Estimates of Area-Averaged Diapycnal Fluxes from Basin-Scale Budgets

Estimates of area-averaged diapycnal fluxes for the southern oceans are derived from basin-scale budgets of mass, heat, and salt using a box inverse model. The diapycnal fluxes are found to be significant terms in the isopycnal budgets of mass, heat, and salt. Dense water entering the subtropical Indian and Pacific basins from the south is returned below the thermocline as less dense deep water. In the Southern Ocean deep water is converted to denser deep and bottom water. Water properties at intermediate depth are substantially modified by diapycnal fluxes of heat and salt, but the modification of intermediate water is not solely driven by interior mixing. The inferred fluxes help explain the changes in temperature–salinity curves observed across each basin, and they are consistent with our understanding of the overall three-dimensional circulation of the Southern Ocean. The fact that area-averaged diapycnal fluxes can be determined from basin-scale budgets using a suitably designed inverse model is encouraging: similar methods applied to the high-quality measurements collected during the World Ocean Circulation Experiment promise to provide the first global maps of diapycnal fluxes derived from ocean observations.