Absolute Velocity, Transport and Stability of the Polar Front Downstream of the Southeast Indian Ridge

Abstract

The fronts of the Antarctic Circumpolar Current (ACC) influence the growth and instability of eddies, eddy-mean flow interaction, and the development of submesoscale processes. We use in situ hydrography and directly measured velocity from a comprehensive shipboard survey of the Polar Front (PF) south of Tasmania to characterize the synoptic mean velocity and transport structure of the PF. A stream-coordinate system, based on dynamic height anomaly (ϕ), is constructed to create an along- and across-stream perspective of the PF that naturally follows the current. We find the surface-intensified along-stream flow carries a full-depth transport of 80.5 Sv over a width of 90 km, with roughly two-thirds carried by the baroclinic flow. The across-stream velocities are largely depth-independent; the mean depth-averaged flow across the PF core is 0.08 m s⁻¹ in the quasi-northward direction, associated with along-isopyncal downwelling and cold water advection. We calculate potential vorticity (PV) to investigate the stability of the PF and find it is susceptible to both baroclinic and barotropic instability. Mean PV is dominated by the thickness PV term $\left(\frac{f{N}^2}{g}\right)$, that is PV is controlled by changes in stratification. While strong upper-ocean PV gradients indicate a barrier to mixing, weaker PV gradients and relatively elevated mixing lengths at mid-depth (≈2,000–2,500 m depth) suggest a deep steering level where eddies can efficiently mix properties across the PF.