Abstract
Abstract. The decrease of the concentration of ozone depleting substances (ODSs) in the stratosphere over the past decade raises the question to what extent observed changes in stratospheric ozone over this period are consistent with known changes in the chemical composition and possible changes in atmospheric transport. Here we present a series of ozone sensitivity calculations with a stratospheric chemistry transport model (CTM) driven by meteorological reanalyses from the European Centre for Medium-Range Weather Forecasts, covering the period 1978–2009. In order to account for the reversal in ODS trends, ozone trends are analysed as piecewise linear trends over two periods, 1979–1999 and 2000–2009. Modelled column ozone (TO3) inter-annual variability and trends are in excellent agreement with observations from the Total Ozone Mapping Spectrometer (TOMS) and Solar Backscatter UV (SBUV/2) as well as the Global Ozone Monitoring Experiment (GOME/GOME2) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instruments. In the period 1979–1999, modelled TO3 trends at mid-latitudes are dominated by changes in in situ gas-phase chemistry, which contribute to about 50% or more of the TO3 trend in most seasons. Changes in meteorology contribute around 35% to mid-latitude TO3 trends, with strong differences between different seasons. In springtime, export of ozone depleted air from polar latitudes contributes about 35–50% to the modelled TO3 trend at SH mid-latitudes and about 15–30% at NH mid-latitudes. Over the period 2000–2009 positive linear trends in modelled TO3, which agree well with observed TO3 trends, are dominated by changes in meteorology, as expected for the yet small decrease in stratospheric halogen loading over this period. While the TO3 trends themselves are not statistically significant over the period 2000–2009, changes in linear trends between 1978–1999 and 2000–2009 are significant at mid- and high latitudes of both hemisphere during most seasons. However, changes in meteorology have contributed substantially to these TO3 trend changes.
Highlights
Stratospheric ozone has shown large decreases during past decades, mainly attributable to the anthropogenic input of ozone depleting substances (ODSs) such as chlorofluorocarbons (CFCs) (e.g., World Meteorological Organization, 2007)
Modelled column ozone (TO3) inter-annual variability and trends are in excellent agreement with observations from the Total Ozone Mapping Spectrometer (TOMS) and Solar Backscatter UV (SBUV/2) as well as the Global Ozone Monitoring Experiment (GOME/GOME2) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instruments
In the period 1979–1999, modelled TO3 trends at mid-latitudes are dominated by changes in in situ gas-phase chemistry, which contribute to about 50% or more of the TO3 trend in most seasons
Summary
Stratospheric ozone has shown large decreases during past decades, mainly attributable to the anthropogenic input of ozone depleting substances (ODSs) such as chlorofluorocarbons (CFCs) (e.g., World Meteorological Organization, 2007). Since the turn of the century, stratospheric concentrations of ODSs have begun to decrease. An onset of recovery of the ozone layer is expected (Newman et al, 2006; World Meteorological Organization, 2007). Some studies have reported the first stage of ozone recovery (i.e., a statistically significant change in ozone trend) (Newchurch et al, 2003; Yang et al, 2008; Reinsel et al, 2005). In order to obtain a better understanding of a possible onset of recovery, a diligent attribution of ozone trends to different contributing factors is necessary. As ozone concentrations feed back on the circulation through changes in radiative heating, “chemical” and “dynamical” effects can never be fully separated
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