On the zonal and meridional circulation and ocean transports between Tasmania and Antarctica

The large scale circulation of the WOCE hydrographic section SR3 between Tasmania and Antarctica is studied through inversion of hydrographic, wind stress and current meter data acquired between 1993 and 1996. These data are assimilated within the framework of a steady state model that is based on hydrostatics, geostrophy, tracer, and potential vorticity conservation. Posterior variances of the major current systems are estimated via the implicit inversion of the Hessian matrix, associated with the assimilation scheme. Analysis of the residuals shows that averaged water mass properties are conserved and that most of the adjustment by the solution is through the uniform vertical displacement of density surfaces within the water column. The inclusion of the tracer and potential vorticity constraints allows parts of the large-scale vertical and meridional circulation to be determined. The upwelling over the Antarctic Divergence is driven mainly by the winds, while the downwelling in the Sub-antarctic Front (SAF) is shown to result from the meridional gradients of the density field caused by the mean Antarctic Circumpolar Current. The shallow meridional and vertical circulation in the SAF is along density surfaces and follows the salinity minimum waters to st depth of 500 dbar. This result suggests a possible mechanism for ventilation of Antarctic Intermediate Water. The total volume transport between Tasmania and Antarctica is estimated as 151+/-50 Sv, with variations between repeat measurements for this section of about 10 Sv. The cross-section circulation pattern can be qualitatively subdivided into five persistent features and their transports and errors are given.