Northern Hemisphere Winter-Spring Ozone Variations during Past 30 Years Using Ozonesonde and the High-Resolution MERRA-2 GMI Simulation

We use O₃ simulations from the high-resolution GEOS-5 Replay run (MERRA-2 GMI at 0.5° model grids, ~50 km) and observations from ozonesondes to investigate factors controlling the interannual variations (IAV) in troposphere and lower stratosphere during the past 30 years. Our work focuses on the winter/spring season over north America and Europe, where strong IAV occurs and most of the ozonesonde observations are available. Both model and ozonesondes show positive ozone anomalies in the lower stratosphere and troposphere after strong El Nino’s, e.g., 1997/1998, caused by an increase in the stratospheric ozone flux. The ozonesonde data show a negative ozone anomaly in 1992-1994 followed by a positive trend in 1990s both in the troposphere and the lower stratosphere following the Pinatubo eruption. We use a stratospheric ozone tracer to gauge the impact of stratospheric ozone variations in different regions of the troposphere. Our results suggest that the influence of the stratospheric IAV is significant in the middle to lower troposphere over N. America, while over Europe, the stratospheric influence is limited to the middle to upper troposphere. We examine mechanisms for downward transport from the stratosphere to the troposphere, with a focus on variations in the role of dynamical IAV (e.g. Arctic Oscillation) on the changes in stratosphere-troposphere exchange to explain the spatial and temporal variations of tropospheric ozone over N. America and Europe.