Impacts of Stratospheric Ozone Recovery on Southern Ocean Temperature and Heat Budget

The stratospheric ozone layer is projected to recover in this century with the phaseout of the ozone depleting substances (ODSs). Previous studies have found that stratospheric ozone recovery causes an equatorward shift of the Southern Hemisphere midlatitude westerlies and the Southern Ocean circulation. However, the impacts of ozone recovery on Southern Ocean heat budget have not been examined before. In this study we investigate the impacts of stratospheric ozone recovery on Southern Ocean surface and interior temperature, heat content, heat uptake, and heat transport using the coupled atmosphere-ocean Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM). Two ensemble simulations, each with 4 members, were performed for 2005-2099: one with fixed 2005-level ODSs and another with projected decreasing ODSs.

In our simulations ozone recovery significantly affects Southern Ocean temperature, with large latitudinal and vertical variations. The Southern Ocean sea surface temperature has a dipole response to stratospheric ozone recovery after 2040 with high latitude warming and midlatitude cooling. The response of the interior ocean temperature is more complicated. The subsurface (above 100 m) cools through the Southern Ocean. Between about 100 m and 3000 m ocean temperature increases south of 60°S and decrease in 40°-60°S. The deep ocean below 3000 m warms across the Southern Ocean. These ocean temperature changes are consistent with ozone-recovery-induced changes in the meridional overturning circulation, which is characterized by an equatorward shift of the upper cell and a weakening of the abyssal cell.

Ozone recovery causes a dipole change of the full-depth ocean heat content with an increase south of 60°S and a decrease between 45°S and 60°S. Integrated over latitudes south of 40°S, Southern Ocean heat content decreases in response to ozone recovery. Heat budget analysis shows that the Southern Ocean heat loss is driven by weakened poleward ocean heat transport across 40°S, which is partly cancelled by enhanced heat uptake. The weakening of poleward ocean heat transport into the Southern Ocean is caused by the ozone-induced equatorward shift of the meridional overturning circulation.