The Stratospheric Contribution to Tropospheric Ozone Variability and Trends

Abstract:

Chemistry-climate models predict an intensification of the stratospheric circulation over the next century in response to greenhouse gas forcing, with uncertain consequences for tropospheric ozone and climate. We examine the role of the stratosphere in present-day tropospheric ozone variability and trends as measured by the Tropospheric Emission Spectrometer (TES) and Microwave Limb Sounder (MLS) onboard NASA’s Aura satellite. Using six years of observations, we find that interannual variability in the stratospheric circulation (diagnosed from measurements of the water vapor tape recorder) of ±40%, which is driven by El Niño /Southern Oscillation (ENSO) and the Quasi-Biennial Oscillation (QBO), leads to changes of ±25 in northern midlatitude lower stratospheric ozone and ±2% in northern midlatitude tropospheric ozone. The stratosphere thus accounts for ~1/2 of the interannual variability in ozone in the mid-troposphere of the northern midlatitudes. While the MLS measurements do not suggest any trend in the stratospheric circulation or zonally averaged lower stratospheric ozone for the 2005-2010 time period, recent modeling work using TES ozone and OMI NO₂ measurements suggests a large trend in the stratospheric contribution to tropospheric ozone over Asia and the Western United States that confounds quantification of emissions-driven changes over these regions. We present the results of these two studies and, more generally, assess the role of the stratosphere in both the variability and trends in tropospheric ozone throughout the extratropics as constrained by the Aura observations and as represented in chemistry-climate models. We also discuss the implications of our work for reducing uncertainties in long-term projections of tropospheric ozone.