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A Mean Synoptic View of the Subantarctic Front South of Australia

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  • 1 Institute of Antarctic and Southern Ocean Studies, University of Tasmania, Hobart, Tasmania, Australia
  • | 2 Antarctic Cooperative Research Centre and CSIRO Division of Marine Research, Hobart, Tasmania, Australia
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Abstract

A mean synoptic view of the subantarctic front (SAF) is obtained from current meter and hydrographic data by averaging absolute and baroclinic velocity measurements in bins defined by a cross-stream coordinate that moves with the current. The cross-stream coordinate is derived from current meter measurements of vertical shear of the horizontal velocity and is expressed in terms of specific volume anomaly at 780 dbar (δ780). By averaging absolute and baroclinic velocity measurements in stream coordinates, the spatial smoothing that results from Eulerian averaging of measurements of a meandering current is avoided. The mean SAF velocity profile is composed of one central peak and several smaller peaks. In the central peak, the 2-yr mean absolute velocity from current meters reaches 52 cm s−1 at 420 dbar; the mean baroclinic velocity from six CTD sections reaches 34 cm s−1. The SAF flow is coherent at all levels, reaches the seafloor, and is at least 220 km wide. The cross-stream structure of baroclinic and absolute transport of the SAF has been characterized for the first time. The integrated mean transport is 116 ± 10 (×106 m3 s−1), of which approximately 14% is barotropic, where barotropic transport is defined to be the velocity at the deepest current meter (3320 dbar) multiplied by the water depth. Comparison of the current meter measurements with hydrographic sections suggest this estimate is likely a lower bound on the absolute transport, as part of the “cold side” of the SAF is not sampled by the moored array. Linear conditions for baroclinic and barotropic instability of the mean flow are satisfied at the array, consistent with earlier work that showed baroclinic conversion is the dominant mechanism for eddy growth at the array, with weaker barotropic conversion also occurring.

Corresponding author address: Dr Helen E. Phillips, Woods Hole Oceanographic Institution, MS21, 360 Woods Hole Road, Woods Hole, MA 02543. Email: hphillips@whoi.edu

* Current affiliation: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

Abstract

A mean synoptic view of the subantarctic front (SAF) is obtained from current meter and hydrographic data by averaging absolute and baroclinic velocity measurements in bins defined by a cross-stream coordinate that moves with the current. The cross-stream coordinate is derived from current meter measurements of vertical shear of the horizontal velocity and is expressed in terms of specific volume anomaly at 780 dbar (δ780). By averaging absolute and baroclinic velocity measurements in stream coordinates, the spatial smoothing that results from Eulerian averaging of measurements of a meandering current is avoided. The mean SAF velocity profile is composed of one central peak and several smaller peaks. In the central peak, the 2-yr mean absolute velocity from current meters reaches 52 cm s−1 at 420 dbar; the mean baroclinic velocity from six CTD sections reaches 34 cm s−1. The SAF flow is coherent at all levels, reaches the seafloor, and is at least 220 km wide. The cross-stream structure of baroclinic and absolute transport of the SAF has been characterized for the first time. The integrated mean transport is 116 ± 10 (×106 m3 s−1), of which approximately 14% is barotropic, where barotropic transport is defined to be the velocity at the deepest current meter (3320 dbar) multiplied by the water depth. Comparison of the current meter measurements with hydrographic sections suggest this estimate is likely a lower bound on the absolute transport, as part of the “cold side” of the SAF is not sampled by the moored array. Linear conditions for baroclinic and barotropic instability of the mean flow are satisfied at the array, consistent with earlier work that showed baroclinic conversion is the dominant mechanism for eddy growth at the array, with weaker barotropic conversion also occurring.

Corresponding author address: Dr Helen E. Phillips, Woods Hole Oceanographic Institution, MS21, 360 Woods Hole Road, Woods Hole, MA 02543. Email: hphillips@whoi.edu

* Current affiliation: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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