Journal articles on the topic 'Antarctic Intermediate Water (AAIW)'
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1
Schmidtko, Sunke, and Gregory C. Johnson. "Multidecadal Warming and Shoaling of Antarctic Intermediate Water*." Journal of Climate 25, no. 1 (January 1, 2012): 207–21. http://dx.doi.org/10.1175/jcli-d-11-00021.1.
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Abstract Antarctic Intermediate Water (AAIW) is a dominant Southern Hemisphere water mass that spreads from its formation regions just north of the Antarctic Circumpolar Current (ACC) to at least 20°S in all oceans. This study uses an isopycnal climatology constructed from Argo conductivity–temperature–depth (CTD) profile data to define the current state of the AAIW salinity minimum (its core) and thence compute anomalies of AAIW core pressure, potential temperature, salinity, and potential density since the mid-1970s from ship-based CTD profiles. The results are used to calculate maps of temporal property trends at the AAIW core, where statistically significant strong circumpolar shoaling (30–50 dbar decade−1), warming (0.05°–0.15°C decade−1), and density reductions [up to −0.03 (kg m−3) decade−1] are found. These trends are strongest just north of the ACC in the southeast Pacific and Atlantic Oceans and decrease equatorward. Salinity trends are generally small, with their sign varying regionally. Bottle data are used to extend the AAIW core potential temperature anomaly analysis back to 1925 in the Atlantic and to ~1960 elsewhere. The modern warm AAIW core conditions appear largely unprecedented in the historical record: biennially and zonally binned median AAIW core potential temperatures within each ocean basin are, with the notable exception of the subtropical South Atlantic in the 1950s–70s, 0.2–1°C colder than modern values. Zonally averaged sea surface temperature anomalies around the AAIW formation latitudes in each ocean and sectoral southern annular mode indices are used to put the AAIW core property trends and variations into context.
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Urakawa, L. Shogo, and Hiroyasu Hasumi. "Eddy-Resolving Model Estimate of the Cabbeling Effect on the Water Mass Transformation in the Southern Ocean." Journal of Physical Oceanography 42, no. 8 (August 1, 2012): 1288–302. http://dx.doi.org/10.1175/jpo-d-11-0173.1.
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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.
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Romahn, S., A. Mackensen, J. Groeneveld, and J. Pätzold. "Deglacial intermediate water reorganization: new evidence from the Indian Ocean." Climate of the Past Discussions 9, no. 4 (July 17, 2013): 4035–63. http://dx.doi.org/10.5194/cpd-9-4035-2013.
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Abstract. The importance of intermediate water masses in climate change and ocean circulation has been emphasized recently. In particular, Antarctic Intermediate Water (AAIW) is thought to have acted as an active interhemispheric transmitter of climate anomalies. Here we reconstruct changes in AAIW signature and spatial and temporal evolution based on a 40 kyr time series of oxygen and carbon isotopes as well as planktic Mg/Ca based thermometry from a site in the western Indian Ocean. Our data suggest that AAIW transmitted Antarctic temperature trends to the equatorial Indian Ocean via the "oceanic tunnel" mechanism. Moreover, our results reveal that deglacial AAIW carried a signature of aged Southern Ocean deep water. We find no evidence of increased formation of intermediate waters during the deglaciation.
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Machín, F., and J. L. Pelegrí. "Northward Penetration of Antarctic Intermediate Water off Northwest Africa." Journal of Physical Oceanography 39, no. 3 (March 1, 2009): 512–35. http://dx.doi.org/10.1175/2008jpo3825.1.
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Abstract In this article, historical and climatological datasets are used to investigate the seasonal northward propagation of Antarctic Intermediate Waters (AAIW) along the eastern margin of the North Atlantic subtropical gyre. A cluster analysis for data north of 26°N shows the presence of a substantial number of hydrographic stations with AAIW characteristics that stretch northeast along the African slope. This water mass extends north during fall, as shown both through the comparison of actual and climatological data, and by applying a mixing analysis to normal-to-shore seasonal sections at both 28.5° and 32°N. The mixing analysis is further used with several fall cruises between 32° and 36°N, and shows that at these latitudes the core of AAIW propagates along the 27.5 isoneutral with contributions that reach as much as 50% at 32.5°N. An idealized Sverdrup-type model is used in combination with climatological hydrographic and wind data to examine what forces this eastern boundary propagation. It is found that column stretching, initiated in the tropical North Atlantic, is the dominant term in the vorticity balance of the AAIW stratum, capable of sustaining a winter–spring–summer northward transport of about 3–4 Sv (1 Sv ≡ 106 m3 s−1) that reaches as far north as the Canary Archipelago (28°N). In fall, this transport may continue beyond 28°N, sustained by a near-slope meridional stretching of this water stratum. AAIW probably fades away in the northeastern region as the result of several processes, specially enhanced double diffusion with surrounding waters and interaction with Mediterranean water lenses.
