scholarly journals Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin

2011 ◽  
Vol 8 (6) ◽  
pp. 2197-2235 ◽  
Author(s):  
K. Shimada ◽  
S. Aoki ◽  
K. I. Ohshima ◽  
S. R. Rintoul
Keyword(s):  
Bottom Water ◽  
Ross Sea ◽  
Potential Temperature ◽  
Volume Transport ◽  
Shelf Water ◽  
Antarctic Bottom Water ◽  
Vertical Diffusion ◽  
Sea Bottom ◽  
The Antarctic ◽  
Source Waters

Abstract. The WOCE Hydrographic Program (WHP) and repeated hydrographic data were used to document overall property changes of the Antarctic Bottom Water (AABW) in the Australian-Antarctic Basin between the 1990s and 2000s. Strong cooling and freshening is observed on isopycnals for layers denser than γn = 28.30. Changes in average salinity and potential temperature below this isopycnal correspond to basin-wide warming of 1300 ± 200 TW and freshening of 24 ± 3 Gt yr−1. While freshening can be explained by freshening of major source waters, i.e., the High Salinity Shelf Water (HSSW) of the Ross Sea and the dense shelf water formed in the Adélie and George V Land (AGVL) region, extensive warming of the AABW cannot be explained by warming of the source waters. A possible cause of warming of the AABW is a decrease in supply of the Ross Sea Bottom Water (RSBW). Hydrographic profiles between the Drygalski Trough of the Western Ross Sea and 150° E were analyzed in the context of a simple advective-diffusive model to assess the causes of the observed changes. The RSBW has also warmed by a larger amount than its source water (the HSSW). The model suggests that the warming of the RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in transport of the RSBW and an enhancement of the vertical diffusion of heat as a result of a 30 ± 7% weakening of the abyssal stratification. Freshening of the HSSW reduced the salinity and density stratification between the bottom water layer and overlying ambient water. Hence, freshening of the HSSW both directly freshened and indirectly warmed the RSBW by enhancing the vertical mixing. A simple box model suggest that changes in property and volume transport (decrease of 6.7% is assumed between the year 1995 and 2005) of the RSBW can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in the AABW. These facts demonstrate that changes in both property and volume transport of the RSBW have contributed to the warming and freshening of the AABW in the Australian-Antarctic Basin.

scholarly journals Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin

Ocean Science ◽  
2012 ◽  
Vol 8 (4) ◽  
pp. 419-432 ◽  
Author(s):  
K. Shimada ◽  
S. Aoki ◽  
K. I. Ohshima ◽  
S. R. Rintoul
Keyword(s):  
Bottom Water ◽  
Ross Sea ◽  
Potential Temperature ◽  
Volume Transport ◽  
Alternative Mechanism ◽  
Shelf Water ◽  
Antarctic Bottom Water ◽  
Vertical Diffusion ◽  
Sea Bottom ◽  
Using Data

Abstract. Changes to the properties of Antarctic Bottom Water in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than γn = 28.30 kg m−3. Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 ± 200 GW and freshening of 24 ± 3 Gt year−1. Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171–158° E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 ± 7% weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7% is assumed between the year 1995 and 2005) can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in AA-AABW.

An east-west contrasting changes of Antarctic Bottom Water properties in the Southern Indian Ocean over the last three decades

2021 ◽  
Author(s):  
Yeon Choi ◽  
SungHyun Nam
Keyword(s):  
Indian Ocean ◽  
Deep Water ◽  
Bottom Water ◽  
Ross Sea ◽  
Weddell Sea ◽  
Southern Indian Ocean ◽  
Antarctic Bottom Water ◽  
Sea Bottom ◽  
East West ◽  
Source Waters

