Dynamics of Southern Hemisphere Super Gyre Response to External Forcings

The subtropical gyres in the Southern Hemisphere are interlinked through the Tasman and Agulhas Leakages and extend throughout the mid-latitude ocean basins of all major oceans. This vast ocean circulation system is called the Southern Hemisphere Super Gyre (SHSG). Previous studies have found a significant strengthening and poleward shift of the SHSG in recent decades. By analyzing multi-member ensembles from the Canadian Earth System Model and the Community Earth System Model, as well as a multi-model ensemble from the Detection and Attribution Model Intercomparison Project, this study investigates the relative contributions of greenhouse gas, aerosol, and ozone depletion forcings to changes in the SHSG since the 1950s. Results show that the strengthening and poleward shift of the SHSG have been dominated by the greenhouse gas forcing, which induces an intensification and poleward shift of the westerlies in the Southern Hemisphere, leading to a southward migration of the zero wind stress curl line and an increase in the positive wind stress curl over the southern SHSG. In contrast, the ozone depletion forcing plays a secondary role in changing the SHSG because its effect on the westerlies is further south than the greenhouse gas forcing. The aerosol forcing has little influence on the SHSG due to its weak effect on the winds in the Southern Hemisphere. The dominance of changes in wind stress curl is further validated through a set of partially coupled experiments in which the contribution of buoyancy and wind stress forcings are separated.