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Graham, Jennifer A., David P. Stevens, and Karen J. Heywood. "Nonlinear Climate Responses to Changes in Antarctic Intermediate Water." Journal of Climate 26, no. 22 (October 29, 2013): 9175–93. http://dx.doi.org/10.1175/jcli-d-12-00767.1.
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Abstract The global impact of changes in Antarctic Intermediate Water (AAIW) properties is demonstrated using idealized perturbation experiments in a coupled climate model. Properties of AAIW were altered between 10° and 20°S in the Atlantic, Pacific, and Indian Oceans separately. Potential temperature was changed by ±1°C, along with density-compensating changes in salinity. For each of the experiments, sea surface temperature responds to changes in AAIW when anomalies surface at higher latitudes (>30°). Anomalous sea-to-air heat fluxes leave density anomalies in the ocean, resulting in nonlinear responses to opposite-sign perturbations. In the Southern Ocean, these affect the meridional density gradient, leading to changes in Antarctic Circumpolar Current transport. The response to cooler, fresher AAIW is both greater in magnitude and significant over a larger area than that for warmer, saltier AAIW. The North Atlantic is particularly sensitive to cool, fresh perturbations, with density anomalies causing reductions in the meridional overturning circulation of up to 1 Sv (1 Sv ≡ 106 m3 s−1). Resultant changes in meridional ocean heat transport, along with surfacing anomalies, cause basinwide changes in the surface ocean and overlying atmosphere on multidecadal time scales.
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Close, Sally E., Alberto C. Naveira Garabato, Elaine L. McDonagh, Brian A. King, Martin Biuw, and Lars Boehme. "Control of Mode and Intermediate Water Mass Properties in Drake Passage by the Amundsen Sea Low." Journal of Climate 26, no. 14 (July 12, 2013): 5102–23. http://dx.doi.org/10.1175/jcli-d-12-00346.1.
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Abstract The evolution of the physical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage region is examined on time scales down to intraseasonal, within the 1969–2009 period. Both SAMW and AAIW experience substantial interannual to interdecadal variability, significantly linked to the action of the Amundsen Sea low (ASL) in their formation areas. Observations suggest that the interdecadal freshening tendency evident in SAMW over the past three decades has recently abated, while AAIW has warmed significantly since the early 2000s. The two water masses have also experienced a substantial lightening since the start of the record. Examination of the mechanisms underpinning water mass property variability shows that SAMW characteristics are controlled predominantly by a combination of air–sea turbulent heat fluxes, cross-frontal Ekman transport of Antarctic surface waters, and the evaporation–precipitation balance in the Subantarctic zone of the southeast Pacific and Drake Passage, while AAIW properties reflect air–sea turbulent heat fluxes and sea ice formation in the Bellingshausen Sea. The recent interdecadal evolution of the ASL is consistent with both the dominance of the processes described here and the response of SAMW and AAIW on that time scale.
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Sloyan, Bernadette M., and Igor V. Kamenkovich. "Simulation of Subantarctic Mode and Antarctic Intermediate Waters in Climate Models." Journal of Climate 20, no. 20 (October 15, 2007): 5061–80. http://dx.doi.org/10.1175/jcli4295.1.
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Abstract The Southern Ocean’s Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are two globally significant upper-ocean water masses that circulate in all Southern Hemisphere subtropical gyres and cross the equator to enter the North Pacific and North Atlantic Oceans. Simulations of SAMW and AAIW for the twentieth century in eight climate models [GFDL-CM2.1, CCSM3, CNRM-CM3, MIROC3.2(medres), MIROC3.2(hires), MRI-CGCM2.3.2, CSIRO-Mk3.0, and UKMO-HadCM3] that provided their output in support of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) have been compared to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atlas of Regional Seas. The climate models, except for UKMO-HadCM3, CSIRO-Mk3.0, and MRI-CGCM2.3.2, provide a reasonable simulation of SAMW and AAIW isopycnal temperature and salinity in the Southern Ocean. Many models simulate the potential vorticity minimum layer and salinity minimum layer of SAMW and AAIW, respectively. However, the simulated SAMW layer is generally thinner and at lighter densities than observed. All climate models display a limited equatorward extension of SAMW and AAIW north of the Antarctic Circumpolar Current. Errors in the simulation of SAMW and AAIW property characteristics are likely to be due to a combination of many errors in the climate models, including simulation of wind and buoyancy forcing, inadequate representation of subgrid-scale mixing processes in the Southern Ocean, and midlatitude diapycnal mixing parameterizations.
