Separating the Effects of Ozone Depletion and Greenhouse Gas Increases on Southern Hemisphere Climate Change

Stratospheric ozone depletion plays a major role in driving climate change in the Southern Hemisphere. However, there are very few studies to quantitatively assess the relative importance of ozone depletion and greenhouse gas increases on Southern Hemisphere climate change, particularly on ocean and sea ice. The purpose of this study is to separate the effects of these two forcings in driving recent Southern Hemisphere climate trends in the stratosphere, troposphere, surface, and the Southern Ocean. Three sets of 1960-2010 ensemble transient simulations are conducted using the coupled ocean version of the Goddard Earth Observing System Model version 5: one control simulation with changing greenhouse gases (GHGs) and ozone depleting substances (ODSs), and two single-forcing simulations in which GHGs and ODSs are respectively fixed at 1960 levels.

Consistent with previous studies, ozone depletion is the dominant driver of Southern Hemisphere stratosphere climate change. In the austral late spring and summer, the ozone hole causes strong Antarctic lower stratospheric cooling and westerly acceleration, and increases the Brewer-Dobson circulation. The stratospheric trends due to ozone depletion and GHG increases are linearly additive: the sum of the trends from the two single-forcing simulations is similar to that from the control simulation. In the troposphere and surface, however, trends are not linearly additive. Ozone depletion and GHG increases produce comparable trends in the summer surface temperature, precipitation, and the Southern Annular Mode, and the sum of the two single-forcing trends is greater than the control simulation trend. We also assess the roles of the two forcings on Southern Ocean and Antarctic sea ice change. Again we find that the simulated Southern Ocean warming, meridional overturning circulation spin-up, and Antarctic sea ice decrease driven by the two forcings are highly nonadditive.