scholarly journals Observation of a Large Lee Wave in the Drake Passage

2017 ◽  
Vol 47 (4) ◽  
pp. 793-810 ◽  
Author(s):  
Jesse M. Cusack ◽  
Alberto C. Naveira Garabato ◽  
David A. Smeed ◽  
James B. Girton
Keyword(s):  
Kinetic Energy ◽  
Southern Ocean ◽  
Drake Passage ◽  
Ocean Dynamics ◽  
Buoyancy Frequency ◽  
Lee Waves ◽  
Lee Wave ◽  
Circumpolar Current ◽  
Shackleton Fracture Zone ◽  
The Antarctic

AbstractLee waves are thought to play a prominent role in Southern Ocean dynamics, facilitating a transfer of energy from the jets of the Antarctic Circumpolar Current to microscale, turbulent motions important in water mass transformations. Two EM-APEX profiling floats deployed in the Drake Passage during the Diapycnal and Isopycnal Mixing Experiment (DIMES) independently measured a 120 ± 20-m vertical amplitude lee wave over the Shackleton Fracture Zone. A model for steady EM-APEX motion is developed to calculate absolute vertical water velocity, augmenting the horizontal velocity measurements made by the floats. The wave exhibits fluctuations in all three velocity components of over 15 cm s−1 and an intrinsic frequency close to the local buoyancy frequency. The wave is observed to transport energy and horizontal momentum vertically at respective peak rates of 1.3 ± 0.2 W m−2 and 8 ± 1 N m−2. The rate of turbulent kinetic energy dissipation is estimated using both Thorpe scales and a method that isolates high-frequency vertical kinetic energy and is found to be enhanced within the wave to values of order 10−7 W kg−1. The observed vertical flux of energy is significantly larger than expected from idealized numerical simulations and also larger than observed depth-integrated dissipation rates. These results provide the first unambiguous observation of a lee wave in the Southern Ocean with simultaneous measurements of its energetics and dynamics.

scholarly journals Detecting and Characterizing Ekman Currents in the Southern Ocean

2015 ◽  
Vol 45 (5) ◽  
pp. 1205-1223 ◽  
Author(s):  
Christopher J. Roach ◽  
Helen E. Phillips ◽  
Nathaniel L. Bindoff ◽  
Stephen R. Rintoul
Keyword(s):  
Southern Ocean ◽  
Classical Theory ◽  
Geographical Area ◽  
Drake Passage ◽  
Current Decay ◽  
Common Value ◽  
Constant Viscosity ◽  
Circumpolar Current ◽  
Reasonable Description ◽  
The Antarctic

AbstractThis study presents a unique array of velocity profiles from Electromagnetic Autonomous Profiling Explorer (EM-APEX) profiling floats in the Antarctic Circumpolar Current (ACC) north of Kerguelen. The authors use these profiles to examine the nature of Ekman spirals, formed by the action of the wind on the ocean’s surface, in light of Ekman’s classical linear theory and more recent enhancements. Vertical decay scales of the Ekman spirals were estimated independently from current amplitude and rotation. Assuming a vertically uniform geostrophic current, decay scales from the Ekman current heading were twice as large as those from the current speed decay, indicating a compressed spiral, consistent with prior observations and violating the classical theory. However, if geostrophic shear is accurately removed, the observed Ekman spiral is as predicted by classical theory and decay scales estimated from amplitude decay and rotation converge toward a common value. No statistically robust relationship is found between stratification and Ekman decay scales. The results indicate that compressed spirals observed in the Southern Ocean arise from aliasing of depth-varying geostrophic currents into the Ekman spiral, as opposed to surface trapping of Ekman currents associated with stratification, and extends the geographical area of similar results from Drake Passage (Polton et al. 2013). Accounting for this effect, the authors find that constant viscosity Ekman models offer a reasonable description of momentum mixing into the upper ocean in the ACC north of Kerguelen. These results demonstrate the effectiveness of a new method and provide additional evidence that the same processes are active for the entire Southern Ocean.

