The Effects of a 1998 Observing System Change on MERRA‐2‐Based Ozone Profile Simulations

Model simulations of ozone (O₃) driven by meteorological reanalyses are useful for filling observational gaps and interpreting observed O₃ variability and trends. However, the transport circulation of reanalysis products is impacted by changes to the observing system (the data assimilated into the reanalyses). We examine the impacts of these changes on simulated O₃ from two models, GMI CTM and M2 GMI Replay, using observations from global ozonesondes (>50,000 profiles) and satellites from 1980-2016. Both models are constrained by meteorology from the NASA MERRA-2 reanalysis, and both use versions of NASA’s GMI chemical mechanism. We focus on an observing system change affecting simulated O₃ after 1998, associated with the assimilation of temperature and humidity data from new microwave profiling satellites. A large post-1998 O₃ increase, mainly confined to 15-20 km altitude, of ~10 DU in midlatitudes occurs in the GMI CTM, worsening the bias compared to observations (see Figure). In contrast, an increase in M2 GMI Replay O₃ of ~10 DU is observed only near -60° latitude, reducing the bias compared to observations. The GMI CTM O₃ high biases display a QBO-like periodicity that result from excessive transport from the tropical stratosphere to the midlatitude lower stratosphere during the QBO westerly phase. We quantify O₃ discontinuities caused by MERRA-2 observing system changes and demonstrate how M2 GMI Replay dampens the effects of these changes and QBO-driven artifacts on simulated lower stratospheric and total O₃. We caution against using simulations driven by a reanalysis to derive multi-decadal O₃ trends, especially prior to 1998.