Frontal Circulation and Submesoscale Variability during the Formation of a Southern Ocean Mesoscale Eddy

AbstractObservations made in the Scotia Sea during the May 2015 Surface Mixed Layer Evolution at Submesoscales (SMILES) research cruise captured submesoscale, O(1–10) km, variability along the periphery of a mesoscale O(10–100) km meander precisely as it separated from the Antarctic Circumpolar Current (ACC) and formed a cyclonic eddy ~120 km in diameter. The meander developed in the Scotia Sea, an eddy-rich region east of the Drake Passage where the Subantarctic and Polar Fronts converge and modifications of Subantarctic Mode Water (SAMW) occur. In situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly formed southern sector of the eddy. A mathematical framework is developed to estimate vertical velocity from collocated drifter and horizontal water velocity time series, under certain simplifying assumptions appropriate for the current dataset. Upwelling (downwelling) rates of O(100) m day−1 are found in the northern (southern) eddy sector. Favorable conditions for submesoscale instabilities are found in the mixed layer, particularly at the beginning of the survey in the vicinity of density fronts. Shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front. Evolution in temperature–salinity (T–S) space indicates modification of water mass properties in the upper 200 m over 2 days. Modifications along σθ = 27–27.2 kg m−3 have climate-related implications for mode and intermediate water transformation in the Scotia Sea on finer spatiotemporal scales than observed previously.

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Spiciness Anomalies of Subantarctic Mode Water in the South Indian Ocean

Journal of Climate ◽

10.1175/jcli-d-20-0482.1 ◽

2021 ◽

Vol 34 (10) ◽

pp. 3927-3953

Author(s):

Motoki Nagura

Keyword(s):

Indian Ocean ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Surface Mixed Layer ◽

The South ◽

South Indian Ocean ◽

Mode Water ◽

Horizontal Advection ◽

South Indian ◽

Subantarctic Mode Water

AbstractThis study investigates spreading and generation of spiciness anomalies of the Subantarctic Mode Water (SAMW) located on 26.6 to 26.8 σθ in the south Indian Ocean, using in situ hydrographic observations, satellite measurements, reanalysis datasets, and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable to that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature and salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

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The Cenozoic tectonic evolution of the Scotia Sea area

10.5194/egusphere-egu2020-178 ◽

2020 ◽

Author(s):

Anne Oldenhage ◽

Anouk Beniest ◽

Wouter P. Schellart

Keyword(s):

Tectonic Evolution ◽

Drake Passage ◽

Sea Level Changes ◽

Scotia Sea ◽

Scotia Ridge ◽

The North ◽

Reconstruction Software ◽

Circumpolar Current ◽

Global Sea Level ◽

The Antarctic

The breakup of the southern edge of Gondwanaland resulted in the formation of the Scotia Plate and the opening of Drake Passage throughout the Cenozoic. During the same period, the Tasman Seaway opened, although the timing of this opening is much better constrained. Rapid cooling of the Antarctic continent followed the openings of Drake Passage and the Tasman Seaway. The opening of Drake Passage or the Tasman seaway allowed the onset of the Antarctic Circumpolar Current, which is held responsible for the late Miocene global cooling, but discussions about the most important opening are still ongoing.The opening of Drake Passage and the development of the Scotia plate have been studied in multitude, but paleogeographic reconstructions show many differences and inconsistencies in both timing of opening Drake Passage as well as paleo-locations of crustal segments. The paleogeographic or tectonic reconstructions of the opening of Drake Passage and the formation of the Scotia plate are hard to compare, because differences in shapes of crustal segments, geographic projections and relative movements of segments chosen by previous authors make it difficult to observe similarities and differences between the different reconstructions.We present a thorough analysis of the previously published paleogeographic reconstructions with the aim to identify agreements and inconsistencies between these reconstructions. We re-defined the crustal segments that formed after the break-up of Gondwanaland by re-interpreting the bathymetry and magnetic anomalies of the study area. We re-modelled and compared georeferenced reconstructions from earlier studies in GPlates plate reconstruction software using our own defined crustal segments.This comparison shows that the different reconstructions agree quite well along the South Scotia Ridge, but that the North Scotia Ridge shows significant variations between different reconstructions or is not even considered in the reconstructions. Also, the nature and age of the crust of the Central Scotia Sea is heavily discussed, resulting in different opening scenarios. We argue that the tectonic evolution of the North Scotia Ridge and Central Scotia Sea is a crucial factor in identifying the timing of the development of an ocean gateway. We made a new tectonic reconstruction of the North Scotia Ridge crustal segments with less overlaps and gaps between the reconstructed crustal segments.The next step would be to compare the global sea-level changes and paleo-bathymetry with the different opening scenarios. Because we standardized all scenarios with the same crustal segments, we will then be able to provide opening ages of Drake Passage for the different scenarios that can be compared in a quantitative way.

