Tropospheric Ozone Variations Governed by ENSO-Driven Changes in the Stratospheric Circulation

Chemistry-climate models robustly predict increases in the large-scale stratospheric circulation and stratosphere-troposphere exchange (STE) in response to increasing greenhouse gases. The impact of these stratospheric dynamical changes on “background” levels of tropospheric ozone, knowledge of which is critical to developing effective air quality policies, has heretofore been unconstrained. Here, we use six years of measurements from the Tropospheric Emission Spectrometer (TES) and Microwave Limb Sounder (MLS) onboard the Aura satellite to provide the first observational estimate of the variability in Northern midlatitude tropospheric ozone attributable to changes in the stratospheric circulation. We show that interannual variability in the stratospheric circulation on the order of +/- 40%, driven largely by El Nino/Southern Oscillation (ENSO) and its interaction with the stratospheric quasi-biennial oscillation (QBO), results in changes of +/- 20% in zonal mean Northern midlatitude stratospheric ozone at 150 hPa. This variability in stratospheric ozone explains 16% of the variance in Northern midlatitude tropospheric ozone at 500 hPa, with a 25% increase in stratospheric ozone leading to a 2% increase in tropospheric ozone approximately one month later. These measurements provide powerful constraints for chemistry-climate models and have the potential to reduce uncertainties in future projections of tropospheric ozone abundances.