Séminaire

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 N.H. mid-latitudes using four ensemble simulations of the Whole Atmosphere Community Climate Model (WACCM). The simulations are driven by time changes in observed sea-surface temperatures, concentrations of greenhouse gases (including methane) and ozone depleting substances. While in the stratosphere WACCM employs a sophisticated chemical mechanism, in the troposphere only the basic tropospheric NOX-CH4 chemistry is simulated, where the NOX emissions remain interannually constant. Despite this simplicity, the simulations capture the measured N.H. background ozone variability to a surprising extent. The variability and trends in the simulated tropospheric ozone from record from 30-90N are explained on both the interannual and decadal timescales by changes in the 150 hPa ozone flux and changes in methane concentration. From 40% (at the surface) to over 80% (at 150 hPa) of the simulated ozone variability from 30-90N is captured by a primary mode (as determined by empirical orthogonal function analysis). This mode of variability shows very strong stratosphere-troposphere coupling, demonstrating the importance of the stratosphere in an attribution of tropospheric ozone variability. Over the long term (1953-2005) the simulated downward residual velocity at 150 hPa increases by 16% from 30-90N. However, the impact of this on tropospheric ozone is modulated by stratospheric ozone depletion. The downward ozone flux at 150 hPa reaches a maximum in the 1960s and a minimum near 1990. From 1990 onward, with the recovery of stratospheric ozone, the flux has increased by approximately 7%. This suggests an increased impact of the stratosphere on tropospheric ozone.