Understanding the Tropospheric Ozone Response to Changes in the Stratospheric Circulation

Abstract:

Chemistry-climate models robustly predict increases in the large-scale stratospheric circulation and stratosphere-troposphere exchange (STE) in response to increasing greenhouse gases. Our previous work has shown that current variability in the stratospheric circulation and stratosphere-to-troposphere ozone flux driven by a combination of El Niño /Southern Oscillation (ENSO) and the stratospheric Quasi-Biennial Oscillation (QBO) provides a “natural experiment” that may reduce uncertainties in predictions of the tropospheric ozone response to future changes in stratospheric transport. Using six years of measurements from the Tropospheric Emission Spectrometer (TES) and Microwave Limb Sounder (MLS) onboard NASA’s Aura satellite, we found that interannual variability in the stratospheric circulation of ~±40% leads to changes of ~±2% in northern midlatitude tropospheric ozone (equaling ~1/2 the total observed interannual variability). Here, we further explore the relationship between the stratospheric circulation and tropospheric ozone variability using two models: the Whole Atmosphere Chemistry-Climate Model (WACCM) and the GEOS-Chem chemistry-transport model (CTM). With the WACCM model, we further explore and untangle the roles of ENSO and the QBO in driving circulation changes and examine small but important differences in the response of the residual vertical velocity and the transport velocity (as measured by the water vapor tape recorder) to these cycles. We also diagnose large differences in the relationship between stratospheric and tropospheric ozone in the specified dynamics and free-running versions of WACCM. With the GEOS-Chem CTM, we use a 30-year simulation to examine the stability of our satellite-derived diagnostics over longer time periods and their sensitivity to changes in meteorology and emissions. We also apply our diagnostics to a 6-year joint 3Dvar assimilation of TES and MLS observations in GEOS-Chem and examine whether the assimilation provides additional constraints on the stratospheric contribution to tropospheric ozone.