scholarly journals Using kinetic energy measurements from altimetry to detect shifts in the positions of fronts in the Southern Ocean

Ocean Science ◽  
2018 ◽  
Vol 14 (1) ◽  
pp. 105-116 ◽  
Author(s):  
Don P. Chambers
Keyword(s):  
Kinetic Energy ◽  
Southern Ocean ◽  
Current System ◽  
Sea Surface ◽  
Significant Shift ◽  
Dynamic Topography ◽  
Circumpolar Current ◽  
Cross Track ◽  
Along Track ◽  
The Antarctic

Abstract. A novel analysis is performed utilizing cross-track kinetic energy (CKE) computed from along-track sea surface height anomalies. The midpoint of enhanced kinetic energy averaged over 3-year periods from 1993 to 2016 is determined across the Southern Ocean and examined to detect shifts in frontal positions, based on previous observations that kinetic energy is high around fronts in the Antarctic Circumpolar Current system due to jet instabilities. It is demonstrated that although the CKE does not represent the full eddy kinetic energy (computed from crossovers), the shape of the enhanced regions along ground tracks is the same, and CKE has a much finer spatial sampling of 6.9 km. Results indicate no significant shift in the front positions across the Southern Ocean, on average, although there are some localized, large movements. This is consistent with other studies utilizing sea surface temperature gradients, the latitude of mean transport, and the probability of jet occurrence, but is inconsistent with studies utilizing the movement of contours of dynamic topography.

scholarly journals Eddies Observed by Argo Floats. Part II: Form Stress and Streamline Length in the Southern Ocean

2017 ◽  
Vol 47 (9) ◽  
pp. 2237-2250 ◽  
Author(s):  
Katsuro Katsumata
Keyword(s):  
Southern Ocean ◽  
Drake Passage ◽  
Meridional Transport ◽  
Argo Floats ◽  
Momentum Budget ◽  
Eddy Transport ◽  
Circumpolar Current ◽  
Topographic Barriers ◽  
The Antarctic ◽  
Zonal Momentum

AbstractArgo floats measure horizontal current velocities at the parking depth and vertical profiles of temperature and salinity. These data were used to study the roles that eddies play in the dynamics of the Antarctic Circumpolar Current (ACC) in the Southern Ocean. A zonal momentum budget was quantified in a box spanning the latitudes of the Drake Passage and bounded by the sea surface and the 1000-dbar depth. The input of eastward zonal momentum from the wind (17.1 × 1011 N) was approximately twice the downward transfer of eastward momentum across the isopycnal whose mean depth was 1000 dbar, which was mediated via form stress carried by eddies [(8.1 ± 1.9) × 1011 N]. The zonal momentum budget was closed to within uncertainty, meaning that the momentum not accounted for by eddies was explained by the Coriolis term associated with meridional transport. The form stress was spatially concentrated near meridional ridges, particularly on their eastern flanks. The localization was extreme: 7% of the total area contributed about 90% of the form stress. Lengths of streamlines were stretched around steady standing meanders. Seven major meanders were found at large topographic barriers along the ACC, with cyclonic meander collocated with the peaks of the topographic barriers. Eddies were found to lengthen the streamlines mostly on the eastern flanks of the meridional ridges, where the eddy transport was southward. Poleward eddy transport on the eastern flanks of meridional ridges is thus highlighted in the ACC dynamics in transferring eastward zonal momentum downward and in adjusting to wind changes by stretching streamlines.

scholarly journals Suppression of Eddy Diffusivity across Jets in the Southern Ocean

2010 ◽  
Vol 40 (7) ◽  
pp. 1501-1519 ◽  
Author(s):  
Raffaele Ferrari ◽  
Maxim Nikurashin
Keyword(s):  
Kinetic Energy ◽  
Southern Ocean ◽  
Antarctic Circumpolar Current ◽  
Eddy Diffusivity ◽  
Simple Expression ◽  
Ongoing Debate ◽  
The Core ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Mean Current