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Variability of Subantarctic Mode Water and Antarctic Intermediate Water in the Drake Passage during the Late-Twentieth and Early-Twenty-First Centuries

Journal of Climate ◽

10.1175/2009jcli2621.1 ◽

2009 ◽

Vol 22 (13) ◽

pp. 3661-3688 ◽

Cited By ~ 75

Author(s):

Alberto C. Naveira Garabato ◽

Loïc Jullion ◽

David P. Stevens ◽

Karen J. Heywood ◽

Brian A. King

Keyword(s):

Sea Ice ◽

Drake Passage ◽

Heat Fluxes ◽

Turbulent Heat ◽

Intermediate Water ◽

The West ◽

Mode Water ◽

Subantarctic Mode Water ◽

Turbulent Heat Fluxes ◽

The Antarctic

Abstract A time series of the physical and biogeochemical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage. Both water masses have experienced substantial variability on interannual to interdecadal time scales. SAMW is formed by winter overturning on the equatorward flank of the Antarctic Circumpolar Current (ACC) in and to the west of the Drake Passage. Its interannual variability is primarily driven by variations in wintertime air–sea turbulent heat fluxes and net evaporation modulated by the El Niño–Southern Oscillation (ENSO). Despite their spatial proximity, the AAIW in the Drake Passage has a very different source than that of the SAMW because it is ventilated by the northward subduction of Winter Water originating in the Bellingshausen Sea. Changes in AAIW are mainly forced by variability in Winter Water properties resulting from fluctuations in wintertime air–sea turbulent heat fluxes and spring sea ice melting, both of which are linked to predominantly ENSO-driven variations in the intensity of meridional winds to the west of the Antarctic Peninsula. A prominent exception to the prevalent modes of SAMW and AAIW formation occurred in 1998, when strong wind forcing associated with constructive interference between ENSO and the southern annular mode (SAM) triggered a transitory shift to an Ekman-dominated mode of SAMW ventilation and a 1–2-yr shutdown of AAIW production. The interdecadal evolutions of SAMW and AAIW in the Drake Passage are distinct and driven by different processes. SAMW warmed (by ∼0.3°C) and salinified (by ∼0.04) during the 1970s and experienced the reverse trends between 1990 and 2005, when the coldest and freshest SAMW on record was observed. In contrast, AAIW underwent a net freshening (by ∼0.05) between the 1970s and the twenty-first century. Although the reversing changes in SAMW were chiefly forced by a ∼30-yr oscillation in regional air–sea turbulent heat fluxes and precipitation associated with the interdecadal Pacific oscillation, with a SAM-driven intensification of the Ekman supply of Antarctic surface waters from the south contributing significantly too, the freshening of AAIW was linked to the extreme climate change that occurred to the west of the Antarctic Peninsula in recent decades. There, a freshening of the Winter Water ventilating AAIW was brought about by increased precipitation and a retreat of the winter sea ice edge, which were seemingly forced by an interdecadal trend in the SAM and regional positive feedbacks in the air–sea ice coupled climate system. All in all, these findings highlight the role of the major modes of Southern Hemisphere climate variability in driving the evolution of SAMW and AAIW in the Drake Passage region and the wider South Atlantic and suggest that these modes may have contributed significantly to the hemispheric-scale changes undergone by those waters in recent decades.

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Control of Mode and Intermediate Water Mass Properties in Drake Passage by the Amundsen Sea Low

Journal of Climate ◽

10.1175/jcli-d-12-00346.1 ◽

2013 ◽

Vol 26 (14) ◽

pp. 5102-5123 ◽

Cited By ~ 15

Author(s):

Sally E. Close ◽

Alberto C. Naveira Garabato ◽

Elaine L. McDonagh ◽

Brian A. King ◽

Martin Biuw ◽

...

