Upper ocean salinity variabilities in the Southern Ocean responding to the recent surface salting in perspective of climate changes

2021 ◽  
Author(s):  
Ling Du ◽  
Xubin Ni
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
Water Mass ◽  
Water Cycle ◽  
Upper Ocean ◽  
Tropical Ocean ◽  
Circumpolar Deep Water ◽  
Salinity Changes ◽  
Ocean Salinity ◽  
The Antarctic

<p>Water cycle have prevailed on upper ocean salinity acting as the climate change fingerprint in the numerous observation and simulation works. Water mass in the Southern Ocean accounted for the increasing importance associated with the heat and salt exchanges between Subantarctic basins and tropical oceans. The circumpolar deep water (CDW), the most extensive water mass in the Southern Ocean, plays an indispensable role in the formation of Antarctic Bottom Water. In our study, the observed CTDs and reanalysis datasets are examined to figure out the recent salinity changes in the three basins around the Antarctica. Significant surface salinity anomalies occurred in the South Indian/Pacific sectors south of 60ºS since 2008, which are connected with the enhanced CDW incursion onto the Antarctic continental shelf. Saltier shelf water was found to expand northward from the Antarctica coast. Meanwhile, the freshening of Upper Circumpolar Deep Water(UCDW), salting and submergence of Subantarctic Mode Water(SAMW) were also clearly observed. The modified vertical salinity structures contributed to the deepen mixed layer and enhanced intermediate stratification between SAMW and UCDW. Their transport of salinity flux attributed to the upper ocean processes responding to the recent atmospheric circulation anomalies, such as the Antarctic Oscillation and Indian Ocean Dipole. The phenomena of SAMW and UCDW salinity anomalies illustrated the contemporaneous changes of the subtropical and polar oceans, which reflected the meridional circulation fluctuation. Salinity changes in upper southern ocean (< 2000m) revealed the influence of global water cycle changes, from the Antarctic to the tropical ocean, by delivering anomalies from high- and middle-latitudes to low-latitudes oceans.</p>

scholarly journals Eddy-Resolving Model Estimate of the Cabbeling Effect on the Water Mass Transformation in the Southern Ocean

2012 ◽  
Vol 42 (8) ◽  
pp. 1288-1302 ◽  
Author(s):  
L. Shogo Urakawa ◽  
Hiroyasu Hasumi
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
Water Mass ◽  
Formation Rate ◽  
Ocean Model ◽  
Intermediate Water ◽  
Mode Water ◽  
Model Estimate ◽  
Circumpolar Deep Water ◽  
Water Mass Transformation

Abstract Cabbeling effect on the water mass transformation in the Southern Ocean is investigated with the use of an eddy-resolving Southern Ocean model. A significant amount of water is densified by cabbeling: water mass transformation rates are about 4 Sv (1 Sv ≡ 106 m3 s−1) for transformation from surface/thermocline water to Subantarctic Mode Water (SAMW), about 7 Sv for transformation from SAMW to Antarctic Intermediate Water (AAIW), and about 5 Sv for transformation from AAIW to Upper Circumpolar Deep Water. These diapycnal volume transports occur around the Antarctic Circumpolar Current (ACC), where mesoscale eddies are active. The water mass transformation by cabbeling in this study is also characterized by a large amount of densification of Lower Circumpolar Deep Water (LCDW) into Antarctic Bottom Water (AABW) (about 9 Sv). Large diapycnal velocity is found not only along the ACC but also along the coast of Antarctica at the boundary between LCDW and AABW. It is found that about 3 Sv of LCDW is densified into AABW by cabbeling on the continental slopes of Antarctica in this study. This densification is not small compared with observational and numerical estimates on the AABW formation rate, which ranges from 10 to 20 Sv.

scholarly journals Carbonate system in the water masses of the Southeast Atlantic sector of the Southern Ocean during February and March 2008

2011 ◽  
Vol 8 (5) ◽  
pp. 1401-1413 ◽  
Author(s):  
M. González-Dávila ◽  
J. M. Santana-Casiano ◽  
R. A. Fine ◽  
J. Happell ◽  
B. Delille ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
Frontal Zone ◽  
Water Masses ◽  
Total Alkalinity ◽  
Carbonate System ◽  
Aragonite Saturation ◽  
Atlantic Sector ◽  
Circumpolar Deep Water ◽  
The Antarctic

