Summary
The total ozone column (TOC) in the 'collar' region outside the Antarctic Ozone Hole exhibits a pronounced wave-1 zonal pattern in austral spring. During the period 1979-2005, the zonal minimum of the pattern exhibited a general eastward shift of approximately 45 degrees of longitude, while the zonal maximum was relatively stable in location [1]. This behaviour has been linked to feedback by the Ozone Hole on the stratospheric wave-1 quasi-stationary planetary wave (QSW) outside the polar vortex [1], and has potentially exposed the biologically productive Weddell Sea region to increased ultraviolet radiation during spring. Here we extend the work of [1] to examine the seasonal trends in TOC zonal asymmetry at high southern latitudes using observational data and simulations of the Chemistry-Climate Model Validation (CCMVal-2) activity. The CCMVal-2 models simulate 3-dimensional atmospheric circulation with fully interactive stratospheric chemistry: greenhouse gases (GHG) are prescribed and ozone is evaluated. We examine a subset of the models, and focus on the austral winter (JJA), spring (SON) and summer (DJF) months for the 'modern era' (1960-2005; REF-B1) and the 'prediction era' (1960-2100; REF-B2) and compare against observations for 1979-2005. Overall we find that the REF-B1 linear trends in the longitude of the TOC minimum and maximum positions are similar to those in the historical observations during spring and summer, and exhibit a general eastward tendency. Smaller trends are also apparent in winter (i.e. before the annual formation of the Ozone Hole). The REF-B2 scenarios suggest a slowing or reversal of eastward tendency during the first half of the 21st century during ozone recovery. However the during the second half of the 21st century, a weak eastward tendency reappears. We conclude that the eastward shift is contributed by both GHG changes and ozone depletion: GHG changes drive a basic long-term shift, which is enhanced in SON and DJF by ozone depletion. We propose that the trends in TOC asymmetry are forced by strengthening of the stratospheric zonal flow due to changes in the meridional temperature gradient, which is interacting with the asymmetry of the Antarctic continent to displace the QSW wave-1 pattern and thus influence the TOC distribution. [1] Grytsai, A.V., O.M. Evtushevsky, O.V. Agapitov, A.R. Klekociuk, and G.P. Milinevsky. Structure and long-term change in the zonal asymmetry in Antarctic total ozone during spring. Annales Geophysicae 25, 361-374, 2007.