Keyword(s):

Water Mass ◽

Drake Passage ◽

Heat Fluxes ◽

Ekman Transport ◽

Turbulent Heat ◽

Intermediate Water ◽

Mode Water ◽

Amundsen Sea ◽

Turbulent Heat Fluxes ◽

Intermediate Water Mass

Abstract The evolution of the physical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage region is examined on time scales down to intraseasonal, within the 1969–2009 period. Both SAMW and AAIW experience substantial interannual to interdecadal variability, significantly linked to the action of the Amundsen Sea low (ASL) in their formation areas. Observations suggest that the interdecadal freshening tendency evident in SAMW over the past three decades has recently abated, while AAIW has warmed significantly since the early 2000s. The two water masses have also experienced a substantial lightening since the start of the record. Examination of the mechanisms underpinning water mass property variability shows that SAMW characteristics are controlled predominantly by a combination of air–sea turbulent heat fluxes, cross-frontal Ekman transport of Antarctic surface waters, and the evaporation–precipitation balance in the Subantarctic zone of the southeast Pacific and Drake Passage, while AAIW properties reflect air–sea turbulent heat fluxes and sea ice formation in the Bellingshausen Sea. The recent interdecadal evolution of the ASL is consistent with both the dominance of the processes described here and the response of SAMW and AAIW on that time scale.

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Circulation and Water Mass Modification in the Brazil–Malvinas Confluence

Journal of Physical Oceanography ◽

10.1175/2009jpo4174.1 ◽

2010 ◽

Vol 40 (5) ◽

pp. 845-864 ◽

Cited By ~ 24

Author(s):

Loic Jullion ◽

Karen J. Heywood ◽

Alberto C. Naveira Garabato ◽

David P. Stevens

Keyword(s):

Deep Water ◽

Subtropical Gyre ◽

Strong Interactions ◽

Intermediate Water ◽

Mode Water ◽

Brazil Current ◽

Subantarctic Mode Water ◽

Circumpolar Current ◽

The Antarctic ◽

Argentine Basin

Abstract The confluence between the Brazil Current and the Malvinas Current [the Brazil–Malvinas Confluence (BMC)] in the Argentine Basin is characterized by a complicated thermohaline structure favoring the exchanges of mass, heat, and salt between the subtropical gyre and the Antarctic Circumpolar Current (ACC). Analysis of thermohaline properties of hydrographic sections in the BMC reveals strong interactions between the ACC and subtropical fronts. In the Subantarctic Front, Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Circumpolar Deep Water (CDW) warm (become saltier) by 0.4° (0.08), 0.3° (0.02), and 0.6°C (0.1), respectively. In the subtropical gyre, AAIW and North Atlantic Deep Water have cooled (freshened) by 0.4° (0.07) and 0.7°C (0.11), respectively. To quantify those ACC–subtropical gyre interactions, a box inverse model surrounding the confluence is built. The model diagnoses a subduction of 16 ± 4 Sv (1 Sv ≡ 106 m3 s−1) of newly formed SAMW and AAIW under the subtropical gyre corresponding to about half of the total subduction rate of the South Atlantic found in previous studies. Cross-frontal heat (0.06 PW) and salt (2.4 × 1012 kg s−1) gains by the ACC in the BMC contribute to the meridional poleward heat and salt fluxes across the ACC. These estimates correspond to perhaps half of the total cross-ACC poleward heat flux. The authors’ results highlight the BMC as a key region in the subtropical–ACC exchanges.

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Antarctic Intermediate Water and Subantarctic Mode Water Formation in the Southeast Pacific: The Role of Turbulent Mixing

Journal of Physical Oceanography ◽

10.1175/2010jpo4114.1 ◽

2010 ◽

Vol 40 (7) ◽

pp. 1558-1574 ◽

Cited By ~ 56

Author(s):

Bernadette M. Sloyan ◽

Lynne D. Talley ◽

Teresa K. Chereskin ◽

Rana Fine ◽

James Holte

Keyword(s):