Abstract. Carbonate system variables were measured in the South Atlantic sector of the Southern Ocean along a transect from South Africa to the southern limit of the Antarctic Circumpolar Current (ACC) from February to March 2008. Eddies detached from the retroflection of the Agulhas Current increased the gradients observed along the fronts. Minima in the fugacity of CO2, fCO2, and maxima in pH on either side of the frontal zone were observed, noting that within the frontal zone fCO2 reached maximum values and pH was at a minimum. Vertical distributions of water masses were described by their carbonate system properties and their relationship to CFC concentrations. Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) offered pHT,25 values of 7.56 and 7.61, respectively. The UCDW also had higher concentrations of CFC-12 (>0.2 pmol kg−1) as compared to deeper waters, revealing that UCDW was mixed with recently ventilated waters. Calcite and aragonite saturation states (Ω) were also affected by the presence of these two water masses with high carbonate concentrations. The aragonite saturation horizon was observed at 1000 m in the subtropical area and north of the Subantarctic Front. At the position of the Polar Front, and under the influence of UCDW and LCDW, the aragonite saturation horizon deepened from 800 m to 1500 m at 50.37° S, and reached 700 m south of 57.5° S. High latitudes proved to be the most sensitive areas to predicted anthropogenic carbon increase. Buffer coefficients related to changes in [CO2], [H+] and Ω with changes in dissolved inorganic carbon (CT) and total alkalinity (AT) offered minima values in the Antarctic Intermediate Water and UCDW layers. These coefficients suggest that a small increase in CT will sharply decrease the status of pH and carbonate saturation. Here we present data that suggest that south of 55° S, surface water will be under-saturated with respect to aragonite within the next few decades.

scholarly journals Carbonate system buffering in the water masses of the Southwest Atlantic sector of the Southern Ocean during February–March 2008

2011 ◽  
Vol 8 (1) ◽  
pp. 435-462
Author(s):  
M. González-Dávila ◽  
J. M. Santana-Casiano ◽  
R. A. Fine ◽  
J. Happell ◽  
B. Delille ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
Frontal Zone ◽  
Water Masses ◽  
Intermediate Water ◽  
Carbonate System ◽  
Atlantic Sector ◽  
Circumpolar Deep Water ◽  
The Antarctic

Abstract. Carbonate system variables were measured in the South Atlantic sector of the Southern Ocean along a transect from South Africa to the southern limit of the Antarctic Circumpolar Current (ACC) in February–March 2008. Eddies detach from retroflection of the Agulhas Current located north of the Subantarctic Front (SAF). The eddies increase the gradients observed at the fronts so that minima in fCO2 and maxima in pH in situ on either side of the frontal zone are observed, while within the frontal zone fCO2 reached maximum values and pH in situ was a minimum. Mixing at the frontal zones, in particular where cyclonic rings were located, brought up CO2-rich water (low pH and high nutrient) that spread out the fronts where recent biological production favored by the nutrient input increases the pH in situ and decreases the fCO2 levels. Vertical distributions of water masses were described by their carbonate system properties and their relationship to CFC concentrations. Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) had pHT,25 values of 7.56 and 7.61, respectively. UCDW also had higher concentrations of CFC-12 (>0.2 pmol kg−1) as compared to deeper waters, revealing the mixing with recently ventilated waters. Calcite and aragonite saturation states (Ω) were also affected by the presence of these two water masses with high carbonate concentration. Ωarag = 1 was observed at 1000 m in the subtropical area and north of the SAF. At the position of the Polar front and under the influence of UCDW and LCDW Ωarag = 1 deepen from 600 m to 1500 m at 50.37° S, and it reaches to 700 m south of 57.5° S. High latitudes are the most sensitive areas under future anthropogenic carbon increase. Buffer coefficients related to changes in [CO2], [H+] and Ω with changes in CT and AT showed the minimum values are found in the Antarctic Intermediate Water (AAIW), and UCDW layers. These coefficients suggest that a small increase in CT will sharply decrease the pH and the carbonate saturation states. Here we present data that are used to suggest that south of 55° S by the year 2045 surface water will be undersaturated in aragonite.

scholarly journals Silicon pool dynamics and biogenic silica export in the Southern Ocean inferred from Si-isotopes

Ocean Science ◽  
2011 ◽  
Vol 7 (5) ◽  
pp. 533-547 ◽  
Author(s):  
F. Fripiat ◽  
A.-J. Cavagna ◽  
F. Dehairs ◽  
S. Speich ◽  
L. André ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Silicic Acid ◽  
Mixed Layer ◽  
Deep Water ◽  
Biogenic Silica ◽  
Polar Front ◽  
Intermediate Water ◽  
Subtropical Zone ◽  
Isotopic Signatures ◽  
The Antarctic

Abstract. Silicon isotopic signatures (δ30Si) of water column silicic acid (Si(OH)4) were measured in the Southern Ocean, along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). This provides the first reported data of a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional water transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determine different mixing interfaces: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products and with the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration northward, across the different interfaces, without significantly changing the AASW δ30Si composition. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, the δ30Si composition of the silicic acid pools is getting slightly, but significantly lighter from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This isotopic trend is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Through the use of δ30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is estimated to be 5.2 ± 1.3 and 1.1 ± 0.3 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively. This is in good agreement with previous estimations. Furthermore, summertime Si-supply into the mixed layer of both zones, via vertical mixing, is estimated to be 1.6 ± 0.4 and 0.1 ± 0.5 mol Si m−2, respectively.

scholarly journals The Role of Southern Ocean Surface Forcings and Mixing in the Global Conveyor

