The Impact of Open Oceanic Processes on the Antarctic Bottom Water Outflows

Abstract The impact of open oceanic processes on the Antarctic Bottom Water (AABW) outflows is investigated using a numerical model with a focus on outflows that occur through deep channels. A major branch of the AABW outflow is known to occur as an overflow from the Filchner Depression to the Weddell Sea through a deep channel a few hundred kilometers wide and a sill roughly 500 m deep. When this overflow enters the Weddell Sea, it encounters the Antarctic Slope Front (ASF) at the shelf break, a density front commonly found along the Antarctic continental shelf break. The presence of an AABW outflow and the ASF create a v-shaped isopycnal structure across the shelf break, indicating an interaction between the overflow and oceanic processes. Model experiments show the overflow transport to increase significantly when an oceanic wind stress increases the depth of the ASF. This enhancement of overflow transport occurs because the channel walls allow a pressure gradient in the along-slope direction to exist and the overflow transport is geostrophically controlled with its ambient oceanic water at the shelf break. Because the ASF is associated with a lighter water mass that reaches the depth close to that of the channel, an increase in its depth increases the density gradient across the shelf break and therefore the geostrophic overflow transport. The enhancement of overflow transport is also likely to result in a lighter overflow water mass, although such an adjustment of density likely occurs on a much longer time scale than the adjustment of transport.

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The features of Antarctic Bottom Water in the fracture zones at 7-10 deg. N of the Mid-Atlantic ridge

10.5194/egusphere-egu2020-19271 ◽

2020 ◽

Author(s):

Valentina Volkova ◽

Alexander Demidov ◽

Fedor Gippius

Keyword(s):

Bottom Water ◽

Historical Data ◽

Water Mass ◽

Main Task ◽

Bottom Relief ◽

Antarctic Bottom Water ◽

Fracture Zones ◽

Mid Atlantic Ridge ◽

Proper Estimation ◽

The Antarctic

Despite the fact that there are numerous estimates of the Antarctic Bottom Water (AABW) formation and transport, its evolution and distribution pathways are still debatable (Morozov E.G. et al., 2010).The main task of this work was to identify the structure and transport of deep and bottom water mass of the fracture zones (7 40', Vernadsky and Doldrums). The research is based on new data (multibeam bottom relief, temperature, salinity, velocity) obtained during the research cruise on the RV "Akademik Nikolaj Strakhov" in October-November 2019 and WODB18 historical data.The main result of the research is proper estimation of the AABW and LNADW transport, which takes into consideration the influence of fracture zone morphometry. Accordingly, the preliminary circulation scheme of water masses is obtained.

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Decadal Freshening of the Antarctic Bottom Water Exported from the Weddell Sea

Journal of Climate ◽

10.1175/jcli-d-12-00765.1 ◽

2013 ◽

Vol 26 (20) ◽

pp. 8111-8125 ◽

Cited By ~ 38

Author(s):

Loïc Jullion ◽

Alberto C. Naveira Garabato ◽

Michael P. Meredith ◽

Paul R. Holland ◽

Peggy Courtois ◽

...

Keyword(s):

Bottom Water ◽

Weddell Sea ◽

Antarctic Bottom Water ◽

Atlantic Sector ◽

Ice Shelves ◽

Anthropogenic Component ◽

Deep Layers ◽

Bottom Waters ◽

Circumpolar Current ◽

The Antarctic

Abstract Recent decadal changes in Southern Hemisphere climate have driven strong responses from the cryosphere. Concurrently, there has been a marked freshening of the shelf and bottom waters across a wide sector of the Southern Ocean, hypothesized to be caused by accelerated glacial melt in response to a greater flux of warm waters from the Antarctic Circumpolar Current onto the shelves of West Antarctica. However, the circumpolar pattern of changes has been incomplete: no decadal freshening in the deep layers of the Atlantic sector has been observed. In this study, the authors document a significant freshening of the Antarctic Bottom Water exported from the Weddell Sea, which is the source for the abyssal layer of the Atlantic overturning circulation, and trace its possible origin to atmospheric-forced changes in the ice shelves and sea ice on the eastern flank of the Antarctic Peninsula that include an anthropogenic component. These findings suggest that the expansive and relatively cool Weddell gyre does not insulate the bottom water formation regions in the Atlantic sector from the ongoing changes in climatic forcing over the Antarctic region.

