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

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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Assessment of the representation of Antarctic Bottom Water properties in the ECCO2 reanalysis

Ocean Science ◽

10.5194/os-10-923-2014 ◽

2014 ◽

Vol 10 (6) ◽

pp. 923-946 ◽

Cited By ~ 9

Author(s):

M. Azaneu ◽

R. Kerr ◽

M. M. Mata

Keyword(s):

Southern Ocean ◽

Ocean Circulation ◽

Bottom Water ◽

Potential Temperature ◽

World Ocean ◽

Volume Transport ◽

Weddell Sea ◽

Antarctic Bottom Water ◽

Water Properties ◽

The Absolute

Abstract. We analyzed the ability of the Estimating the Circulation and Climate of the Ocean – Phase II (ECCO2) reanalysis to represent the hydrographic properties and variability of Antarctic Bottom Water (AABW) in the Southern Ocean. We used a 20-year (1992–2011) observational database to perform comparisons of hydrographic properties and reanalysis output for the same time period. Four case studies based on current meter data and the AABW volume transport estimates previously reported in the literature were also evaluated. The opening and maintenance of an oceanic polynya in the Weddell Sea sector is observed after 2004 in the reanalysis product. Moreover, intense deep water production due to deep convection occurs, which leads to a scenario in which the Weddell Sea is flooded with AABW. For this reason, our analyses focused on the period that was identified as more reliable (1992–2004). The main Southern Ocean oceanographic features, as well as the characteristic shape of the regional potential temperature–salinity (θ–S) diagrams, are coincident with observations. However, the reanalysis output produces surface waters that are generally denser than observations due to the reproduction of waters that are generally saltier than expected, which probably resulted from the strong seasonality of sea ice concentrations. Bottom waters are warmer and less dense, while intermediate waters are statistically closest to the observations. The differences in bottom water properties are partially due to the inability of the reanalysis to properly reproduce the formation and export of dense waters from the shelf and the consequent absence of the densest AABW variety for most of the analyzed period. Despite differences in the absolute values, the upper AABW limit (γn ≥ 28.27 kg m−3) and AABW occupied area estimates are coincident with the observations in the World Ocean Circulation Experiment (WOCE) repeat sections SR2 and SR4. Moreover, the AABW volume export and current velocity variability are correlated with the observed time series in the most important region of dense water export (i.e., the Weddell Sea). Despite the consistency in terms of variability, the absolute volume transport and velocity estimates are underrepresented in all cases.

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Assessment of the ECCO2 reanalysis on the representation of Antarctic Bottom Water properties

Ocean Science Discussions ◽

10.5194/osd-11-1023-2014 ◽

2014 ◽

Vol 11 (2) ◽

pp. 1023-1091 ◽

Cited By ~ 2

Author(s):

M. Azaneu ◽

R. Kerr ◽

M. M. Mata

Keyword(s):

