Estimating the Southern Ocean Overturning Circulation in Thermohaline Coordinates

We demonstrate that it is possible to resolve the imprint of the Southern Ocean overturning circulation by examining the seasonal cycle of water mass formation and destruction in the South Atlantic Ocean using hydrographic data. The variability of water in temperature and salinity classes is compared to the volume changes associated with air-sea heat and freshwater fluxes, allowing the roles of diapycnal and isopycnal mixing to be inferred. This framework enables the isolation of these diabatic processes from the adiabatic effects of heave. We utilize and compare volumetric estimates from Agro profiles, a girdded Argo-based climatology and output from an ocean state estimate. Also used are several air-sea flux climatologies. The analysis emphasizes the role that air-sea heat and freshwater fluxes play in the winter-time formation and the summer-time destruction of Sub-Antarctic Mode Water and Antarctic Winter Water. Countering this variability is the impact of mixing on the winter-time formation of Antarctic Intermediate Water and the winter-time destruction of Upper Circumpolar Deep Water. The balance between water-mass transformation by air/fluxes and mixing is imprinted onto the continual process of wind-driven upwelling/downwelling in the Southern Ocean. The variability of the Southern Ocean overturning is then examined by exploring the fluctuations in the anomalous seasonal formation and destruction of these key water-masses.