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Distribution of Water Masses in the Atlantic Ocean based on GLODAPv2

10.5194/os-2018-140 ◽

2019 ◽

Author(s):

Mian Liu ◽

Toste Tanhua

Keyword(s):

Atlantic Ocean ◽

Bottom Water ◽

Water Mass ◽

Potential Temperature ◽

Weddell Sea ◽

Water Masses ◽

General Distribution ◽

Intermediate Water ◽

The North ◽

The Antarctic

Abstract. The distribution of the main water masses in the Atlantic Ocean are investigated with the Optimal Multi-Parameter (OMP) method. The properties of the main water masses in the Atlantic Ocean are described in a companion article; here these definitions are used to map out the general distribution of those water masses. Six key properties, including conservative (potential temperature and salinity) and non-conservative (oxygen, silicate, phosphate and nitrate), are incorporated into the OMP analysis to determine the contribution of the water masses in the Atlantic Ocean based on the GLODAP v2 observational data. To facilitate the analysis the Atlantic Ocean is divided into four vertical layers based on potential density. Due to the high seasonal variability in the mixed layer, this layer is excluded from the analysis. Central waters are the main water masses in the upper/central layer, generally featuring high potential temperature and salinity and low nutrient concentrations and are easily distinguished from the intermediate water masses. In the intermediate layer, the Antarctic Intermediate Water (AAIW) from the south can be detected to ~30 °N, whereas the Subarctic Intermediate Water (SAIW), having similarly low salinity to the AAIW flows from the north. Mediterranean Overflow Water (MOW) flows from the Strait of Gibraltar as a high salinity water. NADW dominates the deep and overflow layer both in the North and South Atlantic. In the bottom layer, AABW is the only natural water mass with high silicate signature spreading from the Antarctic to the North Atlantic. Due to the change of water mass properties, in this work we renamed to North East Antarctic Bottom Water NEABW north of the equator. Similarly, the distributions of Labrador Sea Water (LSW), Iceland Scotland Overflow Water (ISOW), and Denmark Strait Overflow Water (DSOW) forms upper and lower portion of NADW, respectively roughly south of the Grand Banks between ~50 and 66 °N. In the far south the distributions of Circumpolar Deep Water (CDW) and Weddell Sea Bottom Water (WSBW) are of significance to understand the formation of the AABW.

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Antarctic Bottom Water Warming and Freshening: Contributions to Sea Level Rise, Ocean Freshwater Budgets, and Global Heat Gain*

Journal of Climate ◽

10.1175/jcli-d-12-00834.1 ◽

2013 ◽

Vol 26 (16) ◽

pp. 6105-6122 ◽

Cited By ~ 130

Author(s):

Sarah G. Purkey ◽

Gregory C. Johnson

Keyword(s):

Southern Ocean ◽

Sea Level Rise ◽

Sea Level ◽

Bottom Water ◽

Water Mass ◽

Potential Temperature ◽

Weddell Sea ◽

Freshwater Flux ◽

Antarctic Bottom Water ◽

South Indian

Abstract Freshening and warming of Antarctic Bottom Water (AABW) between the 1980s and 2000s are quantified, assessing the relative contributions of water-mass changes and isotherm heave. The analysis uses highly accurate, full-depth, ship-based, conductivity–temperature–depth measurements taken along repeated oceanographic sections around the Southern Ocean. Fresher varieties of AABW are present within the South Pacific and south Indian Oceans in the 2000s compared to the 1990s, with the strongest freshening in the newest waters adjacent to the Antarctic continental slope and rise indicating a recent shift in the salinity of AABW produced in this region. Bottom waters in the Weddell Sea exhibit significantly less water-mass freshening than those in the other two southern basins. However, a decrease in the volume of the coldest, deepest waters is observed throughout the entire Southern Ocean. This isotherm heave causes a salinification and warming on isobaths from the bottom up to the shallow potential temperature maximum. The water-mass freshening of AABW in the Indian and Pacific Ocean sectors is equivalent to a freshwater flux of 73 ± 26 Gt yr−1, roughly half of the estimated recent mass loss of the West Antarctic Ice Sheet. Isotherm heave integrated below 2000 m and south of 30°S equates to a net heat uptake of 34 ± 14 TW of excess energy entering the deep ocean from deep volume loss of AABW and 0.37 ± 0.15 mm yr−1 of sea level rise from associated thermal expansion.

