Variability of the Antarctic Slope Current System in the Northwestern Weddell Sea

Marina Azaneu,Andrew F Thompson,Bastien Y Queste,Karen J Heywood

doi:10.1175/jpo-d-17-0030.1

Marina Azaneu, Andrew F Thompson + Show 2 more

Open Access

https://doi.org/10.1175/jpo-d-17-0030.1

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Abstract

AbstractThe dense water outflow from the Antarctic continental shelf is closely associated with the strength and position of the Antarctic Slope Front. This study explores the short-term and spatial variability of the Antarctic Slope Front system and the mechanisms that regulate cross-slope exchange using highly temporally and spatially resolved measurements from three ocean gliders deployed in 2012. The 22 sections along the eastern Antarctic Peninsula and west of the South Orkney Islands are grouped regionally and composited by isobaths. There is consistency in the front position around the Powell Basin, varying mostly between the 500- and 800-m isobaths. In most of the study area the flow is bottom intensified. The along-slope transport of the Antarctic Slope Current (upper 1000 m) varies between 0.2 and 5.9 Sv (1 Sv ≡ 106 m3 s−1) and does not exhibit a regional pattern. The magnitude of the velocity field shows substantial variability, up to twice its mean value. Higher eddy kinetic energy (0.003 m2 s−2) is observed in sections with dense water, possibly because of baroclinic instabilities in the bottom layer. Distributions of potential vorticity show an increase toward the shelf along isopycnals and also in the dense water layer. Glider sections located west of the South Orkney Islands indicate a northward direction of the flow associated with the Weddell Front, which differs from previous estimates of the mean circulation. This study provides some of the first observational confirmation of the high-frequency variability associated with an active eddy field that has been suggested by recent numerical simulations in this region.

Highlights

One of the most important aspects of Southern Ocean hydrography is the formation of dense waters on the Antarctic continental shelf, influenced by ocean–atmosphere interaction, addition of ice shelf meltwater, and sea ice formation and melting (Foldvik et al 1985; Nicholls et al 2009)
Even though the water masses constituting the Antarctic Slope Front (ASF) are strongly modified along its pathway from the Joinville Ridge to the northeastern Hesperides Trough, there is still a clear horizontal gradient of the hydrographic properties between 200 and 800 m, centered on the 08C isobath, that is, associated with a strong flow steered by the topography
Our results show that the bottom intensification occurs both in places where there are dense waters over the slope and in areas where it is not present any more, it decreases in intensity as the Antarctic Slope Current (ASC) enters the Hesperides Trough

Summary

Introduction

One of the most important aspects of Southern Ocean hydrography is the formation of dense waters on the Antarctic continental shelf, influenced by ocean–atmosphere interaction, addition of ice shelf meltwater, and sea ice formation and melting (Foldvik et al 1985; Nicholls et al 2009). Fluxes across the slope are regulated by surface wind forcing and by eddy processes The balance between these two components is argued to lead to the typical V shape of the isopycnals at the shelf break found in dense water export areas (Gill 1973; Stewart and Thompson 2013). Recent studies have discussed the role of mesoscale eddies in crossslope mass and property fluxes (Nøst et al 2011; Stewart and Thompson 2015; St-Laurent et al 2013) and have shown that wind-driven and eddy-driven transport make comparable contributions to the Antarctic overturning circulation The combination of these processes regulates the exchange of properties between shelf and oceanic regions (Thompson et al 2014).

Methodology

Glider observations in the east Powell Basin

Findings

Summary