ACC Meanders, Energy Transfer, and Mixed Barotropic–Baroclinic Instability

AbstractAlong-stream variations in the dynamics of the Antarctic Circumpolar Current (ACC) impact heat and tracer transport, regulate interbasin exchange, and influence closure of the overturning circulation. Topography is primarily responsible for generating deviations from zonal-mean properties, mainly through standing meanders associated with regions of high eddy kinetic energy. Here, an idealized channel model is used to explore the spatial distribution of energy exchange and its relationship to eddy geometry, as characterized by both eddy momentum and eddy buoyancy fluxes. Variations in energy exchange properties occur not only between standing meander and quasi-zonal jet regions, but throughout the meander itself. Both barotropic and baroclinic stability properties, as well as the magnitude of energy exchange terms, undergo abrupt changes along the path of the ACC. These transitions are captured by diagnosing eddy fluxes of energy and by adopting the eddy geometry framework. The latter, typically applied to barotropic stability properties, is applied here in the depth–along-stream plane to include information about both barotropic and baroclinic stability properties of the flow. These simulations reveal that eddy momentum fluxes, and thus barotropic instability, play a leading role in the energy budget within a standing meander. This result suggests that baroclinic instability alone cannot capture the dynamics of ACC standing meanders, a challenge for models where eddy fluxes are parameterized.

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Eddy Phase Speeds in a Two-Layer Model of Quasigeostrophic Baroclinic Turbulence with Applications to Ocean Observations

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0192.1 ◽

2016 ◽

Vol 46 (6) ◽

pp. 1963-1985 ◽

Cited By ~ 5

Author(s):

Lei Wang ◽

Malte Jansen ◽

Ryan Abernathey

Keyword(s):

Practical Importance ◽

Phase Speed ◽

Wave Theory ◽

Inverse Cascade ◽

Barotropic Mode ◽

Eddy Fluxes ◽

Circumpolar Current ◽

The Antarctic ◽

Ocean Observations ◽

Shed Light

AbstractThe phase speed spectrum of ocean mesoscale eddies is fundamental to understanding turbulent baroclinic flows. Since eddy phase propagation has been shown to modulate eddy fluxes, an understanding of eddy phase speeds is also of practical importance for the development of improved eddy parameterizations for coarse resolution ocean models. However, it is not totally clear whether and how linear Rossby wave theory can be used to explain the phase speed spectra in various weakly turbulent flow regimes. Using linear analysis, theoretical constraints are identified that control the eddy phase speed in a two-layer quasigeostrophic (QG) model. These constraints are then verified in a series of nonlinear two-layer QG simulations, spanning a range of parameters with potential relevance to the ocean. In the two-layer QG model, the strength of the inverse cascade exerts an important control on the eddy phase speed. If the inverse cascade is weak, the phase speed spectrum is reasonably well approximated by the phase speed of the linearly most unstable mode. A significant inverse cascade instead leads to barotropization, which in turn leads to mean phase speeds closer to those of barotropic-mode Rossby waves. The two-layer QG results are qualitatively consistent with the observed eddy phase speed spectra in the Antarctic Circumpolar Current and may also shed light on the interpretation of phase speed spectra observed in other regions.

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Configuration of a Southern Ocean Storm Track

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0062.1 ◽

2014 ◽

Vol 44 (12) ◽

pp. 3072-3078 ◽

Cited By ~ 16

Author(s):

Tobias Bischoff ◽

Andrew F. Thompson

Keyword(s):

Channel Model ◽

Surface Wind ◽

Storm Track ◽

Chemical Species ◽

Dynamical Structure ◽

Flow Interactions ◽

Order Of Magnitude ◽

Eddy Fluxes ◽

Circumpolar Current ◽

Zonal Average

Abstract Diagnostics of ocean variability that reflect and influence local transport properties of heat and chemical species vary by an order of magnitude along the Southern Ocean’s Antarctic Circumpolar Current (ACC). Topographic “hotspots” are important regions of localized transport anomalies. This study uses a primitive equation channel model to investigate the structure of eddy kinetic energy (EKE), one measure of variability, in an oceanic regime. A storm-track approach emphasizes the importance of stationary eddies, which result from flow interactions with topography, on setting EKE distributions. The influence of these interactions extends far downstream of the topography and impacts EKE patterns through localized convergence and divergence of heat. Unlike for zonal averages, local contributions to the stationary fluxes from terms that integrate to zero in a zonal average are important. The simulations show a strong sensitivity of the zonal structure as well as the distribution and amplitude of stationary eddy fluxes to the surface wind forcing. By focusing on local, time-averaged stationary eddy fluxes, insight into the dynamical structure of the ACC can be gained that is concealed in the averaging procedure associated with traditional zonal or along-stream analyses.

