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  • Author or Editor: R. L. Thompson x
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Carlyn R. Schmidgall
,
Yidongfang Si
,
Andrew L. Stewart
,
Andrew F. Thompson
, and
Andrew McC. Hogg

Abstract

The export of Antarctic Bottom Water (AABW) supplies the bottom cell of the global overturning circulation and plays a key role in regulating climate. This AABW outflow must cross, and is therefore mediated by, the Antarctic Circumpolar Current (ACC). Previous studies present widely varying conceptions of the role of the ACC in directing AABW across the Southern Ocean, suggesting either that AABW may be zonally recirculated by the ACC, or that AABW may flow northward within deep western boundary currents (DWBC) against bathymetry. In this study the authors investigate how the forcing and geometry of the ACC influences the transport and transformation of AABW using a suite of process-oriented model simulations. The model exhibits a strong dependence on the elevation of bathymetry relative to AABW layer thickness: higher meridional ridges suppress zonal AABW exchange, increase the strength of flow in the DWBC, and reduce the meridional variation in AABW density across the ACC. Furthermore, the transport and transformation vary with density within the AABW layer, with denser varieties of AABW being less efficiently transported between basins. These findings indicate that changes in the thickness of the AABW layer, for example, due to changes in Antarctic shelf processes, and tectonic changes in the sea floor shape may alter the pathways and transformation of AABW across the ACC.

Significance Statement

The ocean plays an outsized role in the movement of heat and trace gases around Earth, and the northward export of dense Antarctic Bottom Water is a crucial component of this climate-regulating process. This study aims to understand what sets the pathways of Antarctic Bottom Water as it travels northward across the Antarctic Circumpolar Current, and thus what controls its partitioning between the Atlantic, Indian, and Pacific basins. Our results highlight the importance of seafloor elevation relative to the thickness of the Antarctic Bottom Water layer for directing the flow northward versus between basins. This study motivates future investigation of long-term changes in Antarctic Bottom Water properties and their consequences for its global distribution.

Restricted access
H.J. Freeland
,
F.M. Boland
,
J.A. Church
,
A.J. Clarke
,
A.M.G. Forbes
,
A. Huyer
,
R.L. Smith
,
R.O.R.Y. Thompson
, and
N.J. White

Abstract

The Australian Coastal Experiment (ACE) was conducted in the coastal waters of New South Wales from September 1983 to 1984. The data obtained allow a detailed examination of the dynamics of flow on the continental shelf and slope and in particular allow a description of coastal trapped wave modes propagating within the coastal waveguide.

The trapped-wave signal is contaminated by energy from the East Australia current eddies approaching the continental slope. However, the data do allow a clear separation of the first three coastal trapped wave modes over the range of frequencies appropriate to the weather forcing band. Through that frequency range the phase speed is computed and an empirical dispersion relation determined for each mode. The empirical dispersion relations compare well with the theoretical relations indicating that a large fraction of the variance in current velocities on the continental shelf can be accounted for by coastal trapped wave theory.

Wind forcing of trapped waves is also considered and evidence presented that in the ACE area the motions are dominated by the propagation of free waves through the arrays.

Full access
Maya I. Jakes
,
Helen E. Phillips
,
Annie Foppert
,
Ajitha Cyriac
,
Nathaniel L. Bindoff
,
Stephen R. Rintoul
, and
Andrew F. Thompson

Abstract

Eddy stirring at mesoscale oceanic fronts generates finescale filaments, visible in submesoscale-resolving model simulations and high-resolution satellite images of sea surface temperature, ocean color, and sea ice. Submesoscale filaments have widths of O(1–10) km and evolve on time scales of hours to days, making them extremely challenging to observe. Despite their relatively small scale, submesoscale processes play a key role in the climate system by providing a route to dissipation; altering the stratification of the ocean interior; and generating strong vertical velocities that exchange heat, carbon, nutrients, and oxygen between the mixed layer and the ocean interior. We present a unique set of in situ and satellite observations in a standing meander region of the Antarctic Circumpolar Current (ACC) that supports the theory of cold filamentary intensification—revealing enhanced vertical velocities and evidence of subduction and ventilation associated with finescale cold filaments. We show that these processes are not confined to the mixed layer; EM-APEX floats reveal enhanced downward velocities (>100 m day−1) and evidence of ageostrophic motion extending as deep as 1600 dbar, associated with a ∼20-km-wide cold filament. A finer-scale (∼5 km wide) cold filament crossed by a towed Triaxus is associated with anomalous chlorophyll and oxygen values extending at least 100–200 dbar below the base of the mixed layer, implying recent subduction and ventilation. Energetic standing meanders within the weakly stratified ACC provide an environment conductive to the generation of finescale filaments that can transport water mass properties across mesoscale fronts and deep into the ocean interior.

Open access