scholarly journals Influence of Wind Stress, Wind Stress Curl, and Bottom Friction on the Transport of a Model Antarctic Circumpolar Current

2012 ◽  
Vol 42 (1) ◽  
pp. 207-222 ◽  
Author(s):  
Louis-Philippe Nadeau ◽  
David N. Straub
Keyword(s):  
Wind Stress ◽  
Antarctic Circumpolar Current ◽  
Large Scale ◽  
Drake Passage ◽  
Small Scale ◽  
Wind Stress Curl ◽  
Circumpolar Current ◽  
Quasigeostrophic Model ◽  
Recirculation Gyre ◽  
The Antarctic

Abstract Eddy-permitting simulations of a wind-driven quasigeostrophic model in an idealized Southern Ocean setting are used to attempt to describe what sets the wind-driven circumpolar transport of the Antarctic Circumpolar Current (ACC). For weak forcing, the transport is well described as a linear sum of channel and basin components. The authors’ main focus is on stronger forcing. In this regime, an eddy-driven recirculation appears in the abyssal layer, and all time-mean circumpolar streamlines are found to stem from a Sverdrup-like interior. The Sverdrup flux into Drake Passage latitudes can then be thought of as the sum of one part that feeds the circumpolar current and another that is associated with the recirculation. The relative fractions of this partitioning depend on the bottom drag, the midchannel wind stress, and the wind stress curl. Increasing the strength of the bottom drag reduces the recirculation and increases circumpolar transport. Increasing a zero-curl eastward wind stress reduces the upper-layer expression of the recirculation and increases the transport. Increasing the curl-containing portion of the forcing (while holding the midchannel stress constant) increases the recirculation and decreases the transport. The weakly forced regime is also considered, as are the relative roles of large and small-scale eddies in transporting momentum vertically through the water column in the Drake Passage latitude band. It is found that the vertical momentum flux associated with transient structures can be used to distinguish between different regimes: these structures transmit momentum upward when the dynamics is dominated by the large-scale recirculation gyre and downward when it is not.

scholarly journals A Microscale View of Mixing and Overturning across the Antarctic Circumpolar Current

2016 ◽  
Vol 46 (1) ◽  
pp. 233-254 ◽  
Author(s):  
Alberto C. Naveira Garabato ◽  
Kurt L. Polzin ◽  
Raffaele Ferrari ◽  
Jan D. Zika ◽  
Alexander Forryan
Keyword(s):  
Antarctic Circumpolar Current ◽  
Large Scale ◽  
Drake Passage ◽  
Small Scale ◽  
Overturning Circulation ◽  
Order Of Magnitude ◽  
Circumpolar Current ◽  
Scale Turbulence ◽  
Microstructure Measurements ◽  
The Antarctic

AbstractThe relative roles of isoneutral stirring by mesoscale eddies and dianeutral stirring by small-scale turbulence in setting the large-scale temperature–salinity relation of the Southern Ocean against the action of the overturning circulation are assessed by analyzing a set of shear and temperature microstructure measurements across Drake Passage in a “triple decomposition” framework. It is shown that a picture of mixing and overturning across a region of the Antarctic Circumpolar Current (ACC) may be constructed from a relatively modest number of microstructure profiles. The rates of isoneutral and dianeutral stirring are found to exhibit distinct, characteristic, and abrupt variations: most notably, a one to two orders of magnitude suppression of isoneutral stirring in the upper kilometer of the ACC frontal jets and an order of magnitude intensification of dianeutral stirring in the subpycnocline and deepest layers of the ACC. These variations balance an overturning circulation with meridional flows of O(1) mm s−1 across the ACC’s mean thermohaline structure. Isoneutral and dianeutral stirring play complementary roles in balancing the overturning, with isoneutral processes dominating in intermediate waters and the Upper Circumpolar Deep Water and dianeutral processes prevailing in lighter and denser layers.

scholarly journals On the Sensitivity of the Drake Passage Transport to Air–Sea Momentum Flux

2012 ◽  
Vol 25 (7) ◽  
pp. 2279-2290 ◽  
Author(s):  
Matthew R. Mazloff
Keyword(s):  
Wind Stress ◽  
Spatial Scales ◽  
Accurate Determination ◽  
Drake Passage ◽  
Ocean Modeling ◽  
Wind Stress Curl ◽  
State Estimate ◽  
Circumpolar Current ◽  
The Antarctic

Abstract An eddy-permitting state estimate and its adjoint are used to analyze the influence of wind stress perturbations on the transport of the Antarctic Circumpolar Current (ACC) system through Drake Passage. The transport is found to be sensitive to wind stress perturbations both along the ACC path and also in remote regions. The time scale of influence of wind stress perturbations is on the order of 100 days. Regarding spatial scales, the sensitivity of transport to wind stress is relatively smooth in regions of flat topography. In boundary regions and regions with complex topography, however, the sensitivity is enhanced and characterized by shorter length scales of order 100 km. Positive perturbations to the zonal wind stress usually increase the ACC transport, though the wind stress curl is of primary influence where the currents are steered by topography. Highlighting locations where the ACC is especially responsive to air–sea momentum fluxes reveals where an accurate determination of atmospheric winds may best enhance ocean modeling efforts.

