Large cancellation, due to ozone recovery, of future Southern Hemisphere atmospheric circulation trends

The role of stratospheric ozone recovery in the Southern Hemisphere climate system, in the coming decades, is examined by contrasting two 10-member ensembles of Community Atmospheric Model (CAM3) integrations, over the period 2000 - 2060. Model integrations in the first ensemble are conducted with a complete set of forcings: greenhouse gas concentrations from the A1B scenario, SSTs from corresponding ocean-atmosphere coupled model integrations, and ozone starting with severe depletion over the South Pole and recovering by mid-century. The integrations in the second ensemble are very similar to the first, except that only the transient ozone forcing is specified, and all other forcings are kept at year 2000 levels. Specifying ozone recovery in isolation allows us to determine unambiguously how it impacts the atmospheric circulation. We find that, in DJF, most key indices of atmospheric circulation show significant trends in the second ensemble, due to the closing of the ozone hole. In the first ensemble, however, trends are found to be statistically insignificant for nearly all key circulation indices. This suggests that ozone recovery will result in a nearly complete cancellation (and possible reversal) of the atmospheric circulation effects associated with increasing greenhouse gases, in Southern Hemisphere summer, over the coming half century.