Abstract
Abstract. The EU CANDIDOZ project investigated the chemical and dynamical influences on decadal ozone trends focusing on the Northern Hemisphere. High quality long-term ozone data sets, satellite-based as well as ground-based, and the long-term meteorological reanalyses from ECMWF and NCEP are used together with advanced multiple regression models and atmospheric models to assess the relative roles of chemistry and transport in stratospheric ozone changes. This overall synthesis of the individual analyses in CANDIDOZ shows clearly one common feature in the NH mid latitudes and in the Arctic: an almost monotonic negative trend from the late 1970s to the mid 1990s followed by an increase. In most trend studies, the Equivalent Effective Stratospheric Chlorine (EESC) which peaked in 1997 as a consequence of the Montreal Protocol was observed to describe ozone loss better than a simple linear trend. Furthermore, all individual analyses point to changes in dynamical drivers, such as the residual circulation (responsible for the meridional transport of ozone into middle and high latitudes) playing a key role in the observed turnaround. The changes in ozone transport are associated with variations in polar chemical ozone loss via heterogeneous ozone chemistry on PSCs (polar stratospheric clouds). Synoptic scale processes as represented by the new equivalent latitude proxy, by conventional tropopause altitude or by 250 hPa geopotential height have also been successfully linked to the recent ozone increases in the lowermost stratosphere. These show significant regional variation with a large impact over Europe and seem to be linked to changes in tropospheric climate patterns such as the North Atlantic Oscillation. Some influence in recent ozone increases was also attributed to the rise in solar cycle number 23. Changes from the late 1970s to the mid 1990s were found in a number of characteristics of the Arctic vortex. However, only one trend was found when more recent years are also considered, namely the tendency for cold winters to become colder.
Highlights
The possible depletion of the ozone layer was raised in the early 1970s (Crutzen, 1971; Johnston, 1971; Molina and Rowland, 1974; Stolarski and Cicerone, 1974)
The conclusions of these statistical analyses of observations are supported by a SLIMCAT modelling study which compared total ozone observations with total ozone for a simulation driven by ERA-40 winds and temperatures, but without chemical ozone depletion from Ozone Depleting Substances (ODS) (Fig. 3, updated from Hadjinicolaou et al, 2005)
The importance of dynamical influences on ozone trends has been recognised for some time, the difficulties in quantifying the impact has led to past trends being generally attributed to halogen chemistry with an implicit assumption that any uncertainties resulting from interannual changes in dynamics are adequately represented in the error bars associated with the trend estimates
Summary
The possible depletion of the ozone layer was raised in the early 1970s (Crutzen, 1971; Johnston, 1971; Molina and Rowland, 1974; Stolarski and Cicerone, 1974). The implementation of the Montreal Protocol, its adjustments and amendments has successfully resulted in reduced global production of ODS (and, with a small delay, emissions) from the end of the 1980s (Fig. 1, top panel) This has led to a more recent decline of the effective stratospheric chlorine loading (EESC) by about 6% since its peak in the late 1990s (Fig. 1, middle panel). Quantification of dynamical influences on stratospheric ozone changes was highlighted as an outstanding issue in ozone research, for which the level of scientific understanding was quoted as medium/medium-low in WMO 2002 (Table 4.5 in Chipperfield and Randel, 2003). This uncertainty strongly limits the interpretation of the past evolution of the. The 2006 Assessment was written during the preparation of this paper and is only referred to for specific points
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