Southern Ocean ◽

Case Studies ◽

Bottom Water ◽

Potential Temperature ◽

Volume Transport ◽

Reanalysis Data ◽

Weddell Sea ◽

Good Representation ◽

Antarctic Bottom Water ◽

Water Properties

Abstract. We analyzed the ability of the Estimating the Circulation and Climate of the Ocean – Phase II (ECCO2) reanalysis to represent the hydrographic properties and variability of the Antarctic Bottom Water (AABW) in the Southern Ocean. We used a twenty-year observational database to perform comparisons of hydrographic properties and reanalysis data for the same time period (1992–2011). In addition, we evaluated four case studies based on current meter data and the AABW volume transport estimates previously reported in the literature. The main Southern Ocean oceanographic features, as well as the characteristic shape of the regional potential temperature–salinity (θ–S) diagrams, are adequately represented by the reanalysis. However, the opening of an oceanic polynya in the Weddell Sea Sector, which has been clearly visible since 2005, contributed to an unrealistic representation of the hydrographic properties of the Southern Ocean primarily after 2004. In this sense, our analyses focused on the period that was identified as more reliable (1992–2004). In general, the reanalysis data showed surface waters that were warmer, saltier, and denser than observations, which may have resulted from the absence of Ice Shelf Water and from the overestimation of sea ice concentrations that limit oceanic heat loss during austral winters. Intermediate waters were generally colder, fresher, and denser than observations, whereas deep waters were warmer and less dense. These differences in deep water properties were partially a result of the inability to reproduce the densest AABW variety by reanalysis for most of the analyzed period and also because of the model's relatively coarse vertical resolution. Despite differences in absolute values, the upper AABW limit (γn ≥ 28.27 kg m−3) and AABW occupied area were well represented in the WOCE repeat sections SR2 and SR4 for the studied periods. In section WOCE SR3, however, the estimates from the differences were not as well correlated, and the AABW layer thickness was underrepresented. The case studies showed a good representation of the AABW volume export and current velocity variability in the most important region of dense water export (i.e., the Weddell Sea). The exception is the AABW volume transport near the Kerguelen Plateau, in which the rugged local bathymetry and the relatively coarse model resolution hampered a fair representation of the transport variability by the reanalysis. Despite the consistency in terms of variability, absolute volume transport, and velocity, estimates were underrepresented in all cases. Moreover, the reanalysis was capable of reproducing the general variability pattern and trends of the AABW hydrographic properties reported by previous studies. Therefore, the ECCO2 data from the 1992–2004 period was considered adequate for investigating the circulation of the AABW and variability of the hydrographic properties, whereas data from the latter period (2005–2011) must be given careful attention.

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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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Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO3.4

Geoscientific Model Development ◽

10.5194/gmd-8-3119-2015 ◽

2015 ◽

Vol 8 (10) ◽

pp. 3119-3130 ◽

Cited By ~ 18

Author(s):

C. Heuzé ◽

J. K. Ridley ◽

D. Calvert ◽

D. P. Stevens ◽

K. J. Heywood

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Surface Waters ◽

Bottom Water ◽

Climate Model ◽

Deep Convection ◽

Mixed Layer Depth ◽

Vertical Mixing ◽

Weddell Sea ◽

Antarctic Bottom Water

Abstract. Most CMIP5 (Coupled Model Intercomparison Project Phase 5) models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross seas. To identify the mechanisms triggering Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO3.4 forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing give warmer surface waters, open larger polynyas causing more saline surface waters and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

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Evidence for northward expansion of Antarctic Bottom Water mass in the Southern Ocean during the last glacial inception

Paleoceanography ◽

10.1029/2008pa001603 ◽

2009 ◽

Vol 24 (1) ◽

pp. n/a-n/a ◽

Cited By ~ 51

Author(s):

Aline Govin ◽

Elisabeth Michel ◽

Laurent Labeyrie ◽

Claire Waelbroeck ◽

Fabien Dewilde ◽

...

Keyword(s):

Southern Ocean ◽

Bottom Water ◽

Water Mass ◽

Antarctic Bottom Water ◽

Last Glacial ◽

Northward Expansion ◽

The Last Glacial

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

Journal of Physical Oceanography ◽

10.1175/2011jpo4571.1 ◽

2011 ◽

Vol 41 (10) ◽

pp. 1941-1957 ◽

Cited By ~ 15

Author(s):

Shinichiro Kida

Keyword(s):

Bottom Water ◽

Water Mass ◽

Shelf Break ◽

Weddell Sea ◽

Antarctic Bottom Water ◽

Antarctic Continental Shelf ◽

Deep Channel ◽

Major Branch ◽

The Antarctic ◽

The Impact

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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Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO

Geoscientific Model Development Discussions ◽

10.5194/gmdd-8-2949-2015 ◽

2015 ◽

Vol 8 (3) ◽

pp. 2949-2972 ◽

Cited By ~ 2

Author(s):

