Strengthening of the Southern Ocean Carbon Sink through Recent Changes in Freshwater Forcing

Water masses sourced in the Southern Ocean freshened significantly in recent decades indicating substantial changes in surface freshwater fluxes in their source region or changes in ocean circulation. Yet, it is not very well understood how these changes in freshwater forcing affect the Southern Ocean carbon sink, particularly in interaction with the strong shifts in wind forcing. Here, we contrast the impact of these two changing forcings on the net CO₂ exchange over the Southern Ocean by performing sensitivity experiments with an eddy-resolving regional ocean model (ROMS) that is coupled to an ecological/biogeochemical model (BEC). For the first time we include in our model, additionally to the atmospheric forcing, also observation-based regionally and temporally varying buoyancy fluxes from sea and land ice including their long-term trends. These newly considered forcings improve not only the model’s reproduction of the complex hydrography of the Southern Ocean, they also enable us to demonstrate that the changes in freshwater fluxes from sea ice and glacial melting are largely responsible for the observed salinity changes. These changes cause a significant increase in the vertical stratification of the surface waters by up to 20% in the coastal and open ocean of the Pacific and the high-latitude Atlantic. Based on our model experiments, we argue that this stabilization reduces the entrainment of carbon-rich deep waters into the surface layer owing to a longer persistence of the summer-time stable surface layer and a shallower winter-time convection. If this is the case in reality, then this process enhances the Southern Ocean carbon sink, opposing the effect stemming from the increased wind-driven upwelling that has been suggested to saturate the Southern Ocean carbon sink. In fact, we argue that the freshwater forcing changes are an important contributor to the recently observed reinvigoration of the Southern Ocean carbon sink.