Near cancellation of Southern Hemisphere atmospheric circulation trends in 2000–2060 owing to stratospheric ozone recovery: An attribution study with CAM

We examine two, ten-member ensembles of CAM3 integrations that were carried out as part of the Large Ensemble experiment (Deser, Journal of Climate, 2010, submitted). Specifically, in one set the atmospheric model is integrated over the period 2000-2060, with a complete set of forcings: SSTs from a 40-member set of ocean-atmosphere coupled model integrations, greehouse gas concentrations from the A1B scenario, and a stratospheric ozone field that starts with severe depletion over the South Pole and recovers to near 1960 values by mid-century, as expected from the banning of CFCs by the Montreal protocol. In the second set, by contrast, only the transient ozone field is included, and all other forcings are kept at year 2000 levels. We find that, in DJF, all key indices of atmospheric circulation (e.g. the latitudinal position of the eddy driven jet, or the extent of the Hadley cell) show significant negative trends in the second set, owing to the closing of the ozone hole. In the first set however, all key circulation indices become statistically insignificant. This suggest a nearly complete cancellation between the effects of increasing greenhouse gases and those of stratospheric ozone depletion. While such cancellations have recently been reported in the context of the upcoming 2011 Ozone Assessment Report, using Chemistry-Climate-Models (CCMs), this study offers an independent confirmation using an CMIP3-class model, i.e. one without a well resolved stratosphere or interactive ozone chemistry, and the first clear attribution of the reduced trends as a consequence of ozone recovery.