Abstract
Abstract. While chemistry-climate models are able to reproduce many characteristics of the global total column ozone field and its long-term evolution, they have fared less well in simulating the commonly used diagnostic of the area of the Antarctic ozone hole i.e. the area within the 220 Dobson Unit (DU) contour. Two possible reasons for this are: (1) the underlying Global Climate Model (GCM) does not correctly simulate the size of the polar vortex, and (2) the stratospheric chemistry scheme incorporated into the GCM, and/or the model dynamics, results in systematic biases in the total column ozone fields such that the 220 DU contour is no longer appropriate for delineating the edge of the ozone hole. Both causes are examined here with a view to developing ozone hole area diagnostics that better suit measurement-model inter-comparisons. The interplay between the shape of the meridional mixing barrier at the edge of the vortex and the meridional gradients in total column ozone across the vortex edge is investigated in measurements and in 5 chemistry-climate models (CCMs). Analysis of the simulation of the polar vortex in the CCMs shows that the first of the two possible causes does play a role in some models. This in turn affects the ability of the models to simulate the large observed meridional gradients in total column ozone. The second of the two causes also strongly affects the ability of the CCMs to track the observed size of the ozone hole. It is shown that by applying a common algorithm to the CCMs for selecting a delineating threshold unique to each model, a more appropriate diagnostic of ozone hole area can be generated that shows better agreement with that derived from observations.
Highlights
The phrase “the ozone hole” is used to describe the severe annual Antarctic spring-time ozone depletion that was first observed more than 25 years ago
The primary objective of this paper is to examine how the two factors mentioned above affect the simulation of the ozone hole area in five chemistry-climate models (CCMs)
There is a sharp decrease in observed total column ozone at umn ozone from SOCOL version 2.0 shown here is a sigthe edge of the vortex between 60◦ S and 70◦ S equivalent nificant improvement on the results shown in Eyring et al
Summary
The phrase “the ozone hole” is used to describe the severe annual Antarctic spring-time ozone depletion that was first observed more than 25 years ago. The ozone hole is most commonly defined as the region at high southern latitudes enclosed by the 220 DU total column ozone contour (Newman et al, 2004) Diagnostics such as the daily ozone hole area and the annual maximum area are widely reported and are used as general measures of the severity of Antarctic ozone depletion. The primary objective of this paper is to examine how the two factors mentioned above (biases in total column ozone and the size of the circumpolar vortex) affect the simulation of the ozone hole area in five CCMs. The CCMs’ simulation of the position and width of the mixing barrier at the edge of the Antarctic polar vortex is compared with meteorological reanalyses using a metric that highlights the region of reduced meridional mixing at the edge of the Antarctic polar vortex (Bodeker et al, 2002; Wang et al, 2005). Combining the results from the comparison of the polar vortex edge with estimates of the total column ozone biases in the CCMs, two diagnostics of the areal extent of Antarctic ozone loss which preserve the ozone hole concept but allow for a more consistent comparison between models and observations are introduced and discussed
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