Eddy-mean flow interactions and vertical eddy energy redistribution associated with the standing meander in the Antarctic Circumpolar Current

Abstract Maps of the gradient of sea surface height (SSH) and sea surface temperature (SST) reveal that the Antarctic Circumpolar Current (ACC) consists of multiple jets or frontal filaments. The braided and patchy nature of the gradient fields seems at odds with the traditional view, derived from hydrographic sections, that the ACC is made up of three continuous circumpolar fronts. By applying a nonlinear fitting procedure to 638 weekly maps of SSH gradient (∇SSH), it is shown that the distribution of maxima in ∇SSH (i.e., fronts) is strongly peaked at particular values of absolute SSH (i.e., streamlines). The association between the jets and particular streamlines persists despite strong topographic and eddy–mean flow interactions, which cause the jets to merge, diverge, and fluctuate in intensity along their path. The SSH values corresponding to each frontal branch are nearly constant over the sector of the Southern Ocean between 100°E and 180°. The front positions inferred from SSH agree closely with positions inferred from hydrographic sections using traditional water mass criteria. Recognition of the multiple branches of the Southern Ocean fronts helps to reconcile differences between front locations determined by previous studies. Weekly maps of SSH are used to characterize the structure and variability of the ACC fronts and filaments. The path, width, and intensity of the frontal branches are influenced strongly by the bathymetry. The “meander envelopes” of the fronts are narrow on the northern slope of topographic ridges, where the sloping topography reinforces the β effect, and broader over abyssal plains.

Download Full-text

Time-Mean Flow as the Prevailing Contribution to the Poleward Heat Flux across the Southern Flank of the Antarctic Circumpolar Current: A Case Study in the Fawn Trough, Kerguelen Plateau

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0125.1 ◽

2013 ◽

Vol 43 (3) ◽

pp. 583-601 ◽

Cited By ~ 12

Author(s):

H. Sekma ◽

Y.-H. Park ◽

F. Vivier

Keyword(s):

Heat Flux ◽

Southern Ocean ◽

Antarctic Circumpolar Current ◽

Mean Flow ◽

Kerguelen Plateau ◽

The Mean ◽

The Cross ◽

Circumpolar Current ◽

The Antarctic ◽

Southern Flank

Abstract The major mechanisms of the oceanic poleward heat flux in the Southern Ocean are still in debate. The long-standing belief stipulates that the poleward heat flux across the Antarctic Circumpolar Current (ACC) is mainly due to mesoscale transient eddies and the cross-stream heat flux by time-mean flow is insignificant. This belief has recently been challenged by several numerical modeling studies, which stress the importance of mean flow for the meridional heat flux in the Southern Ocean. Here, this study analyzes moored current meter data obtained recently in the Fawn Trough, Kerguelen Plateau, to estimate the cross-stream heat flux caused by the time-mean flow and transient eddies. It is shown that the poleward eddy heat flux in this southern part of the ACC is negligible, while that from the mean flow is overwhelming by two orders of magnitude. This is due to the unusual anticlockwise turning of currents with decreasing depth, which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough. The circumpolar implications of these local observations are discussed in terms of the depth-integrated linear vorticity budget, which suggests that the six topographic features along the southern flank of the ACC equivalent to the Fawn Trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region. As eddy activity on the southern flank of the ACC is too weak to transport sufficient heat poleward, the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task.

Download Full-text

Heat flux carried by the Antarctic Circumpolar Current mean flow

Journal of Geophysical Research Atmospheres ◽

10.1029/2001jc001187 ◽

2002 ◽

Vol 107 (C9) ◽

Cited By ~ 27

Author(s):

Che Sun

Keyword(s):

Heat Flux ◽

Antarctic Circumpolar Current ◽

Mean Flow ◽

Circumpolar Current ◽

The Antarctic

Download Full-text

Southern Ocean overturning across streamlines in an eddying simulation of the Antarctic Circumpolar Current

Ocean Science ◽

10.5194/os-3-491-2007 ◽

2007 ◽

Vol 3 (4) ◽

pp. 491-507 ◽

Cited By ~ 45

Author(s):

A. M. Treguier ◽

M. H. England ◽

S. R. Rintoul ◽

G. Madec ◽

J. Le Sommer ◽

...

