Water-mass Transformations in the Southern Ocean Diagnosed from Observations: Contrasting Effects of Surface Buoyancy Fluxes and Interior Mixing

The transformation rates of water masses in the Southern Ocean are estimated in an isopycnal framework using air-sea fluxes of heat and freshwater together with in situ estimates of diapycnal mixing. The air-sea fluxes are taken from two different air-sea flux climatologies and a reanalysis dataset, while the diapycnal mixing is estimated from a mixing parameterization applied to five years of ARGO float data. Air-sea fluxes lead to a large transformation of typically -59 to -77 Sv, directed toward lighter waters centred at neutral density gamma = 27.1. Diapycnal mixing leads to two weaker peaks in transformation acting in the interior, 9 Sv towards denser waters centred at gamma = 27.6 and -18 Sv towards lighter waters centred at gamma = 27.9. Hence, air-sea fluxes and interior diapycnal mixing are important in transforming different water masses within the Southern Ocean. In terms of the wider connections, the lightening of dense waters within the Southern Ocean is slightly larger, though comparable in magnitude, to the air-sea transformation of dense waters in the North Atlantic. The pattern of surface and interior fluxes responsible for these water mass transformations partly relates to the strength of the Antarctic Circumpolar Current in density space: the surface fluxes exhibit a positive correlation with the current speed and the diapycnal diffusivity positively correlates with the inferred current speed at a depth of 1400 m.