Abstract
Abstract. The effect of the winter Brewer-Dobson circulation (BDC) on the seasonal and decadal evolution of total ozone in both hemispheres is investigated using satellite total ozone data from the merged GOME/SCIAMACHY/GOME-2 (GSG) data set (1995–2010) and outputs from two chemistry-climate models (CCM), the FUB-EMAC and DLR-E39C-A models. Combining data from both hemispheres a linear relationship between the winter average extratropical 100 hPa eddy heat flux and the ozone ratio with respect to fall ozone levels exists and is statistically significant for tropical as well as polar ozone. The high correlation at high latitudes persists well into the summer months until the onset of the next winter season. The anti-correlation of the cumulative eddy heat flux with tropical ozone ratios, however, breaks down in spring as the polar vortex erodes and changes to a weak positive correlation similar to that observed at high latitudes. The inter-annual variability and decadal evolution of ozone in each hemisphere in winter, spring, and summer are therefore driven by the cumulative effect of the previous winter's meridional circulation. This compact linear relationship is also found in both CCMs used in this study indicating that current models realistically describe the variability in stratospheric circulation and its effect on total ozone. Both models show a positive trend in the winter mean eddy heat flux (and winter BDC strength) in both hemispheres until year 2050, however the inter-annual variability (peak-to-peak) is two to three times larger than the mean change between 1960 and 2050. It is, nevertheless, possible to detect a shift in this compact linear relationship related to past and future changes in the stratospheric halogen load. Using the SBUV/TOMS/OMI (MOD V8) merged data set (1980–2010), it can be shown that from the decade 1990–1999 to 2000–2010 this linear relationship remained unchanged (before and after the turnaround in the stratospheric halogen load), while a shift is evident between 1980–1989 (upward trend in stratospheric halogen) and the 1990s, which is a clear sign that an onset of recovery is detectable despite the large variability in polar ozone. Because of the large variability from year to year in the BDC circulation substantial polar ozone depletion may still occur in coming decades in selected winters with weak BDC and very low polar stratospheric temperatures.
Highlights
For many years the main focus in studies of long-term ozone changes was the impact on chemical ozone loss from halogen containing anthropogenic ozone depleting substances (ODS) (WMO, 1999)
In this study model runs from 1960 until 2050 with prescribed GHG concentrations from the A1B scenario (IPCC, 2000) and surface mixing ratios of ozone depleting substances (ODS) based on the halogen A1 scenario from WMO (2007) are used
A compact linear relationship between polar spring-to-fall ozone ratio and the modelled winter eddy heat flux is seen in both models, clearly indicating that the overall Brewer-Dobson circulation (BDC) pattern on total ozone is well reproduced by the models (Figs. 9 and 10)
Summary
For many years the main focus in studies of long-term ozone changes was the impact on chemical ozone loss from halogen containing anthropogenic ozone depleting substances (ODS) (WMO, 1999). This close coupling between atmospheric dynamics and chemistry is evident on interannual time scales as shown by the compact linear relationship between the cumulative hemispheric winter eddy heat flux and spring-to-fall polar total ozone ratio combining data from both hemispheres as reported by Weber et al (2003). They showed a corresponding anti-correlation with OClO columns from GOME observations, an indication of chlorine activation inside the polar vortex, that prove year-to-year variability in polar ozone loss are driven by coupled variations in atmospheric dynamics and polar chemistry (Weber et al, 2003; Tegtmeier et al, 2008). The implications on future changes in the ozone – BDC interaction are discussed followed by conclusions from this study
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