Relative influences of atmospheric chemistry and transport on Arctic ozone trends

The reduction in the amount of ozone in the atmospheric column over the Arctic region, observed during the 1990s1,2, resembles the onset of the Antarctic ozone ‘hole’ in the mid-1980s, but the two polar regions differ significantly with respect to the relative contributions of chemistry and atmospheric dynamics to the ozone abundance. In the strong, cold Antarctic vortex, rapid springtime chemical ozone loss occurs throughout a large region of the lower stratosphere, whereas in the Arctic, although chemical ozone depletion has been observed3,4,5,6,7,8,9,10,11, the vortex is generally much smaller, weaker and more variable12. Here we report a model-based analysis of the relative importance of dynamics and chemistry in causing the Arctic ozone trend in the 1990s, using a state-of-the-art three-dimensional stratospheric chemistry–transport model. North of 63°?N we find that, on average, dynamical variations dominate the interannual variability, with little evidence for a trend towards more wintertime chemical depletion. However, increases in the burden of atmospheric halogens since the early 1970s are responsible for a large (14%) reduction in the average March column ozone, but this effect is mostly caused by increased destruction throughout the year rather than by halogen chemistry associated with wintertime polar statospheric clouds. Any influence of climate change on future average Arctic ozone amounts may thus be dominated by possible circulation changes, rather than by changes in chemical loss.