Mixed Layer ◽

Austral Summer ◽

Buoyancy Flux ◽

Intermediate Water ◽

Austral Winter ◽

Mode Water ◽

Subantarctic Mode Water ◽

Southeast Pacific ◽

Water Formation ◽

Summer Survey

Abstract During the 2005 austral winter (late August–early October) and 2006 austral summer (February–mid-March) two intensive hydrographic surveys of the southeast Pacific sector of the Southern Ocean were completed. In this study the turbulent kinetic energy dissipation rate ε, diapycnal diffusivity κ, and buoyancy flux Jb are estimated from the CTD/O2 and XCTD profiles for each survey. Enhanced κ of O(10−3 to 10−4 m2 s−1) is found near the Subantarctic Front (SAF) during both surveys. During the winter survey, enhanced κ was also observed north of the “subduction front,” the northern boundary of the winter deep mixed layer north of the SAF. In contrast, the summer survey found enhanced κ across the entire region north of the SAF below the shallow seasonal mixed layer. The enhanced κ below the mixed layer decays rapidly with depth. A number of ocean processes are considered that may provide the energy flux necessary to support the observed diffusivity. The observed buoyancy flux (4.0 × 10−8 m2 s−3) surrounding the SAF during the summer survey is comparable to the mean buoyancy flux (0.57 × 10−8 m2 s−3) associated with the change in the interior stratification between austral summer and autumn, determined from Argo profiles. The authors suggest that reduced ocean stratification during austral summer and autumn, by interior mixing, preconditions the water column for the rapid development of deep mixed layers and efficient Antarctic Intermediate Water and Subantarctic Mode Water formation during austral winter and early spring.

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The Effects of Mesoscale Eddies on the Main Subtropical Thermocline

Journal of Physical Oceanography ◽

10.1175/jpo2639.1 ◽

2004 ◽

Vol 34 (11) ◽

pp. 2428-2443 ◽

Cited By ~ 32

Author(s):

Cara C. Henning ◽

Geoffrey K. Vallis

Keyword(s):

Mixed Layer ◽

Mesoscale Eddy ◽

Mesoscale Eddies ◽

Western Boundary ◽

Residual Circulation ◽

Mean Flow ◽

Vertical Diffusion ◽

Mode Water ◽

Water Region ◽

The Mean

Abstract The effects of mesoscale eddies on the main subtropical thermocline are explored using a simply configured wind- and buoyancy-driven primitive equation numerical model in conjunction with transformed Eulerian mean diagnostics and simple scaling ideas and closure schemes. If eddies are suppressed by a modest but nonnegligible horizontal diffusion and vertical diffusion is kept realistically small, the model thermocline exhibits a familiar two-regime structure with an upper, advectively dominated ventilated thermocline and a lower, advective– diffusive internal thermocline, and together these compose the main thermocline. If the horizontal resolution is sufficiently high and the horizontal diffusivity is sufficiently low, then a vigorous mesoscale eddy field emerges. In the mixed layer and upper-mode-water regions, the divergent eddy fluxes are manifestly across isopycnals and so have a diabatic effect. Beneath the mixed layer, the mean structure of the upper (i.e., ventilated) thermocline is still found to be dominated by mean advective terms, except in the “mode water” region and close to the western boundary current. The eddies are particularly strong in the mode-water region, and the low-potential-vorticity pool of the noneddying case is partially eroded away as the eddies try to flatten the isopycnals and reduce available potential energy. The intensity of the eddies decays with depth more slowly than does the mean flow, leading to a three-way balance among eddy flux convergence, mean flow advection, and diffusion in the internal thermocline. Eddies subduct water along isopycnals from the surface into the internal thermocline, replenishing its water masses and maintaining its thickness. Just as in the noneddying case, the dynamics of the internal thermocline can be usefully expressed as an advective–diffusive balance, but where advection is now by the residual (eddy-induced plus Eulerian mean) circulation. The eddy-induced advection partially balances the mean upwelling through the base of the thermocline, and this leads to a slightly thicker thermocline than in the noneddying case. The results suggest that as the diffusivity goes to zero, the residual circulation will go to zero but the thickness of the internal thermocline may remain finite, provided eddy activity persists.

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Diagnosing Surface Mixed Layer Dynamics from High-Resolution Satellite Observations: Numerical Insights

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0136.1 ◽

2013 ◽

Vol 43 (7) ◽

pp. 1345-1355 ◽

Cited By ~ 22

Author(s):

Aurelien L. Ponte ◽

Patrice Klein ◽

Xavier Capet ◽

Pierre-Yves Le Traon ◽

Bertrand Chapron ◽

...

