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 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 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 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 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.

scholarly journals Coccolithophore populations and their contribution to carbonate export during an annual cycle in the Australian sector of the Antarctic zone

2018 ◽  
Vol 15 (6) ◽  
pp. 1843-1862 ◽  
Author(s):  
Andrés S. Rigual Hernández ◽  
José A. Flores ◽  
Francisco J. Sierro ◽  
Miguel A. Fuertes ◽  
Lluïsa Cros ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Ocean Acidification ◽  
Deep Ocean ◽  
Sediment Traps ◽  
Global Ocean ◽  
Field Observations ◽  
Scotia Sea ◽  
Allochthonous Species ◽  
Circumpolar Current ◽  
The Antarctic

Abstract. The Southern Ocean is experiencing rapid and relentless change in its physical and biogeochemical properties. The rate of warming of the Antarctic Circumpolar Current exceeds that of the global ocean, and the enhanced uptake of carbon dioxide is causing basin-wide ocean acidification. Observational data suggest that these changes are influencing the distribution and composition of pelagic plankton communities. Long-term and annual field observations on key environmental variables and organisms are a critical basis for predicting changes in Southern Ocean ecosystems. These observations are particularly needed, since high-latitude systems have been projected to experience the most severe impacts of ocean acidification and invasions of allochthonous species. Coccolithophores are the most prolific calcium-carbonate-producing phytoplankton group playing an important role in Southern Ocean biogeochemical cycles. Satellite imagery has revealed elevated particulate inorganic carbon concentrations near the major circumpolar fronts of the Southern Ocean that can be attributed to the coccolithophore Emiliania huxleyi. Recent studies have suggested changes during the last decades in the distribution and abundance of Southern Ocean coccolithophores. However, due to limited field observations, the distribution, diversity and state of coccolithophore populations in the Southern Ocean remain poorly characterised. We report here on seasonal variations in the abundance and composition of coccolithophore assemblages collected by two moored sediment traps deployed at the Antarctic zone south of Australia (2000 and 3700 m of depth) for 1 year in 2001–2002. Additionally, seasonal changes in coccolith weights of E. huxleyi populations were estimated using circularly polarised micrographs analysed with C-Calcita software. Our findings indicate that (1) coccolithophore sinking assemblages were nearly monospecific for E. huxleyi morphotype B/C in the Antarctic zone waters in 2001–2002; (2) coccoliths captured by the traps experienced weight and length reduction during summer (December–February); (3) the estimated annual coccolith weight of E. huxleyi at both sediment traps (2.11 ± 0.96 and 2.13 ± 0.91 pg at 2000 and 3700 m) was consistent with previous studies for morphotype B/C in other Southern Ocean settings (Scotia Sea and Patagonian shelf); and (4) coccolithophores accounted for approximately 2–5 % of the annual deep-ocean CaCO3 flux. Our results are the first annual record of coccolithophore abundance, composition and degree of calcification in the Antarctic zone. They provide a baseline against which to monitor coccolithophore responses to changes in the environmental conditions expected for this region in coming decades.

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