C. Heuzé ◽

J. K. Ridley ◽

D. Calvert ◽

D. P. Stevens ◽

K. J. Heywood

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Surface Waters ◽

Bottom Water ◽

Deep Convection ◽

Mixed Layer Depth ◽

Vertical Mixing ◽

Weddell Sea ◽

Cmip5 Models ◽

Antarctic Bottom Water

Abstract. Most CMIP5 models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross Seas. To identify the triggering mechanisms leading to Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing are warmer, open larger polynyas and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

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Recent Freshening, Warming, and Contraction of the Antarctic Bottom Water in the Indian Sector of the Southern Ocean

Frontiers in Marine Science ◽

10.3389/fmars.2021.730630 ◽

2021 ◽

Vol 8 ◽

Author(s):

N Anilkumar ◽

Babula Jena ◽

Jenson V. George ◽

Sabu P ◽

Kshitija S ◽

...

Keyword(s):

Southern Ocean ◽

Layer Thickness ◽

Bottom Water ◽

Water Mass ◽

Global Climate ◽

Recent Decade ◽

Global Ocean ◽

Antarctic Bottom Water ◽

Ice Shelves ◽

Water Characteristics

High saline and cold Antarctic Bottom Water (AABW) forms around the continental margin of Antarctica that ventilates into the global ocean. To study the recent changes in AABW, we have analyzed the in situ observations collected from Indian Ocean expeditions to the Southern Ocean during 2010, 2011, 2017, 2018, and 2020. A comprehensive analysis of these observations indicated recent freshening, warming, and contraction in the layer thickness of the AABW. Even though the AABW depicted inter-annual variability, it changed to moderately fresher and lighter water mass at the end of the recent decade. The characteristics of AABW exhibited a contraction in its layer thickness (∼50–120 m) during recent years. The water mass showed its freshening (∼0.002) and warming (∼0.04°C) tendency from 2018 to 2020. The recent warming (∼0.3°C) of Circumpolar Deep Water (CDW) near the Prydz Bay suggests enhanced melting of ice shelves. It is hypothesized that the combined influences of onshore intrusion of warm CDW, upper ocean warming, sea ice decline, wind forcing, polynya, and calving events possibly caused the freshening and reduction in the thickness of AABW. The continued changes in the ocean-atmospheric environmental conditions and the subsequent changes in the bottom water characteristics likely influence the global climate, overturning circulation, and the biogeochemical cycle.

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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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Global Contraction of Antarctic Bottom Water between the 1980s and 2000s*

Journal of Climate ◽

10.1175/jcli-d-11-00612.1 ◽

2012 ◽

Vol 25 (17) ◽

pp. 5830-5844 ◽

Cited By ~ 121

Author(s):

Sarah G. Purkey ◽

Gregory C. Johnson

Keyword(s):

Southern Ocean ◽

Bottom Water ◽

Potential Temperature ◽

Deep Ocean ◽

Global Scale ◽

Meridional Overturning Circulation ◽

Antarctic Bottom Water ◽

Ocean Heat Uptake ◽

Near Surface ◽

The North

Abstract A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the meridional overturning circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW. Rates of change in AABW-related circulation are estimated in most of the world’s deep-ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water below θ = 0°C at a rate of −8.2 (±2.6) × 106 m3 s−1. This bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near-surface recovery, apparently through a southward surge of Circumpolar Deep Water from the north. To the north, smaller losses of bottom waters are seen along three of the four main northward outflow routes of AABW. Volume and heat budgets below deep, cold θ surfaces within the Brazil and Pacific basins are not in steady state. The observed changes in volume and heat of the coldest waters within these basins could be accounted for by small decreases to the volume transport or small increases to θ of their inflows, or fractional increases in deep mixing. The budget calculations and global contraction pattern are consistent with a global-scale slowdown of the bottom, southern limb of the MOC.

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