Mean stream coordinates structure of the Subantarctic Front: Temperature, salinity, and absolute velocity

Christopher S Meinen,Douglas S Luther,Karen L Tracey,D Randolph Watts,Alan D Chave

doi:10.1029/2002jc001545

Abstract

The mean synoptic structure of the northern, strongest branch of the Antarctic Circumpolar Current southwest of Tasmania, at the Subantarctic Front (SAF), is estimated by a stream coordinates analysis of data from overlapping arrays of Inverted Echo Sounders (IESs) and Horizontal Electric Field Recorders deployed during the 1995–1997 Sub‐Antarctic Flux and Dynamics Experiment. The stream coordinates are derived from a daily objective mapping of the temperature field obtained from combining the IES travel time measurements with an empirical look‐up table constructed from the extensive hydrography acquired during WOCE. Full‐water‐column stream‐coordinates sections of temperature, Salinity, and absolute velocity are presented and compared with prior observations. The along‐stream current has a single peak with surface velocities reaching about 50 cm s−1. The vertical structure of the along‐stream velocity is roughly consistent with a combined external and first internal normal mode description that is adapted to the buoyancy frequency as it varies across the front, although there are some significant differences. The cross‐stream structure of along‐stream velocity is very nearly symmetric about the jet axis, but the lateral shear magnitude is slightly larger on the cold side of the SAF. Separating the baroclinic and barotropic currents reveals that the SAF currents are diffluent, primarily baroclinically, in the cross‐stream direction. Baroclinic cross‐stream diffluence of approximately 0.23 Sv per km (Sv = 106 m3 s−1), or about 16 Sv per degree of longitude at 51°S. The 2‐year mean total SAF transport is 75 Sv (for a 220 km width); the barotropic contribution is small (8 Sv) but not negligible.

Highlights

The present study focuses on the measurements made by the 16 Inverted Echo Sounders (IESs) and 7 Horizontal Electric Field Recorders (HEFRs) which were located within the main array
The distance between the two points was defined as the crossstream distance, while the tangent to the Z6 = 500 dbar contour line at XZ6 provided the direction of downstream flow for the rotation of the velocities into along-stream and cross-stream components
[41] Using 2 years of temperature, salinity, and absolute velocity profiles obtained from the combination of hydrography, IESs, and HEFRs, a stream coordinates representation of the temperature, salinity, and absolute velocity of the SAF has been developed

Summary

Introduction

[2] Historically, in situ observations of the Antarctic Circumpolar Current (ACC) have been limited by the inhospitable weather and sea-state conditions in the Southern Ocean during much of the year. In the early 1990s, a 2-year study utilizing four current meter moorings (the AUSSAF array) was completed south of Australia [Phillips and Rintoul, 2000, 2002] These moored velocity measurements provided the first look at the mean absolute velocity structure of the SAF south of Australia. Using historical hydrography from the region of study, characteristic relationships between t and other oceanic variables (e.g., temperature, salinity, specific volume anomaly) can be developed and combined with the IES measured t to estimate full water column profiles of these variables [Meinen and Watts, 2000; Watts et al, 2001]. The complete details of how the SAFDE IES and HEFR measurements were combined to provide daily (noon UT) time series of absolute velocity profiles have been presented by Meinen et al [2002]

Motivation and Methods

Results

Multiple Branches to the SAF?

93 Sv 73 Sv