scholarly journals Basin and Channel Contributions to a Model Antarctic Circumpolar Current

2009 ◽  
Vol 39 (4) ◽  
pp. 986-1002 ◽  
Author(s):  
Louis-Philippe Nadeau ◽  
David N. Straub
Keyword(s):  
Wind Stress ◽  
Antarctic Circumpolar Current ◽  
Robustness Analysis ◽  
Drake Passage ◽  
Western Boundary ◽  
Analytic Model ◽  
Saturation Regime ◽  
Zonal Jets ◽  
Circumpolar Current ◽  
Quasigeostrophic Model

Abstract The idea that basinlike dynamics may play a major role in determining the Antarctic Circumpolar Current (ACC) transport is revisited. A simple analytic model is developed to describe the relationship between the wind stress and transport. At very low-wind stress, a nonzero minimum is predicted. This is followed by two distinct dynamical regimes for stronger forcing: 1) a Stommel regime in which transport increases linearly with forcing strength; and 2) a saturation regime in which the transport levels off. The baroclinic structure of the Sverdrup flux into the Drake Passage latitude band is central to the analytic model, and the geometry of characteristics, or geostrophic contours, is key to predicting the transition between the two regimes. A robustness analysis is performed using an eddy-permitting quasigeostrophic model in idealized geometries. Many simulations were carried out in large domains across a range of forcing strengths. The simulations agree qualitatively with the analytic model, with two main discrepancies being related to zonal jet structures and to a western boundary inertial recirculation. Eddy fluxes associated with zonal jets modify the baroclinic structure and lower the saturation transport value. Inertial effects increase the transport, although this effect is mainly limited to smaller domains.

scholarly journals Comparing Idealized and Complex Topographies in Quasigeostrophic Simulations of an Antarctic Circumpolar Current

2013 ◽  
Vol 43 (8) ◽  
pp. 1821-1837 ◽  
Author(s):  
Louis-Philippe Nadeau ◽  
David N. Straub ◽  
David M. Holland
Keyword(s):  
Antarctic Circumpolar Current ◽  
Large Scale ◽  
Drake Passage ◽  
Horizontal Resolution ◽  
Complex Topography ◽  
Form Drag ◽  
Scotia Ridge ◽  
Circumpolar Current ◽  
Recirculation Gyre ◽  
Realistic Topography

Abstract The circumpolar transport of a wind-driven quasigeostrophic Antarctic Circumpolar Current is considered. Simple theory suggests transport in a strongly forced regime—the focus of this study—is largely determined by a partitioning of the southward Sverdrup flux into Drake Passage latitudes: some streamlines feed a “basin contribution” to the circumpolar transport and others feed a large-scale recirculation gyre. Simulations assuming an idealized Scotia Ridge topography are considered to test for sensitivity to resolution. Considerable sensitivity to both vertical and horizontal resolution is found, and associated with this is a tight stationary eddy trapped on the western flank of the ridge. That is, this eddy is sensitive to resolution and exerts an influence that acts to reduce the circumpolar transport. Simulations using the Scotia Ridge–like topography are also compared to others using more realistic topography. In the idealized (ridge) topography experiments, there is only a single ridge against which topographic form drag can act to remove eastward momentum from the system; in the complex topography experiments, there are many. It is found that the experiments assuming realistic topography do not develop an analog to the single topographically trapped eddy prevalent in the Scotia Ridge topography simulations. Additionally, circumpolar transport in these simulations agrees better with the theory. Whether this agreement is simply fortuitous, however, is unclear. To address this, a series of simulations assumes topography that varies smoothly between the idealized ridge and realistic configurations.

scholarly journals Estimating a Submesoscale Diffusivity Using a Roughness Measure Applied to a Tracer Release Experiment in the Southern Ocean

2015 ◽  
Vol 45 (6) ◽  
pp. 1610-1631 ◽  
Author(s):  
Emma J. D. Boland ◽  
Emily Shuckburgh ◽  
Peter H. Haynes ◽  
James R. Ledwell ◽  
Marie-José Messias ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Antarctic Circumpolar Current ◽  
Drake Passage ◽  
Velocity Fields ◽  
Small Scale ◽  
The Pacific ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Mesoscale Processes ◽  
Submesoscale Processes

AbstractThe use of a measure to diagnose submesoscale isopycnal diffusivity by determining the best match between observations of a tracer and simulations with varying small-scale diffusivities is tested. Specifically, the robustness of a “roughness” measure to discriminate between tracer fields experiencing different submesoscale isopycnal diffusivities and advected by scaled altimetric velocity fields is investigated. This measure is used to compare numerical simulations of the tracer released at a depth of about 1.5 km in the Pacific sector of the Southern Ocean during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) field campaign with observations of the tracer taken on DIMES cruises. The authors find that simulations with an isopycnal diffusivity of ~20 m2 s−1 best match observations in the Pacific sector of the Antarctic Circumpolar Current (ACC), rising to ~20–50 m2 s−1 through Drake Passage, representing submesoscale processes and any mesoscale processes unresolved by the advecting altimetry fields. The roughness measure is demonstrated to be a statistically robust way to estimate a small-scale diffusivity when measurements are relatively sparse in space and time, although it does not work if there are too few measurements overall. The planning of tracer measurements during a cruise in order to maximize the robustness of the roughness measure is also considered. It is found that the robustness is increased if the spatial resolution of tracer measurements is increased with the time since tracer release.