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Estimates of Eddy Heat Flux Crossing the Antarctic Circumpolar Current from Observations in Drake Passage

Journal of Physical Oceanography ◽

10.1175/jpo-d-16-0029.1 ◽

2016 ◽

Vol 46 (7) ◽

pp. 2103-2122 ◽

Cited By ~ 6

Author(s):

D. Randolph Watts ◽

Karen L. Tracey ◽

Kathleen A. Donohue ◽

Teresa K. Chereskin

Keyword(s):

Heat Flux ◽

Heat Transport ◽

Antarctic Circumpolar Current ◽

Baroclinic Instability ◽

Drake Passage ◽

Heat Fluxes ◽

Poleward Heat Transport ◽

Eddy Heat Flux ◽

Circumpolar Current ◽

The Antarctic

AbstractThe 4-yr measurements by current- and pressure-recording inverted echo sounders in Drake Passage produced statistically stable eddy heat flux estimates. Horizontal currents in the Antarctic Circumpolar Current (ACC) turn with depth when a depth-independent geostrophic current crosses the upper baroclinic zone. The dynamically important divergent component of eddy heat flux is calculated. Whereas full eddy heat fluxes differ greatly in magnitude and direction at neighboring locations within the local dynamics array (LDA), the divergent eddy heat fluxes are poleward almost everywhere. Case studies illustrate baroclinic instability events that cause meanders to grow rapidly. In the southern passage, where eddy variability is weak, heat fluxes are weak and not statistically significant. Vertical profiles of heat flux are surface intensified with ~50% above 1000 m and uniformly distributed with depth below. Summing poleward transient eddy heat transport across the LDA of −0.010 ± 0.005 PW with the stationary meander contribution of −0.004 ± 0.001 PW yields −0.013 ± 0.005 PW. A comparison metric, −0.4 PW, represents the total oceanic heat loss to the atmosphere south of 60°S. Summed along the circumpolar ACC path, if the LDA heat flux occurred at six “hot spots” spanning similar or longer path segments, this could account for 20%–70% of the metric, that is, up to −0.28 PW. The balance of ocean poleward heat transport along the remaining ACC path should come from weak eddy heat fluxes plus mean cross-front temperature transports. Alternatively, the metric −0.4 PW, having large uncertainty, may be high.

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Is the Coefficient of Eddy Potential Vorticity Diffusion Positive? Part I: Barotropic Zonal Channel

Journal of Physical Oceanography ◽

10.1175/jpo-d-17-0229.1 ◽

2018 ◽

Vol 48 (7) ◽

pp. 1589-1607 ◽

Cited By ~ 1

Author(s):

V. O. Ivchenko ◽

V. B. Zalesny ◽

B. Sinha

Keyword(s):

Potential Vorticity ◽

Independent Solution ◽

Zonal Flow ◽

Momentum Balance ◽

Eddy Fluxes ◽

The Mean ◽

Zonal Momentum Balance ◽

Circumpolar Current ◽

The Antarctic ◽

Zonal Momentum

AbstractThe question of whether the coefficient of diffusivity of potential vorticity by mesoscale eddies is positive is studied for a zonally reentrant barotropic channel using the quasigeostrophic approach. The topography is limited to the first mode in the meridional direction but is unlimited in the zonal direction. We derive an analytic solution for the stationary (time independent) solution. New terms associated with parameterized eddy fluxes of potential vorticity appear both in the equations for the mean zonal momentum balance and in the kinetic energy balance. These terms are linked with the topographic form stress exerted by parameterized eddies. It is demonstrated that in regimes with zonal flow (analogous to the Antarctic Circumpolar Current), the coefficient of eddy potential vorticity diffusivity must be positive.

