Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning

Ocean overturning circulation requires a continuous thermodynamic transformation of the buoyancy of seawater. The steeply sloping isopycnals of the Southern Ocean provide a pathway for Circumpolar Deep Water to upwell from mid depth without strong diapycnal mixing1, 2, 3, where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch4, 5, 6. While brine rejection from sea ice is thought to contribute to the lower branch7, the role of sea ice in the upper branch is less well understood, partly due to a paucity of observations of sea-ice thickness and transport8, 9. Here we quantify the sea-ice freshwater flux using the Southern Ocean State Estimate, a state-of-the-art data assimilation that incorporates millions of ocean and ice observations. We then use the water-mass transformation framework10 to compare the relative roles of atmospheric, sea-ice, and glacial freshwater fluxes, heat fluxes, and upper-ocean mixing in transforming buoyancy within the upper branch. We find that sea ice is a dominant term, with differential brine rejection and ice melt transforming upwelled Circumpolar Deep Water at a rate of ~22 × 106 m3 s−1. These results imply a prominent role for Antarctic sea ice in the upper branch and suggest that residual overturning and wind-driven sea-ice transport are tightly coupled.

Details