Dynamical Drivers of Recent Boreal Winter Ozone Trends in the Northern Hemisphere Lower Stratosphere

Previous studies have identified a decrease in lower stratospheric ozone since the year 1998 that is robust in the Northern Hemisphere across multiple independent observing platforms. Although internal variability appears to be the primary driver of these ozone changes the exact mechanism by which ozone is changing is not well understood. Here we use the Goddard Earth Observing System (GEOS) general circulation model, constrained with meteorological fields from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), to investigate the drivers of recent ozone changes, with a focus on the Northern Hemisphere lower stratosphere (NH LS) during boreal winter. Analysis of an idealized tracer that covaries closely with ozone on interannual and decadal timescales reveals that recent ozone decreases in the NH subtropics are associated with a poleward expansion of upwelling in the NH LS, with reduced (enhanced) downwelling over northern subtropics (midlatitudes). We also perform a ten-member ensemble of free-running simulations and show that the MERRA-2 ozone trends are qualitatively within the free-running ensemble spread, although the magnitude of the free-running simulations’ trends are significantly weaker. Finally, we place our GEOS-based results in the broader context of the hindcast simulations performed as part of Phase 1 of the Chemistry Climate Modeling Initiative.