Mechanisms Linked to Recent Ozone Decreases in the Northern Hemisphere Lower Stratosphere. Issue 9 (2nd May 2020)

• Record Type: Journal Article
• Authors: Orbe, Clara; Wargan, Krzysztof; Pawson, Steven; Oman, Luke D.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 9(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-05-02
• Subjects: Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2019JD031631
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Previous studies have robustly identified a decrease since 1998 in lower stratospheric (LS) ozone in the Northern Hemisphere (NH). While this ozone decrease is qualitatively explained as resulting from changes in the large‐scale circulation, there is not yet a quantitative mechanistic explanation of these changes. Here, we explore the drivers of recent ozone changes using two different configurations of the Goddard Earth Observing System (GEOS) general circulation model. The first configuration of GEOS includes a full chemistry module and is constrained with meteorological fields from the Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2). This configuration (M2GMI) is used to analyze an idealized tracer that covaries closely with ozone on interannual and decadal timescales, revealing 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). The second configuration of GEOS is a free‐running version of the GEOS Chemistry Climate Model (CCM) that is used to perform a 10‐member ensemble of free‐running simulations. Comparisons of the two configurations reveal that, while the free‐running model can produce negative ozone changes in the NH LS, the magnitude of these changes is significantly weaker, relative to both M2GMI and MERRA‐2; moreover, these weaker ozone decreases are consistent with weaker simulated changes in the residual circulation. Finally, we examine the GEOS model results in the broader context of the hindcast simulations performed as part of Phase 1 of the Chemistry Climate Modeling Initiative. We show that the majority of the free‐running simulations considered here also exhibit weaker long‐term residual circulation changes, compared to reanalyses. Key Points: Free‐running and MERRA‐2‐constrained GEOS runs are used to identify mechanisms for ozone decreases in the lower stratosphere over 1998–2018 Analysis of an idealized e90 tracer indicates that ozone decreases are associated with a poleward expansion of the residual mean circulation Free‐running GEOS simulations produce weaker residual circulation—and associated ozone—changes over 1998–2018, compared to reanalyses