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Bottom water production and its links with the thermohaline circulation

Antarctic Science ◽

10.1017/s095410200400224x ◽

2004 ◽

Vol 16 (4) ◽

pp. 427-437 ◽

Cited By ~ 139

Author(s):

STANLEY S. JACOBS

Keyword(s):

Temporal Variability ◽

Bottom Water ◽

Thermohaline Circulation ◽

World Ocean ◽

Antarctic Bottom Water ◽

Water Production ◽

Ongoing Work ◽

Antarctic Continental Shelf ◽

The World ◽

The Antarctic

For more than a century it has been known that the abyssal basins of the world ocean are primarily occupied by relatively cold and fresh waters that originate in the Southern Ocean. Their distinguishing characteristics are acquired by exposure of surface and shelf waters to ‘ventilation’ by the polar atmosphere and to the melting and freezing of ice over and near the Antarctic continental shelf. Subsequent mixing with deep water over the continental slope results in ‘Bottom Water’ that forms the southern sinking limb of the global ‘Thermohaline Circulation.’ Over recent decades, oceanographers have wrestled with a variety of bottom water and thermohaline circulation problems, ranging from basic definitions to forcing and formation sites, source components and properties, generation processes and rates, mixing and sinking, pathways and transports. A brief review of these efforts indicates both advances and anomalies in our understanding of Antarctic Bottom Water production and circulation. Examples from ongoing work illustrate increasing interest in the temporal variability of bottom water in relation to climate change.

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Ventilation of the abyss in the Atlantic sector of the Southern Ocean

Scientific Reports ◽

10.1038/s41598-021-86043-2 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Camille Hayatte Akhoudas ◽

Jean-Baptiste Sallée ◽

F. Alexander Haumann ◽

Michael P. Meredith ◽

Alberto Naveira Garabato ◽

...

Keyword(s):

Southern Ocean ◽

Bottom Water ◽

Climate Models ◽

Deep Ocean ◽

Analytical Framework ◽

Weddell Sea ◽

Global Ocean ◽

Shelf Water ◽

Antarctic Bottom Water ◽

Atlantic Sector

AbstractThe Atlantic sector of the Southern Ocean is the world’s main production site of Antarctic Bottom Water, a water-mass that is ventilated at the ocean surface before sinking and entraining older water-masses—ultimately replenishing the abyssal global ocean. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in this region have led to a strikingly broad range of results, with water transport-based calculations (8.4–9.7 Sv) yielding larger rates than tracer-based estimates (3.7–4.9 Sv). Here, we reconcile these conflicting views by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly quantified. We show that the layer of Antarctic Bottom Water denser than 28.36 kg m$$^{-3}$$ - 3 $$\gamma _{n}$$ γ n is exported northward at a rate of 8.4 ± 0.7 Sv, composed of 4.5 ± 0.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 0.5 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.4 ± 0.6 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region is well ventilated and thus draws down heat and carbon into the deep ocean. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector, and define a baseline, process-discerning target for its realistic representation in climate models.

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Attenuated carbon sequestration by Weddell Sea dense waters over the 21st century – an assessment with FESOM-REcoM

10.5194/egusphere-egu21-6248 ◽

2021 ◽

Author(s):

Cara Nissen ◽

Ralph Timmermann ◽

Mario Hoppema ◽

Judith Hauck

Keyword(s):

