Analysis of Ozone Variations in the Troposphere and Lower Stratosphere during the Past 30 Years Using Observations and the High-Resolution MERRA-2 GMI Simulations

We use O₃ simulations from the high resolution GEOS-5 Replay simulation (MERRA-2 GMI at 0.5° horizontal resolution) and observations from ozonesondes and Aura instruments to investigate the factors controlling the observed interannual and trend variations in tropospheric O₃ during the past 30 years. Our work focuses on the Northern Hemisphere mid-high latitudes, where 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. The model fully reproduces the observed negative anomalies in the upper troposphere and lower stratosphere, but underestimates these anomalies in the middle and lower troposphere. We carry out a series of diagnostic tests to examine the variations of chemistry and transport processes over past 30 years in the model including using a stratospheric ozone tracer, which gauges the impact of stratospheric ozone variations in different regions of the troposphere.