Abstract Geostrophic eddies control the meridional mixing of heat, carbon, and other climatically important tracers in the Southern Ocean. The rate of eddy mixing is typically quantified through an eddy diffusivity. There is an ongoing debate as to whether eddy mixing in enhanced in the core of the Antarctic Circumpolar Current or on its flanks. A simple expression is derived that predicts the rate of eddy mixing, that is, the eddy diffusivity, as a function of eddy and mean current statistics. This novel expression predicts suppression of the cross-jet eddy diffusivity in the core of the Antarctic Circumpolar Current, despite enhanced values of eddy kinetic energy. The expression is qualitatively and quantitatively validated by independent estimates of eddy mixing from altimetry observations. This work suggests that the meridional eddy diffusivity across the Antarctic Circumpolar Current is weaker than presently assumed because of the suppression of eddy mixing by the strong zonal current.

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 Southern Ocean circulation

2005 ◽  
Vol 32 (2) ◽  
pp. 265-280 ◽  
Author(s):  
Stuart A. Cunningham
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Antarctic Circumpolar Current ◽  
Three Dimensional ◽  
Deep Ocean ◽  
Ocean Basin ◽  
Ocean Dynamics ◽  
Circumpolar Current ◽  
Earth’S Climate ◽  
The Antarctic

The Discovery Investigations of the 1930s provided a compelling description of the main elements of the Southern Ocean circulation. Over the intervening years, this has been extended to include ideas on ocean dynamics based on physical principles. In the modern description, the Southern Ocean has two main circulations that are intimately linked: a zonal (west-east) circumpolar circulation and a meridional (north-south) overturning circulation. The Antarctic Circumpolar Current transports around 140 million cubic metres per second west to east around Antarctica. This zonal circulation connects the Atlantic, Indian and Pacific Oceans, transferring and blending water masses and properties from one ocean basin to another. For the meridional circulation, a key feature is the ascent of waters from depths of around 2,000 metres north of the Antarctic Circumpolar Current to the surface south of the Current. In so doing, this circulation connects deep ocean layers directly to the atmosphere. The circumpolar zonal currents are not stable: meanders grow and separate, creating eddies and these eddies are critical to the dynamics of the Southern Ocean, linking the zonal circumpolar and meridional circulations. As a result of this connection, a global three-dimensional ocean circulation exists in which the Southern Ocean plays a central role in regulating the Earth's climate.

scholarly journals Using kinetic energy measurements from altimetry to detect shifts in the positions of fronts in the Southern Ocean

2017 ◽  
Author(s):  
Don P. Chambers
Keyword(s):  
Kinetic Energy ◽  
Southern Ocean ◽  
Current System ◽  
Sea Surface ◽  
Significant Shift ◽  
Dynamic Topography ◽  
Circumpolar Current ◽  
Cross Track ◽  
Along Track ◽  
The Antarctic

Abstract. A novel analysis is performed utilizing cross-track kinetic energy (CKE) computed from sea surface height anomalies derived from along-track satellite altimetry. The mid-point of enhanced kinetic energy averaged over three-year periods from 1993 to 2015 is determined across the Southern Ocean and examined to detect shifts in frontal positions, based on previous observations that kinetic energy is largest along fronts and jets in the Antarctic Circumpolar Current system. It is demonstrated that although the CKE does not represent the full eddy kinetic energy (computed from crossovers), the shape of the enhanced regions along groundtracks is the same, and CKE has a much finer spatial sampling of 6.9 km. Results indicate no significant shift in the front positions across the Southern Ocean, on average, although there are some localized, large movements. This is consistent with other studies utilizing sea surface temperature gradients and the latitude of mean transport, but inconsistent with studies utilizing the movement of contours of dynamic topography.

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.

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