Carbon Sequestration ◽

Sea Ice ◽

Continental Shelf ◽

Bottom Water ◽

Water Mass ◽

Weddell Sea ◽

Ice Shelf ◽

Deep Waters ◽

Water Formation ◽

Water Mass Transformation

Deep and bottom water formation regions have long been recognized to be efficient vectors for carbon transfer to depth, leading to carbon sequestration on time scales of centuries or more. Precursors of Antarctic Bottom Water (AABW) are formed on the Weddell Sea continental shelf as a consequence of buoyancy loss of surface waters at the ice-ocean or atmosphere-ocean interface, which suggests that any change in water mass transformation rates in this area affects global carbon cycling and hence climate. Many of the models previously used to assess AABW formation in present and future climates contained only crude representations of ocean-ice shelf interaction. Numerical simulations often featured spurious deep convection in the open ocean, and changes in carbon sequestration have not yet been assessed at all. Here, we present results from the global model FESOM-REcoM, which was run on a mesh with elevated grid resolution in the Weddell Sea and which includes an explicit representation of sea ice and ice shelves. Forcing this model with ssp585 scenario output from the AWI Climate Model, we assess changes over the 21st century in the formation and northward export of dense waters and the associated carbon fluxes within and out of the Weddell Sea. We find that the northward transport of dense deep waters (&#963;2>37.2 kg m-3 below 2000 m) across the SR4 transect, which connects the tip of the Antarctic Peninsula with the eastern Weddell Sea, declines from 4 Sv to 2.9 Sv by the year 2100. Concurrently, despite the simulated continuous increase in surface ocean CO2 uptake in the Weddell Sea over the 21st century, the carbon transported northward with dense deep waters declines from 3.5 Pg C yr-1 to 2.5 Pg C yr-1, demonstrating the dominant role of dense water formation rates for carbon sequestration. Using the water mass transformation framework, we find that south of SR4, the formation of downwelling dense waters declines from 3.5 Sv in the 1990s to 1.6 Sv in the 2090s, a direct result of the 18% lower sea-ice formation in the area, the increased presence of modified Warm Deep Water on the continental shelf, and 50% higher ice shelf basal melt rates. Given that the reduced formation of downwelling water masses additionally occurs at lighter densities in FESOM-REcoM in the 2090s, this will directly impact the depth at which any additional oceanic carbon uptake is stored, with consequences for long-term carbon sequestration.

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New geophysical evidences of erosion at the base of the Walvis Ridge by the Antarctic Bottom Water (AABW)

10.3997/2214-4609-pdb.172.sbgf0448_07 ◽

2007 ◽

Author(s):

M.A. Gorini ◽

K. Owolabi ◽

J.R. Derntl ◽

C.G. Silva

Keyword(s):

Bottom Water ◽

Antarctic Bottom Water ◽

Walvis Ridge ◽

The Antarctic

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On the deep convection events and Antarctic Bottom Water formation in ocean reanalysis products

10.5194/os-2017-9 ◽

2017 ◽

Cited By ~ 1

Author(s):

Wilton Aguiar ◽

Mauricio M. Mata ◽

Rodrigo Kerr

Keyword(s):

General Circulation ◽

Bottom Water ◽

Deep Convection ◽

General Circulation Models ◽

Open Ocean ◽

Weddell Sea ◽

Antarctic Bottom Water ◽

Ocean Reanalysis ◽

Ocean General Circulation Models ◽

Bottom Water Formation

Abstract. Deep convection in open ocean polynyas are common sources of error on the representation of Antarctic Bottom Water (AABW) formation in Ocean General Circulation Models. Even though those events are well described in non-assimilatory ocean simulations, recent appearance of open ocean polynya in Estimating the Circulation and Climate of the Ocean Phase II reanalysis product raises a question if this spurious event is also found in state-of-art reanalysis products. In order to answer this question, we evaluate how three recently released high-resolution ocean reanalysis form AABW in their simulations. We found that two of them (ECCO2 and SoSE) create AABW by open ocean deep convection events in Weddell Sea, showing that assimilation of sea ice has not been enough to avoid open ocean polynya appearance. The third reanalysis – My Ocean University Reading – actually creates AABW by a rather dynamically accurate mechanism, depicting both continental shelf convection, and exporting of Dense Shelf Water to open ocean. Although the accuracy of the AABW formation in this reanalysis allows an advance in represent this process, the differences found between the real ocean and the simulated one suggests that ocean reanalysis still need substantial improvements to accurately represent AABW formation.

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