2008 ◽  
Vol 38 (7) ◽  
pp. 1377-1400 ◽  
Author(s):  
Daniele Iudicone ◽  
Gurvan Madec ◽  
Bruno Blanke ◽  
Sabrina Speich
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Deep Water ◽  
Thermohaline Circulation ◽  
North Atlantic Deep Water ◽  
Surface Fluxes ◽  
Weddell Sea ◽  
Mode Water ◽  
Circumpolar Deep Water ◽  
Subantarctic Mode Water

Abstract Despite the renewed interest in the Southern Ocean, there are yet many unknowns because of the scarcity of measurements and the complexity of the thermohaline circulation. Hence the authors present here the analysis of the thermohaline circulation of the Southern Ocean of a steady-state simulation of a coupled ice–ocean model. The study aims to clarify the roles of surface fluxes and internal mixing, with focus on the mechanisms of the upper branch of the overturning. A quantitative dynamical analysis of the water-mass transformation has been performed using a new method. Surface fluxes, including the effect of the penetrative solar radiation, produce almost 40 Sv (1 Sv ≡ 106 m3 s−1) of Subantarctic Mode Water while about 5 Sv of the densest water masses (γ > 28.2) are formed by brine rejection on the shelves of Antarctica and in the Weddell Sea. Mixing transforms one-half of the Subantarctic Mode Water into intermediate water and Upper Circumpolar Deep Water while bottom water is produced by Lower Circumpolar Deep Water and North Atlantic Deep Water mixing with shelf water. The upwelling of part of the North Atlantic Deep Water inflow is due to internal processes, mainly downward propagation of the surface freshwater excess via vertical mixing at the base of the mixed layer. A complementary Lagrangian analysis of the thermohaline circulation will be presented in a companion paper.

scholarly journals Ocean processes at the Antarctic continental slope

2014 ◽  
Vol 372 (2019) ◽  
pp. 20130047 ◽  
Author(s):  
Karen J. Heywood ◽  
Sunke Schmidtko ◽  
Céline Heuzé ◽  
Jan Kaiser ◽  
Timothy D. Jickells ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Continental Shelf ◽  
Continental Slope ◽  
Climate Models ◽  
Water Mass ◽  
Ocean Model ◽  
Shelf Break ◽  
Small Scale ◽  
Exchange Processes ◽  
The Antarctic

The Antarctic continental shelves and slopes occupy relatively small areas, but, nevertheless, are important for global climate, biogeochemical cycling and ecosystem functioning. Processes of water mass transformation through sea ice formation/melting and ocean–atmosphere interaction are key to the formation of deep and bottom waters as well as determining the heat flux beneath ice shelves. Climate models, however, struggle to capture these physical processes and are unable to reproduce water mass properties of the region. Dynamics at the continental slope are key for correctly modelling climate, yet their small spatial scale presents challenges both for ocean modelling and for observational studies. Cross-slope exchange processes are also vital for the flux of nutrients such as iron from the continental shelf into the mixed layer of the Southern Ocean. An iron-cycling model embedded in an eddy-permitting ocean model reveals the importance of sedimentary iron in fertilizing parts of the Southern Ocean. Ocean gliders play a key role in improving our ability to observe and understand these small-scale processes at the continental shelf break. The Gliders: Excellent New Tools for Observing the Ocean (GENTOO) project deployed three Seagliders for up to two months in early 2012 to sample the water to the east of the Antarctic Peninsula in unprecedented temporal and spatial detail. The glider data resolve small-scale exchange processes across the shelf-break front (the Antarctic Slope Front) and the front's biogeochemical signature. GENTOO demonstrated the capability of ocean gliders to play a key role in a future multi-disciplinary Southern Ocean observing system.

scholarly journals Silicon pool dynamics and biogenic silica export in the Southern Ocean, inferred from Si-isotopes

2011 ◽  
Vol 8 (2) ◽  
pp. 639-674 ◽  
Author(s):  
F. Fripiat ◽  
A.-J. Cavagna ◽  
F. Dehairs ◽  
S. Speich ◽  
L. André ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Silicic Acid ◽  
Mixed Layer ◽  
Deep Water ◽  
Biogenic Silica ◽  
Water Masses ◽  
Intermediate Water ◽  
Subtropical Zone ◽  
Isotopic Signatures ◽  
The Antarctic

Abstract. Water column silicon isotopic signatures (δ30Si) of silicic acid (Si(OH)4) in the Southern Ocean were measured along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). These data are the first reported for a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determined different mixing interfaces between ACC water masses: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and the thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products. With the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration across the different interfaces northward without significantly changing the AASW δ30Si. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, there is a slight but significant Si-isotopic lightening of the silicic acid pools from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This eastward lightening is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Using the Si-isotopic constraint, we estimate for the Greenwich Meridian a net biogenic silica production which should be representative of the annual export, at 4.5 ± 1.1 and 1.5 ± 0.4 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively, in agreement with previous estimations. The summertime Si-supply into the mixed layer via vertical mixing was also assessed at 1.5 ± 0.4 and 0.1 ± 0.5 mol Si m−2, respectively.

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