Keyword(s):

High Resolution ◽

Mixed Layer ◽

Vertical Mixing ◽

Mesoscale Eddy ◽

Three Dimensional ◽

Satellite Observations ◽

Scaling Analysis ◽

Surface Mixed Layer ◽

Surface Layers ◽

Velocity Scale

Abstract High-resolution numerical experiments of ocean mesoscale eddy turbulence show that the wind-driven mixed layer (ML) dynamics affects mesoscale motions in the surface layers at scales lower than O(60 km). At these scales, surface horizontal currents are still coherent to, but weaker than, those derived from sea surface height using geostrophy. Vertical motions, on the other hand, are stronger than those diagnosed using the adiabatic quasigeotrophic (QG) framework. An analytical model, based on a scaling analysis and on simple dynamical arguments, provides a physical understanding and leads to a parameterization of these features in terms of vertical mixing. These results are valid when the wind-driven velocity scale is much smaller than that associated with eddies and the Ekman number (related to the ratio between the Ekman and ML depth) is not small. This suggests that, in these specific situations, three-dimensional ML motions (including the vertical velocity) can be diagnosed from high-resolution satellite observations combined with a climatological knowledge of ML conditions and interior stratification.

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A New Algorithm for Finding Mixed Layer Depths with Applications to Argo Data and Subantarctic Mode Water Formation*

Journal of Atmospheric and Oceanic Technology ◽

10.1175/2009jtecho543.1 ◽

2009 ◽

Vol 26 (9) ◽

pp. 1920-1939 ◽

Cited By ~ 147

Author(s):

James Holte ◽

Lynne Talley

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Gradient Methods ◽

Temperature Threshold ◽

Intermediate Water ◽

Mode Water ◽

Southwest Atlantic ◽

Argo Data ◽

Southeastern Pacific ◽

Subantarctic Mode Water

Abstract A new hybrid method for finding the mixed layer depth (MLD) of individual ocean profiles models the general shape of each profile, searches for physical features in the profile, and calculates threshold and gradient MLDs to assemble a suite of possible MLD values. It then analyzes the patterns in the suite to select a final MLD estimate. The new algorithm is provided in online supplemental materials. Developed using profiles from all oceans, the algorithm is compared to threshold methods that use the C. de Boyer Montégut et al. criteria and to gradient methods using 13 601 Argo profiles from the southeast Pacific and southwest Atlantic Oceans. In general, the threshold methods find deeper MLDs than the new algorithm and the gradient methods produce more anomalous MLDs than the new algorithm. When constrained to using only temperature profiles, the algorithm offers a clear improvement over the temperature threshold and gradient methods; the new temperature algorithm MLDs more closely approximate the density algorithm MLDs than the temperature threshold and gradient MLDs. The algorithm is applied to profiles from a formation region of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The density algorithm finds that the deepest MLDs in this region routinely reach 500 dbar and occur north of the A. H. Orsi et al. mean Subantarctic Front in the southeastern Pacific Ocean. The deepest MLDs typically occur in August and September and are congruent with the subsurface salinity minimum, a signature of AAIW.

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Simulation of Subantarctic Mode and Antarctic Intermediate Waters in Climate Models

Journal of Climate ◽

10.1175/jcli4295.1 ◽

2007 ◽

Vol 20 (20) ◽

pp. 5061-5080 ◽

Cited By ~ 41

Author(s):

Bernadette M. Sloyan ◽

Igor V. Kamenkovich

Keyword(s):

Southern Ocean ◽

Climate Models ◽

Intermediate Water ◽

Mixing Processes ◽

Intergovernmental Panel ◽

Mode Water ◽

Buoyancy Forcing ◽

The North ◽

Circumpolar Current ◽

Ipcc Ar4

Abstract The Southern Ocean’s Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are two globally significant upper-ocean water masses that circulate in all Southern Hemisphere subtropical gyres and cross the equator to enter the North Pacific and North Atlantic Oceans. Simulations of SAMW and AAIW for the twentieth century in eight climate models [GFDL-CM2.1, CCSM3, CNRM-CM3, MIROC3.2(medres), MIROC3.2(hires), MRI-CGCM2.3.2, CSIRO-Mk3.0, and UKMO-HadCM3] that provided their output in support of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) have been compared to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atlas of Regional Seas. The climate models, except for UKMO-HadCM3, CSIRO-Mk3.0, and MRI-CGCM2.3.2, provide a reasonable simulation of SAMW and AAIW isopycnal temperature and salinity in the Southern Ocean. Many models simulate the potential vorticity minimum layer and salinity minimum layer of SAMW and AAIW, respectively. However, the simulated SAMW layer is generally thinner and at lighter densities than observed. All climate models display a limited equatorward extension of SAMW and AAIW north of the Antarctic Circumpolar Current. Errors in the simulation of SAMW and AAIW property characteristics are likely to be due to a combination of many errors in the climate models, including simulation of wind and buoyancy forcing, inadequate representation of subgrid-scale mixing processes in the Southern Ocean, and midlatitude diapycnal mixing parameterizations.

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