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Vertical Eddy Fluxes in the Southern Ocean

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0178.1 ◽

2013 ◽

Vol 43 (5) ◽

pp. 941-955 ◽

Cited By ~ 32

Author(s):

Jan D. Zika ◽

Julien Le Sommer ◽

Carolina O. Dufour ◽

Jean-Marc Molines ◽

Bernard Barnier ◽

...

Keyword(s):

Southern Ocean ◽

Southern Annular Mode ◽

Ekman Transport ◽

Ekman Pumping ◽

Light Water ◽

Dense Water ◽

Vertical Fluxes ◽

Eddy Fluxes ◽

Circumpolar Current ◽

The Antarctic

Abstract The overturning circulation of the Southern Ocean has been investigated using eddying coupled ocean–sea ice models. The circulation is diagnosed in both density–latitude coordinates and in depth–density coordinates. Depth–density coordinates follow streamlines where the Antarctic Circumpolar Current is equivalent barotropic, capture the descent of Antarctic Bottom Water, follow density outcrops at the surface, and can be interpreted energetically. In density–latitude coordinates, wind-driven northward transport of light water and southward transport of dense water are compensated by standing meanders and to a lesser degree by transient eddies, consistent with previous results. In depth–density coordinates, however, wind-driven upwelling of dense water and downwelling of light water are compensated more strongly by transient eddy fluxes than fluxes because of standing meanders. Model realizations are discussed where the wind pattern of the southern annular mode is amplified. In density–latitude coordinates, meridional fluxes because of transient eddies can increase to counter changes in Ekman transport and decrease in response to changes in the standing meanders. In depth–density coordinates, vertical fluxes because of transient eddies directly counter changes in Ekman pumping.

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Connecting flow–topography interactions, vorticity balance, baroclinic instability and transport in the Southern Ocean: the case of an idealized storm track

Ocean Science ◽

10.5194/os-16-1207-2020 ◽

2020 ◽

Vol 16 (5) ◽

pp. 1207-1223

Author(s):

Julien Jouanno ◽

Xavier Capet

Keyword(s):

Southern Ocean ◽

Antarctic Circumpolar Current ◽

Baroclinic Instability ◽

World Ocean ◽

Storm Track ◽

Comprehensive Treatment ◽

Global Circulation Models ◽

The World ◽

Circumpolar Current ◽

The Antarctic

Abstract. The dynamical balance of the Antarctic Circumpolar Current and its implications on the functioning of the world ocean are not fully understood and poorly represented in global circulation models. In this study, the sensitivities of an idealized Southern Ocean (SO) storm track are explored with a set of eddy-rich numerical simulations. The classical partition between barotropic and baroclinic modes is sensitive to current–topography interactions in the mesoscale range 10–100 km, as comparisons between simulations with rough or smooth bathymetry reveal. Configurations with a rough bottom have weak barotropic motions, ubiquitous bottom form stress/pressure torque, no wind-driven gyre in the lee of topographic ridges, less efficient baroclinic turbulence and, thus, larger circumpolar transport rates. The difference in circumpolar transport produced by topographic roughness depends on the strength with which (external) thermohaline forcings by the rest of the world ocean constrain the stratification at the northern edge of the SO. The study highlights the need for a more comprehensive treatment of the Antarctic Circumpolar Current (ACC) interactions with the ocean floor, including realistic fields of bottom form stress and pressure torque. It also sheds some light on the behavior of idealized storm tracks recently modeled: (i) the saturation mechanism, whereby the circumpolar transport does not depend on wind intensity, is a robust and generic attribute of ACC-like circumpolar flows; (ii) the adjustment toward saturation can take place over widely different timescales (from months to years) depending on the possibility (or not) for barotropic Rossby waves to propagate signals of wind change and accelerate/decelerate SO wind-driven gyres. The real SO having both gyres and ACC saturation timescales typical of our “no gyre” simulations may be in an intermediate regime in which mesoscale topography away from major ridges provides partial and localized support for bottom form stress/pressure torque.

