropospheric ozone record from 30–90° N at background surface measurement sites, 500 hPa measurement sites and in the area average is largely explained on interannual timescales by changes in the 150 hPa 30–90° N ozone flux and changes in tropospheric methane concentrations. The average sensitivity of tropospheric ozone to methane (percent change in ozone to a percent change in methane) from 30–90° N is 0.17 at 500 hPa and 0.21 at the surface; the average sensitivity of tropospheric ozone to the 150 hPa ozone flux (percent change in ozone to a percent change in the ozone flux) from 30–90° N is 0.19 at 500 hPa and 0.11 at the surface. The 30–90° N simulated downward residual velocity at 150 hPa increased by 15% between 1953 and 2005. However, the impact of this on the 30–90° N 150 hPa ozone flux is modulated by the long-term changes in stratospheric ozone. The ozone flux decreases from 1965 to 1990 due to stratospheric ozone depletion, but increases again by approximately 7% from 1990–2005. The first empirical orthogonal function of interannual ozone variability explains from 40% (at the surface) to over 80% (at 150 hPa) of the simulated ozone interannual 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 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. We find little relation between the El Nino Southern Oscillation (ENSO) index and large-scale tropospheric ozone variability over the long-term record.