Relative Roles of Buoyancy and Wind Forcing in Southern Ocean Climate Change

Observations show that over the past several decades the Southern Ocean has warmed and freshened, which can be attributed to two components of surface forcing: poleward-intensified westerly winds and increased buoyancy flux. Here we aim to isolate and quantify the effects of these two forcings on Southern Ocean change by using a partial coupling technique in CESM. Results from abrupt quadrupling of CO₂ experiments show that buoyancy forcing dominates the Southern Ocean warming, with wind stress change reinforcing the warming between 40˚S to 55˚S and reducing the warming at higher and lower latitudes. The wind-induced warming/cooling pattern is primarily associated with the deepening/shoaling of isopycnals. For salinity change, we find that the strengthened residual meridional overturning circulation due to intensified westerlies brings saltier deep water upward, leading to a surface salinity increase over the Southern Ocean. This wind-induced increase of salinity partially offsets surface freshening due to amplification of global hydrological cycle. Intensified winds increase upwelling of relatively warm deep water, reducing sea ice formation and extent and further affecting the surface salinity redistribution. These changes in Southern Ocean density structure also affect ACC transport. We find that buoyancy forcing causes an ACC transport increase (3.1±1.6 Sv) by increasing the meridional density gradient across the ACC in the upper 2000m, while the wind-induced response is more barotropic, affecting the whole column transport (8.7±2.3 Sv). In addition, the mean ACC position shifts southward, by less than 0.5˚ for wind and buoyancy forcing, separately, indicating insensitivity of the mean ACC position to the surface forcing. These results provide some insight into how the Southern Ocean might change under global warming and lay the groundwork for improved projections.