The Transient Response of the Southern Ocean to Stratospheric Ozone Depletion

The impact of stratospheric ozone depletion on the climate of the Southern Ocean is evaluated using an ensemble of GFDL ESM2Mc coupled climate model simulations. By imposing a step change from 1860 to 2000 ozone concentrations we are able to estimate response functions associated with this change. Ozone depletion results in a poleward shift of the extratropical jet; a shift which is larger when daily rather than monthly ozone concentrations are prescribed. Two time scales are found for the ocean response, an initial cooling centered in the Southwest Pacific followed by cooling in the Pacific sector and then warming in both sectors. The physical processes that drive this response are different across time periods and locations, as is the sign of the response itself. Initial cooling in the Pacific sector is not just driven by the increased winds pushing cold water northward, but also by a decrease in surface salinity reducing wintertime mixing and increased ice and clouds reflecting more shortwave radiation back to space. The decrease in salinity is primarily driven by a southward shift of precipitation associated with a shifting storm track, coupled with decreased evaporation associated with colder surface temperatures. A subsurface increase in heat associated with this reduction in mixing then upwells along the Antarctic coast, producing a subsequent warming. Similar changes in convective activity occur in the Weddell Sea but are offset in time. The time scale for the initial cooling is much longer than that seen in the NCAR CCSM3.5 model, possibly a result of differences in the parameterization of vertical mixing and the representation of cloud feedbacks in the two models.