<p>Physical properties of Antarctic Bottom Water (AABW) derived from mixture of multiple source waters of different properties, are significantly affected by and contribute to the climate change. This study reveals a contrasting east-west pattern of changes in AABW temperature and salinity in the Southern Indian Ocean (SIO), which continues to become warmer (0.04 ± 0.01<sup></sup>°C/decade) and more saline (0.002 ± 0.001 kg/g/decade) in the western SIO whereas warmer (0.03 ± 0.01<sup></sup>°C/decade) and fresher (-0.004 ± 0.001 kg/g/decade) in the eastern SIO over the past three decades, based on repeat hydrographic observations along meridional lines (1993, 1996, 2008, and 2019 in the western SIO and 1995, 2004, and 2012 in the eastern SIO). Warming and salinification of AABW consisting of the Cape Darnley Bottom Water (CDBW), Weddell Sea Deep Water (WSDW), and Lower Circumpolar Deep Water (LCDW) in the western SIO, are explained by changing proportion of source waters during the period, e.g., decreasing portion of relatively fresh CDBW (from 68% to 59%), and increasing portions of saline WSDW (from 30% to 34%) and warm and saline LCDW (from 2% to 7%). In contrast, in the eastern SIO, warming and freshening of the AABW consisting of the Ross Sea Bottom Water (RSBW), Adélie Land Bottom Water (ALBW), and LCDW are not explained by the changing proportion but properties of the source waters during the period, e.g., warming and freshening of RSBW (0.08°C/decade and -0.013 kg/g/decade) and ALBW (0.01°C/decade and -0.008 kg/g/decade). The east-west contrasting changes of AABW properties (eastern freshening and western salinification) over the last three decades have important consequences within and beyond the SIO.</p>

scholarly journals Rebound of shelf water salinity in the Ross Sea

2019 ◽  
Vol 10 (1) ◽  
Author(s):  
Pasquale Castagno ◽  
Vincenzo Capozzi ◽  
Giacomo R. DiTullio ◽  
Pierpaolo Falco ◽  
Giannetta Fusco ◽  
...  
Keyword(s):  
Large Scale ◽  
Bottom Water ◽  
High Salinity ◽  
Ross Sea ◽  
Shelf Water ◽  
Antarctic Bottom Water ◽  
Sea Bottom ◽  
Salinity Increase ◽  
Interannual Fluctuations ◽  
Observational Record

AbstractAntarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation and ventilates the abyssal ocean. In recent decades, AABW has warmed, freshened and reduced in volume. Ross Sea Bottom Water (RSBW), the second largest source of AABW, has experienced the largest freshening. Here we use 23 years of summer measurements to document temporal variability in the salinity of the Ross Sea High Salinity Shelf Water (HSSW), a precursor to RSBW. HSSW salinity decreased between 1995 and 2014, consistent with freshening observed between 1958 and 2008. However, HSSW salinity rebounded sharply after 2014, with values in 2018 similar to those observed in the mid-late 1990s. Near-synchronous interannual fluctuations in salinity observed at five locations on the continental shelf suggest that upstream preconditioning and large-scale forcing influence HSSW salinity. The rate, magnitude and duration of the recent salinity increase are unusual in the context of the (sparse) observational record.

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

Ocean Science ◽  
2014 ◽  
Vol 10 (6) ◽  
pp. 923-946 ◽  
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.

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

2014 ◽  
Vol 11 (2) ◽  
pp. 1023-1091 ◽  
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.

Recent recovery of Antarctic Bottom Water formation in the Ross Sea driven by climate anomalies

2021 ◽  
Author(s):  
Alessandro Silvano ◽  
Annie Foppert ◽  
Steve Rintoul ◽  
Paul Holland ◽  
Takeshi Tamura ◽  
...  
Keyword(s):  
Sea Ice ◽  
Continental Shelf ◽  
Bottom Water ◽  
Ross Sea ◽  
Ice Sheet ◽  
Ice Formation ◽  
Depth Range ◽  
Antarctic Bottom Water ◽  
Climate Anomalies ◽  
The Antarctic

