| 1. |
- Naveira Garabato, Alberto C., et al.
(author)
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Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf
- 2017
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In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 542:7640, s. 219-222
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Journal article (peer-reviewed)abstract
- The instability and accelerated melting of the Antarctic Ice Sheet are among the foremost elements of contemporary global climate change la. The increased freshwater output from Antarctica is important in determining sea level rise(1,3), the fate of Antarctic sea ice and its effect on the Earth's albedo(4,5), ongoing changes in global deep-ocean ventilation(3,6), and the evolution of Southern Ocean ecosystems and carbon cycling(7,8). A key uncertainty in assessing and predicting the impacts of Antarctic Ice Sheet melting concerns the vertical distribution of the exported meltwater. This is usually represented by climate-scale models(3-5,9) as a near-surface freshwater input to the ocean, yet measurements around Antarctica reveal the meltwater to be concentrated at deeper levels(10-14). Here we use observations of the turbulent properties of the meltwater outflows from beneath a rapidly melting Antarctic ice shelf to identify the mechanism responsible for the depth of the meltwater. We show that the initial ascent of the meltwater outflow from the ice shelf cavity triggers a centrifugal overturning instability that grows by extracting kinetic energy from the lateral shear of the background oceanic flow. The instability promotes vigorous lateral export, rapid dilution by turbulent mixing, and finally settling of meltwater at depth. We use an idealized ocean circulation model to show that this mechanism is relevant to a broad spectrum of Antarctic ice shelves. Our findings demonstrate that the mechanism producing meltwater at depth is a dynamically robust feature of Antarctic melting that should be incorporated into climate-scale models.
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| 2. |
- Brannigan, Liam, et al.
(author)
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Submesoscale Instabilities in Mesoscale Eddies
- 2017
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In: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 47:12, s. 3061-3085
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Journal article (peer-reviewed)abstract
- Submesoscale processes have been extensively studied in observations and simulations of fronts. Recent idealized simulations show that submesoscale instabilities also occur in baroclinic mesoscale cyclones and anticyclones. The instabilities in the anticyclone grow faster and at coarser grid resolution than in the cyclone. The instabilities lead to larger restratification in the anticyclone than in the cyclone. The instabilities also lead to changes in the mean azimuthal jet around the anticyclone from 2-km resolution, but a similar effect only occurs in the cyclone at 0.25-km resolution. A numerical passive tracer experiment shows that submesoscale instabilities lead to deeper subduction in the interior of anticyclonic than cyclonic eddies because of outcropping isopycnals extending deeper into the thermocline in anticyclones. An energetic analysis suggests that both vertical shear production and vertical buoyancy fluxes are important in anticyclones but primarily vertical buoyancy fluxes occur in cyclones at these resolutions. The energy sources and sinks vary azimuthally around the eddies caused by the asymmetric effects of the Ekman buoyancy flux. Glider transects of a mesoscale anticyclone in the Tasman Sea show that water with low stratification and high oxygen concentrations is found in an anticyclone, in a manner that may be consistent with the model predictions for submesoscale subduction in mesoscale eddies.
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| 3. |
- Dotto, Tiago S., et al.
(author)
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Control of the Oceanic Heat Content of the Getz-Dotson Trough, Antarctica, by the Amundsen Sea Low
- 2020
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In: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291. ; 125:8
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Journal article (peer-reviewed)abstract
- The changing supply of warm Circumpolar Deep Water (CDW) to the West Antarctic continental shelf is responsible for the basal melting and thinning of the West Antarctic ice shelves that has occurred in recent decades. Here we assess the variability in CDW supply, and its drivers, from a multiyear mooring deployed in, and a regional ocean model spanning, the Getz-Dotson Trough, Amundsen Sea. Between 2010 to 2015, the CDW within the trough underwent a pronounced cooling and freshening, associated with changes in thermohaline properties on isopycnals. Variability in the rate of CDW inflow is controlled by local wind forcing of a shelf break undercurrent, which determines the hydrographic properties of inflowing CDW via tilting of density surfaces above the continental slope. Local wind is coupled to the Amundsen Sea Low (ASL) low-pressure system, which is modulated by large-scale climatic modes via atmospheric teleconnections. For the period analyzed, a deeper ASL was associated with westward wind anomaly at the shelf break. Changes in the sea surface slope decelerated the shelf break undercurrent, resulting in less heat accessing the continental shelf and, consequently, a cooling of the Getz-Dotson Trough. Therefore, the present work suggests that the fate of the West Antarctic ice shelves is closely tied to the future evolution of the ASL.
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| 4. |
- Gülk, Birte, 1994, et al.