Keyword(s):

Ocean Circulation ◽

Antarctic Circumpolar Current ◽

Meridional Circulation ◽

Dimensional Structure ◽

Residual Circulation ◽

Mean Flow ◽

Buoyancy Forcing ◽

Circumpolar Current ◽

Surface Buoyancy ◽

The Antarctic

Abstract. An eddying global model is used to study the characteristics of the Antarctic Circumpolar Current (ACC) in a streamline-following framework. Previous model-based estimates of the meridional circulation were calculated using zonal averages: this method leads to a counter-intuitive poleward circulation of the less dense waters, and underestimates the eddy effects. We show that on the contrary, the upper ocean circulation across streamlines agrees with the theoretical view: an equatorward mean flow partially cancelled by a poleward eddy mass flux. Two model simulations, in which the buoyancy forcing above the ACC changes from positive to negative, suggest that the relationship between the residual meridional circulation and the surface buoyancy flux is not as straightforward as assumed by the simplest theoretical models: the sign of the residual circulation cannot be inferred from the surface buoyancy forcing only. Among the other processes that likely play a part in setting the meridional circulation, our model results emphasize the complex three-dimensional structure of the ACC (probably not well accounted for in streamline-averaged, two-dimensional models) and the distinct role of temperature and salinity in the definition of the density field. Heat and salt transports by the time-mean flow are important even across time-mean streamlines. Heat and salt are balanced in the ACC, the model drift being small, but the nonlinearity of the equation of state cannot be ignored in the density balance.

Download Full-text

Heat fluxes across the Antarctic Circumpolar Current in Drake Passage: Mean flow and eddy contributions

Journal of Geophysical Research Oceans ◽

10.1002/2014jc010201 ◽

2014 ◽

Vol 119 (9) ◽

pp. 6381-6402 ◽

Cited By ~ 8

Author(s):

Ramiro Ferrari ◽

Christine Provost ◽

Young-Hyang Park ◽

Nathalie Sennéchael ◽

Zoé Koenig ◽

...

Keyword(s):

Antarctic Circumpolar Current ◽

Drake Passage ◽

Heat Fluxes ◽

Mean Flow ◽

Circumpolar Current ◽

The Antarctic

Download Full-text

The Antarctic Circumpolar Current in Three Dimensions

Journal of Physical Oceanography ◽

10.1175/jpo2893.1 ◽

2006 ◽

Vol 36 (4) ◽

pp. 651-669 ◽

Cited By ~ 18

Author(s):

Timour Radko ◽

John Marshall

Keyword(s):

Antarctic Circumpolar Current ◽

Three Dimensional ◽

Perturbation Expansion ◽

Dimensional Structure ◽

Residual Circulation ◽

Mean Flow ◽

Overturning Circulation ◽

The Mean ◽

Circumpolar Current ◽

The Antarctic

Abstract A simple theory is developed for the large-scale three-dimensional structure of the Antarctic Circumpolar Current and the upper cell of its overturning circulation. The model is based on a perturbation expansion about the zonal-average residual-mean model developed previously by Marshall and Radko. The problem is solved using the method of characteristics for idealized patterns of wind and buoyancy forcing constructed from observations. The equilibrium solutions found represent a balance between the Eulerian meridional overturning, eddy-induced circulation, and downstream advection by the mean flow. Depth and stratification of the model thermocline increase in the Atlantic–Indian Oceans sector where the mean wind stress is large. Residual circulation in the model is characterized by intensification of the overturning circulation in the Atlantic–Indian sector and reduction in strength in the Pacific Ocean region. Predicted three-dimensional patterns of stratification and residual circulation in the interior of the ACC are compared with observations.

