Recent Decline in Extratropical Lower Stratospheric Ozone Attributed to Circulation Changes

The 1998-2016 ozone trends in the lower stratosphere are examined using the Modern-Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2) and related National Aeronautics and Space Administration products. After removing biases resulting from step changes in the MERRA-2 ozone observations, a discernible negative trend of -1.67 +/- 0.54Dobson units per decade (DU/decade) is found in the 10-km layer above the tropopause between 20 degrees N and 60 degrees N. A weaker but statistically significant trend of -1.17 +/- 0.33DU/decade exists between 50 degrees S and 20 degrees S. In the Tropics, a positive trend is seen in a 5-km layer above the tropopause. Analysis of an idealized tracer in a model simulation constrained by MERRA-2 meteorological fields provides strong evidence that these trends are driven by enhanced isentropic transport between the tropical (20 degrees S-20 degrees N) and extratropical lower stratosphere in the past two decades. This is the first time that a reanalysis data set has been used to detect and attribute trends in lower stratospheric ozone. Plain Language Summary Stratospheric ozone shields the biosphere from harmful ultraviolet radiation and affects the Earth's radiative budget. Observational data show evidence that concentrations of ozone in the upper stratosphere have increased in the last 15years. This is an expected result of the implementation of the Montreal Protocol and its amendments banning emissions of ozone depleting substances into the atmosphere. The evolution of stratospheric ozone is also impacted by climate change through its dependence on temperature and circulation, which can be different at different altitudes. These effects are less well understood. This study uses the National Aeronautics and Space Administration's data and computer models to analyze the long-term changes in ozone since 1998. It is shown that the increase in the upper stratospheric ozone has been partially offset by a small but discernible decline of ozone concentrations in the lowermost stratosphere, in qualitative agreement with one recent study. A chemistry model simulation forced by meteorological data provides strong evidence that the primary mechanism driving this negative trend is an intensification of transport of ozone-poor air from the Tropics into the extratropics, indicative of a systematic change in the lower stratospheric circulation between 1998 and 2016.