(author)
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Impacts of Vertical Convective Mixing Schemes and Freshwater Forcing on the 2016-2017 Maud Rise Polynya Openings in a Regional Ocean Simulation
- 2024
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In: JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS. - 1942-2466. ; 16:5
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Journal article (peer-reviewed)abstract
- The correct representation of the Maud Rise open-ocean polynya in the Weddell Sea remains a challenge for ocean models. Here we reproduce the most recent polynya openings in 2016-2017 using a regional configuration, and assess their dependencies on vertical convective mixing schemes and freshwater forcing, both separately and in combination. We test three vertical convective mixing schemes: the enhanced vertical diffusion (EVD), the Eddy-Diffusivity Mass-Flux (EDMF) parameterization, and a modified version of EDMF accounting for thermobaric effects. Using simulations for the period 2007-2017, we find that the modified EDMF reproduces the observed climatological evolution of the mixed layer depth better than the original EDMF and the EVD, but a polynya fails to open due to excessive freshwater forcing. We thus use the modified EDMF to perform sensitivity experiments with reduced precipitation during 2012-2017. The imposed freshwater forcing strongly affects the number of years with polynyas. The simulation with the best representation of the 2016-2017 polynyas is analyzed to evaluate the triggering mechanisms. The 2016 polynya was induced by the action of thermobaric instabilities on a weak ambient stratification. This opening preconditioned the water column for 2017, which produced a stronger polynya. By examining the impacts of the different convective mixing schemes, we show that the modified EDMF generates more realistic patterns of deep convection. Our results highlight the importance of surface freshwater forcing and thermobaricity in governing deep convection around Maud Rise, and the need to represent thermobaric instabilities to realistically model Maud Rise polynyas. We investigate the impacts of representing numerical vertical mixing and surface freshwater forcing in a regional ocean model on polynyas (large openings in the pack ice) at Maud Rise, Southern Ocean. Maud Rise is prone to hosting polynyas, often associated with deep convection, which is a local vertical mixing process homogenizing the water column between surface and depths of several hundred meters. Numerical models often use simplistic strategies to represent this process, but improved parameterizations have recently become available. In this work, we test the impact of the representation of convective mixing in a particularly sensitive region. The last Maud Rise polynyas were observed in 2016 and 2017. Our regional simulation is capable of reproducing these polynyas, which has long been a challenge for ocean-sea ice models. We show that the 2016 polynya resulted from the action of a vertical instability at depth acting on weak ambient stratification. This event preconditioned the stronger 2017 polynya and deep convection. We conclude that representing convective plumes as a sub-grid scale process in models leads to a more realistic representation of open-ocean polynyas and associated convection events. The Eddy-Diffusivity Mass-Flux (EDMF) parameterization is tested in a regional simulation of the ocean around Maud Rise Thermobaric effects on convective plumes are enabled by modifying the EDMF parameterization Simulations of Maud Rise polynyas are highly sensitive to freshwater forcing and mixing schemes
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| 5. |
- Rabe, Benjamin, et al.
(author)
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Polar Fresh Water in a Changing Global Climate Linking Arctic and Southern Ocean Processes
- 2023
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In: Bulletin of the American Meteorological Society. - 0003-0007 .- 1520-0477. ; 104:5
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Journal article (peer-reviewed)abstract
- NORP-SORP Workshop on Polar Fresh Water: Sources, Pathways and Impacts of Freshwater in Northern and Southern Polar Oceans and Seas (SPICE-UP) What: Up to 60 participants at a time and more than twice as many registrants in total from 20 nations and across experience levels met to discuss the current status of research on freshwater in both polar regions, future directions, and synergies between the Arctic and Southern Ocean research communities When: 19–21 September 2022 Where: Online
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| 6. |
- Silvano, Alessandro, et al.
(author)
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Observing Antarctic Bottom Water in the Southern Ocean
- 2023
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In: Frontiers in Marine Science. - 2296-7745. ; 10
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Research review (peer-reviewed)abstract
- Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW's key role in regulating Earth's climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system.
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| 7. |
- Yu, Xiaolong, et al.
(author)
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An Annual Cycle of Submesoscale Vertical Flow and Restratification in the Upper Ocean
- 2019
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In: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 49:6, s. 1439-1461
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Journal article (peer-reviewed)abstract
- Numerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification and thus restratify the upper ocean via vertical flow. However, the observational evidence for this restratifying process has been lacking due to the difficulty in measuring such ephemeral phenomena, particularly over periods of months to years. This study presents an annual cycle of the vertical velocity and associated restratification estimated from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical midocean area of the northeast Atlantic. Vertical velocities inferred using the nondiffusive density equation are substantially stronger at submesoscales (horizontal scales of 1-10 km) than at mesoscales (horizontal scales of 10-100 km), with respective root-mean-square values of 38.0 +/- 6.9 and 22.5 +/- 3.3 m day(-1). The largest submesoscale vertical velocities and rates of restratification occur in events of a few days' duration in winter and spring, and extend down to at least 200 m below the mixed layer base. These events commonly coincide with the enhancement of submesoscale lateral buoyancy gradients, which is itself associated with persistent mesoscale frontogenesis. This suggests that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence that restratifies the upper ocean. The upper-ocean restratification induced by submesoscale motions integrated over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling, indicating that submesoscale flows play a significant role in determining the climatological upper-ocean stratification in the study area.
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