Direct observations of the Antarctic Slope Current transport at 113°E

B Peña‐Molino,M S Mccartney,S R Rintoul

doi:10.1002/2015jc011594

Abstract

AbstractThe Antarctic Slope Current (ASC), defined here as the region of westward flow along the continental slope off Antarctica, forms the southern limb of the subpolar gyres. It regulates the exchange of water across the shelf break and provides a path for interbasin westward transport. Despite its significance, the ASC remains largely unobserved around most of the Antarctic continent. Here we present direct velocity observations from a 17 month current meter moored array deployed across the continental slope between the 1000 and the 4200 m isobaths, in the southeastern Indian Ocean near 113°E. The observed time‐mean flow consists of a surface‐intensified jet associated with the Antarctic Slope Front (ASF) and a broader bottom‐intensified westward flow that extends out to approximately the 4000 m isobath and is strongest along the upper slope. The time‐mean transport of the ASC is −29.2 Sv. Fluctuations in the transport are large, typically exceeding the mean by a factor of 2. They are mainly due to changes in the northward extent of the current over the lower slope. However, seasonal changes in the wind also drive variations in the transport of the ASF and the flow in the upper slope. Both mean and variability are largely barotropic, thus invisible to traditional geostrophic methods.

Highlights

The Antarctic Slope Current (ASC) is a more or less continuous band of westward flow that surrounds most of the Antarctic continent
Across the broad region of westward flow that defines the ASC, the isopycnal tilt is gentle and often upward toward the south, as a result of the upwelling driven by the divergence of the winds south of the Antarctic Circumpolar Current (ACC)
Bindoff et al [2000] characterized the circulation over the slope between 80 and 1508E based on a combination of synoptic observations, including surface drifters, shipboard ADCP (SADCP), and geostrophy

Summary

Introduction

The Antarctic Slope Current (ASC) is a more or less continuous band of westward flow that surrounds most of the Antarctic continent. The geostrophic shear associated with a positive slope in the isopycnals is positive, consistent with surfaceintensified eastward flow Despite this positive shear around much of the Antarctic continent, models and observations consistently show westward flow along the continental slope, indicating that the ASC has a strong barotropic component to the west that is opposite sign to the shear. This barotropic nature of the ASC is not surprising, since the wind stress curl south of the ACC is negative, and the response of a weakly stratified ocean to such forcing is in the form of a barotropic cyclonic gyre [Morrow et al, 2008]

Results

Discussion

Conclusion