Separating the effects of ozone depletion/recovery and climate change on the dynamics of the Southern Hemisphere stratosphere and troposphere

The Southern Hemisphere (SH) stratosphere and troposphere have undergone significant changes in recent decades as a result of stratospheric ozone depletion. As the ozone layer recovers many of these changes are expected to reverse, but the degree to which this happens will depend upon the effects of climate change. In order to understand the relative impacts of ozone depletion/recovery and climate change a comprehensive middle atmosphere model is required. Here we use a version of the Canadian Middle Atmosphere Model (CMAM) that is coupled to an ocean general circulation model. Three sets of transient simulations extending from 1960 to 2100 are examined: one in which greenhouse gas (GHG) concentrations are held fixed at 1960 levels, a second in which ozone depleting substances (ODSs) are held fixed at 1960 levels, and a third in which both GHGs and ODSs are allowed to vary in time. SH circulation changes induced by the ODS forcing occur primarily in austral summer, with past and future trends being opposite in sign, while the GHG forcing produces more uniform changes over the year, with past and future trends having the same sign. The ODS forcing dominates past summertime trends, while the ODS and GHG forcings contribute nearly equally (but in the opposite sense) to future summertime trends. The SH response of various dynamical quantities to the GHG and ODS forcings is shown to be additive in both the stratosphere and troposphere, that is, trends computed from the sum of the first two simulations are equal to trends from the third. Additivity is shown to hold for a number of quantities, including zonal mean zonal wind, lower stratospheric vertical mass flux, wave drag, tropopause pressure, Hadley cell boundary, and annular mode index.