<div> <div> <div> <p>Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation, ventilates the abyssal ocean and sequesters heat and carbon on multidecadal to millennial timescales. AABW originates on the Antarctic continental shelf, where strong winter cooling and brine released during sea ice formation produce Dense Shelf Water, which sinks to the deep ocean. The salinity, density and volume of AABW have decreased over the last 50 years, with the most marked changes observed in the Ross Sea. These changes have been attributed to increased melting of the Antarctic Ice Sheet. Here we use in situ observations to document a recovery in the salinity, density and thickness (that is, depth range) of AABW formed in the Ross Sea, with properties in 2018–2019 similar to those observed in the 1990s. The recovery was caused by increased sea ice formation on the continental shelf. Increased sea ice formation was triggered by anomalous wind forcing associated with the unusual combination of positive Southern Annular Mode and extreme El Niño conditions between 2015 and 2018. Our study highlights the sensitivity of AABW formation to remote forcing and shows that climate anomalies can drive episodic increases in local sea ice formation that counter the tendency for increased ice-sheet melt to reduce AABW formation.</p> </div> </div> </div>

scholarly journals Variability and Mixing of the Filchner Overflow Plume on the Continental Slope, Weddell Sea

2019 ◽  
Vol 49 (1) ◽  
pp. 3-20 ◽  
Author(s):  
K. Daae ◽  
I. Fer ◽  
E. Darelius
Keyword(s):  
Continental Slope ◽  
Bottom Water ◽  
Large Fraction ◽  
Volume Transport ◽  
Weddell Sea ◽  
Water Masses ◽  
Shelf Water ◽  
Antarctic Bottom Water ◽  
Ambient Water ◽  
Mixing Rates

AbstractA large fraction of Antarctic Bottom Water is produced in the Weddell Sea, through mixing between the cold and dense shelf water masses and the warm and saline off-shelf water. We present observations of the dense Filchner overflow plume from one mooring at the Filchner sill and two moorings located downstream, on the continental slope. The plume variability over the continental slope at a monthly time scale is related to upstream conditions at the Filchner sill, with a high correlation in density. Revised column-integrated volume transport calculations across the Filchner sill indicate 50% higher values in 2010 compared with the earlier estimates available from 1985. Over the continental slope, the plume thickness fluctuates strongly between less than 25 m and more than 250 m. Observations of elevated temperature variance and high Froude numbers at the plume interface imply high mixing rates and entrainment of ambient water masses. The mixing events typically coincide with shear spikes across the plume. The shear spikes appear quasi-periodically, when counterrotating oscillations with periods of 24 and 72 h align. The clockwise 24-h oscillation is related to diurnal, barotropic tidal currents and topographic vorticity waves, whereas the counterclockwise 72-h oscillation is related to vortex stretching or topographic vorticity waves.

Chemical evidence of the changes of the Antarctic Bottom Water ventilation in the western Ross Sea between 1997 and 2003

2010 ◽  
Vol 57 (5) ◽  
pp. 639-652 ◽  
Author(s):  
Paola Rivaro ◽  
Serena Massolo ◽  
Andrea Bergamasco ◽  
Pasquale Castagno ◽  
Giorgio Budillon
Keyword(s):  
Bottom Water ◽  
Ross Sea ◽  
Antarctic Bottom Water ◽  
The Antarctic ◽  
Western Ross Sea ◽  
Chemical Evidence

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

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.

Recent recovery of Antarctic Bottom Water formation in the Ross Sea driven by climate anomalies

2020 ◽  
Vol 13 (12) ◽  
pp. 780-786 ◽  
Author(s):  
Alessandro Silvano ◽  
Annie Foppert ◽  
Stephen R. Rintoul ◽  
Paul R. Holland ◽  
Takeshi Tamura ◽  
...  
Keyword(s):  
Bottom Water ◽  
Ross Sea ◽  
Antarctic Bottom Water ◽  
Climate Anomalies ◽  
Water Formation ◽  
Bottom Water Formation
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