The Polar Front in Drake Passage: A Composite-mean Stream-coordinate View

The Polar Front (PF) in Drake Passage is studied using 4 years of data collected by a line of current and pressure recording inverted echo sounders complemented with satellite altimetry. Two preferred locations of the PF are found. A northern and southern PF are separated geographically by a seafloor ridge - the Shackleton Fracture Zone - and hydrographically by 17 cm of geopotential height. Expressed in stream coordinates, vertical structures of buoyancy were determined with a gravest empirical mode analysis. Baroclinic velocity referenced to zero at 3500 dbar, width, and full transport (about 70 Sv) of the jets are statistically indistinguishable; the two jets alternately carry the full transport rather than coexisting. The influence of local bathymetry and deep cyclogenesis manifests as differences in deep reference velocity structures. Downstream reference velocities of the PF-N and PF-S reach maximum speeds of 0.09 and 0.06 m/s, respectively. Buoyancy fields are indicative of upwelling and poleward residual circulation at the PF. Based on potential vorticity fields and mixing length estimates, the northern and southern PF both act as a barrier to cross-frontal exchange.