scholarly journals Intrinsic variability of the Antarctic Circumpolar Current system: low- and high-frequency fluctuations of the Argentine Basin flow

Ocean Science ◽  
2014 ◽  
Vol 10 (2) ◽  
pp. 201-213 ◽  
Author(s):  
G. Sgubin ◽  
S. Pierini ◽  
H. A. Dijkstra
Keyword(s):  
High Frequency ◽  
Antarctic Circumpolar Current ◽  
Current System ◽  
Low Frequency ◽  
Drake Passage ◽  
Ocean Model ◽  
Intrinsic Variability ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Argentine Basin

Abstract. In this paper, the variability of the Antarctic Circumpolar Current system produced by purely intrinsic nonlinear oceanic mechanisms is studied through a sigma-coordinate ocean model, implemented in a large portion of the Southern Ocean at an eddy-permitting resolution under steady surface heat and momentum fluxes. The mean transport through the Drake Passage and the structure of the main Antarctic Circumpolar Current fronts are well reproduced by the model. Intrinsic variability is found to be particularly intense in the Subantarctic Front and in the Argentine Basin, on which further analysis is focused. The low-frequency variability at interannual timescales is related to bimodal behavior of the Zapiola Anticyclone, with transitions between a strong and collapsed anticyclonic circulation in substantial agreement with altimeter observations. Variability on smaller timescales shows clear evidence of topographic Rossby-wave propagation along the eastern and southern flanks of the Zapiola Rise and of mesoscale eddies, also in agreement with altimeter observations. The analysis of the relationship between the low- and high-frequency variability suggests possible mechanisms of mutual interaction.

scholarly journals Topographic Control of Southern Ocean Gyres and the Antarctic Circumpolar Current: A Barotropic Perspective

2019 ◽  
Vol 49 (12) ◽  
pp. 3221-3244 ◽  
Author(s):  
Ryan D. Patmore ◽  
Paul R. Holland ◽  
David R. Munday ◽  
Alberto C. Naveira Garabato ◽  
David P. Stevens ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Antarctic Circumpolar Current ◽  
Large Scale ◽  
Volume Transport ◽  
Topographic Features ◽  
Conservation Of Momentum ◽  
Ridge Width ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Ridge Geometry

AbstractIn the Southern Ocean the Antarctic Circumpolar Current is significantly steered by large topographic features, and subpolar gyres form in their lee. The geometry of topographic features in the Southern Ocean is highly variable, but the influence of this variation on the large-scale flow is poorly understood. Using idealized barotropic simulations of a zonal channel with a meridional ridge, it is found that the ridge geometry is important for determining the net zonal volume transport. A relationship is observed between ridge width and volume transport that is determined by the form stress generated by the ridge. Gyre formation is also highly reliant on the ridge geometry. A steep ridge allows gyres to form within regions of unblocked geostrophic (f/H) contours, with an increase in gyre strength as the ridge width is reduced. These relationships among ridge width, gyre strength, and net zonal volume transport emerge to simultaneously satisfy the conservation of momentum and vorticity.

scholarly journals Coupling Influences of ENSO and PDO on the Inter-Decadal SST Variability of the ACC around the Western South Atlantic

2019 ◽  
Vol 11 (18) ◽  
pp. 4853
Author(s):  
You-Lin Wang ◽  
Yu-Chen Hsu ◽  
Chung-Pan Lee ◽  
Chau-Ron Wu
Keyword(s):  
Antarctic Circumpolar Current ◽  
Water Mass ◽  
Southern Oscillation ◽  
Drake Passage ◽  
South Atlantic ◽  
Sst Variability ◽  
The Pacific ◽  
Circumpolar Current ◽  
The Antarctic

The Antarctic Circumpolar Current (ACC) plays an important role in the climate as it balances heat energy and water mass between the Pacific and Atlantic Oceans through the Drake Passage. However, because the historical measurements and observations are extremely limited, the decadal and long-term variations of the ACC around the western South Atlantic Ocean are rarely studied. By analyzing reconstructed sea surface temperatures (SSTs) in a 147-year period (1870–2016), previous studies have shown that SST anomalies (SSTAs) around the Antarctic Peninsula and South America had the same phase change as the El Niño Southern Oscillation (ENSO). This study further showed that changes in SSTAs in the regions mentioned above were enlarged when the Pacific Decadal Oscillation (PDO) and the ENSO were in the same warm or cold phase, implying that changes in the SST of higher latitude oceans could be enhanced when the influence of the ENSO is considered along with the PDO.

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