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Estimates and Implications of Surface Eddy Diffusivity in the Southern Ocean Derived from Tracer Transport

Journal of Physical Oceanography ◽

10.1175/jpo2949.1 ◽

2006 ◽

Vol 36 (9) ◽

pp. 1806-1821 ◽

Cited By ~ 161

Author(s):

John Marshall ◽

Emily Shuckburgh ◽

Helen Jones ◽

Chris Hill

Keyword(s):

Southern Ocean ◽

Antarctic Circumpolar Current ◽

Surface Wind ◽

Effective Diffusivity ◽

Eddy Diffusivity ◽

Tracer Transport ◽

Near Surface ◽

Surface Wind Stress ◽

Circumpolar Current ◽

The Antarctic

Abstract Near-surface “effective diffusivities” associated with geostrophic eddies in the Southern Ocean are estimated by numerically monitoring the lengthening of idealized tracer contours as they are strained by surface geostrophic flow observed by satellite altimetry. The resulting surface diffusivities show considerable spatial variability and are large (2000 m2 s−1) on the equatorward flank of the Antarctic Circumpolar Current and are small (500 m2 s−1) at the jet axis. Regions of high and low effective diffusivity are shown to be collocated with regions of, respectively, weak and strong isentropic potential vorticity gradients. The maps of diffusivity are used, along with climatological estimates of surface wind stress and air–sea buoyancy flux, to estimate surface meridional residual flows and the relative importance of Eulerian and eddy-induced circulation in the streamwise-averaged dynamics of the Antarctic Circumpolar Current.

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Scales, Growth Rates, and Spectral Fluxes of Baroclinic Instability in the Ocean

Journal of Physical Oceanography ◽

10.1175/2011jpo4404.1 ◽

2011 ◽

Vol 41 (6) ◽

pp. 1057-1076 ◽

Cited By ~ 117

Author(s):

Ross Tulloch ◽

John Marshall ◽

Chris Hill ◽

K. Shafer Smith

Keyword(s):

Growth Rates ◽

Large Scale ◽

Baroclinic Instability ◽

Local Approximation ◽

Zero Crossing ◽

Zonal Velocity ◽

Return Flows ◽

Circumpolar Current ◽

The Antarctic ◽

The Kuroshio

Abstract An observational, modeling, and theoretical study of the scales, growth rates, and spectral fluxes of baroclinic instability in the ocean is presented, permitting a discussion of the relation between the local instability scale; the first baroclinic deformation scale Rdef; and the equilibrated, observed eddy scale. The geography of the large-scale, meridional quasigeostrophic potential vorticity (QGPV) gradient is mapped out using a climatological atlas, and attention is drawn to asymmetries between midlatitude eastward currents and subtropical return flows, the latter of which has westward and eastward zonal velocity shears. A linear stability analysis of the climatology, under the “local approximation,” yields the growth rates and scales of the fastest-growing modes. Fastest-growing modes on eastward-flowing currents, such as the Kuroshio and the Antarctic Circumpolar Current, have a scale somewhat larger (by a factor of about 2) than Rdef. They are rapidly growing (e folding in 1–3 weeks) and deep reaching, and they can be characterized by an interaction between interior QGPV gradients, with a zero crossing in the QGPV gradient at depth. In contrast, fastest-growing modes in the subtropical return flows (as well as much of the gyre interiors) have a scale smaller than Rdef (by a factor of between 0.5 and 1), grow more slowly (e-folding scale of several weeks), and owe their existence to the interaction of a positive surface QGPV gradient and a negative gradient beneath. These predictions of linear theory under the local approximation are then compared to observed eddy length scales and spectral fluxes using altimetric data. It is found that the scale of observed eddies is some 2–3 times larger than the instability scale, indicative of a modest growth in horizontal scale. No evidence of an inverse cascade over decades in scale is found. Outside of a tropical band, the eddy scale varies with latitude along with but somewhat less strongly than Rdef. Finally, exactly the same series of calculations is carried out on fields from an idealized global eddying model, enabling study in a more controlled setting. Broadly similar conclusions are reached, thus reinforcing inferences made from the data.

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