Download Full-text

Equilibration of the Antarctic Circumpolar Current by Standing Meanders

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0163.1 ◽

2014 ◽

Vol 44 (7) ◽

pp. 1811-1828 ◽

Cited By ~ 59

Author(s):

Andrew F. Thompson ◽

Alberto C. Naveira Garabato

Keyword(s):

Antarctic Circumpolar Current ◽

Relative Vorticity ◽

Alternative Mechanism ◽

Mean Flow ◽

Spatially Correlated ◽

Eddy Characteristics ◽

Rapid Changes ◽

The Mean ◽

Circumpolar Current ◽

The Antarctic

Abstract The insensitivity of the Antarctic Circumpolar Current (ACC)’s prominent isopycnal slope to changes in wind stress is thought to stem from the action of mesoscale eddies that counterbalance the wind-driven Ekman overturning—a framework verified in zonally symmetric circumpolar flows. Substantial zonal variations in eddy characteristics suggest that local dynamics may modify this balance along the path of the ACC. Analysis of an eddy-resolving ocean GCM shows that the ACC can be broken into broad regions of weak eddy activity, where surface winds steepen isopycnals, and a small number of standing meanders, across which the isopycnals relax. Meanders are coincident with sites of (i) strong eddy-induced modification of the mean flow and its vertical structure as measured by the divergence of the Eliassen–Palm flux and (ii) enhancement of deep eddy kinetic energy by up to two orders of magnitude over surrounding regions. Within meanders, the vorticity budget shows a balance between the advection of relative vorticity and horizontal divergence, providing a mechanism for the generation of strong vertical velocities and rapid changes in stratification. Temporal fluctuations in these diagnostics are correlated with variability in both the Eliassen–Palm flux and bottom speed, implying a link to dissipative processes at the ocean floor. At larger scales, bottom pressure torque is spatially correlated with the barotropic advection of planetary vorticity, which links to variations in meander structure. From these results, it is proposed that the “flexing” of standing meanders provides an alternative mechanism for reducing the sensitivity of the ACC’s baroclinicity to changes in forcing, separate from an ACC-wide change in transient eddy characteristics.

Download Full-text

Direct Estimate of Lateral Eddy Diffusivity Upstream of Drake Passage

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0120.1 ◽

2014 ◽

Vol 44 (10) ◽

pp. 2593-2616 ◽

Cited By ~ 37

Author(s):

Ross Tulloch ◽

Raffaele Ferrari ◽

Oliver Jahn ◽

Andreas Klocker ◽

Joseph LaCasce ◽

...

Keyword(s):

Antarctic Circumpolar Current ◽

Model Simulation ◽

Phase Speed ◽

Drake Passage ◽

Eddy Diffusivity ◽

Flow Speed ◽

Mean Flow ◽

Direct Estimate ◽

Circumpolar Current ◽

The Antarctic

Abstract The first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s−1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s−1 and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. These vertical variations are not captured by ocean models presently used for climate studies, but they significantly affect the ventilation of different water masses.

Download Full-text

Brief communication "On one mechanism of low frequency variability of the Antarctic Circumpolar Current"

Nonlinear Processes in Geophysics ◽

10.5194/npg-18-361-2011 ◽

2011 ◽

Vol 18 (3) ◽

pp. 361-365 ◽

Cited By ~ 1

Author(s):

O. G. Derzho ◽

B. de Young

Keyword(s):

Rossby Wave ◽

Antarctic Circumpolar Current ◽

Large Scale ◽

Wave Train ◽

Low Frequency ◽

Wave Number ◽

Mean Flow ◽

Zonal Wave Number ◽

Circumpolar Current ◽

The Antarctic

Abstract. In this paper we present a simple analytical model for low frequency and large scale variability of the Antarctic Circumpolar Current (ACC). The physical mechanism of the variability is related to temporal and spatial variations of the cyclonic mean flow (ACC) due to circularly propagating nonlinear barotropic Rossby wave trains. It is shown that the Rossby wave train is a fundamental mode, trapped between the major fronts in the ACC. The Rossby waves are predicted to rotate with a particular angular velocity that depends on the magnitude and width of the mean current. The spatial structure of the rotating pattern, including its zonal wave number, is defined by the specific form of the stream function-vorticity relation. The similarity between the simulated patterns and the Antarctic Circumpolar Wave (ACW) is highlighted. The model can predict the observed sequence of warm and cold patches in the ACW as well as its zonal number.

Download Full-text