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Naveira Garabato, Alberto C., Loïc Jullion, David P. Stevens, Karen J. Heywood, and Brian A. King. "Variability of Subantarctic Mode Water and Antarctic Intermediate Water in the Drake Passage during the Late-Twentieth and Early-Twenty-First Centuries." Journal of Climate 22, no. 13 (July 1, 2009): 3661–88. http://dx.doi.org/10.1175/2009jcli2621.1.
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Abstract A time series of the physical and biogeochemical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage. Both water masses have experienced substantial variability on interannual to interdecadal time scales. SAMW is formed by winter overturning on the equatorward flank of the Antarctic Circumpolar Current (ACC) in and to the west of the Drake Passage. Its interannual variability is primarily driven by variations in wintertime air–sea turbulent heat fluxes and net evaporation modulated by the El Niño–Southern Oscillation (ENSO). Despite their spatial proximity, the AAIW in the Drake Passage has a very different source than that of the SAMW because it is ventilated by the northward subduction of Winter Water originating in the Bellingshausen Sea. Changes in AAIW are mainly forced by variability in Winter Water properties resulting from fluctuations in wintertime air–sea turbulent heat fluxes and spring sea ice melting, both of which are linked to predominantly ENSO-driven variations in the intensity of meridional winds to the west of the Antarctic Peninsula. A prominent exception to the prevalent modes of SAMW and AAIW formation occurred in 1998, when strong wind forcing associated with constructive interference between ENSO and the southern annular mode (SAM) triggered a transitory shift to an Ekman-dominated mode of SAMW ventilation and a 1–2-yr shutdown of AAIW production. The interdecadal evolutions of SAMW and AAIW in the Drake Passage are distinct and driven by different processes. SAMW warmed (by ∼0.3°C) and salinified (by ∼0.04) during the 1970s and experienced the reverse trends between 1990 and 2005, when the coldest and freshest SAMW on record was observed. In contrast, AAIW underwent a net freshening (by ∼0.05) between the 1970s and the twenty-first century. Although the reversing changes in SAMW were chiefly forced by a ∼30-yr oscillation in regional air–sea turbulent heat fluxes and precipitation associated with the interdecadal Pacific oscillation, with a SAM-driven intensification of the Ekman supply of Antarctic surface waters from the south contributing significantly too, the freshening of AAIW was linked to the extreme climate change that occurred to the west of the Antarctic Peninsula in recent decades. There, a freshening of the Winter Water ventilating AAIW was brought about by increased precipitation and a retreat of the winter sea ice edge, which were seemingly forced by an interdecadal trend in the SAM and regional positive feedbacks in the air–sea ice coupled climate system. All in all, these findings highlight the role of the major modes of Southern Hemisphere climate variability in driving the evolution of SAMW and AAIW in the Drake Passage region and the wider South Atlantic and suggest that these modes may have contributed significantly to the hemispheric-scale changes undergone by those waters in recent decades.
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Roque, David, Ivan Parras-Berrocal, Miguel Bruno, Ricardo Sánchez-Leal, and Francisco Javier Hernández-Molina. "Seasonal variability of intermediate water masses in the Gulf of Cádiz: implications of the Antarctic and subarctic seesaw." Ocean Science 15, no. 5 (October 22, 2019): 1381–97. http://dx.doi.org/10.5194/os-15-1381-2019.
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Abstract. Global circulation of intermediate water masses has been extensively studied; however, its regional and local circulation along continental margins and variability and implications on sea floor morphologies are still not well known. In this study the intermediate water mass variability in the Gulf of Cádiz (GoC) and adjacent areas has been analysed and its implications discussed. Remarkable seasonal variations of the Antarctic Intermediate Water (AAIW) and the Subarctic Intermediate Water (SAIW) are determined. During autumn a greater presence of the AAIW seems to be related to a reduction in the presence of SAIW and Eastern North Atlantic Central Water (ENACW). This interaction also affects the Mediterranean Water (MW), which is pushed by the AAIW toward the upper continental slope. In the rest of the seasons, the SAIW is the predominant water mass reducing the presence of the AAIW. This seasonal variability for the predominance of these intermediate water masses is explained in terms of the concatenation of several wind-driven processes acting during the different seasons. Our finding is important for a better understanding of regional intermediate water mass variability with implications in the Atlantic Meridional Overturning Circulation (AMOC), but further research is needed in order to decode their changes during the geological past and their role, especially related to the AAIW, in controlling both the morphology and the sedimentation along the continental slopes.
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Downes, Stephanie M., Anand Gnanadesikan, Stephen M. Griffies, and Jorge L. Sarmiento. "Water Mass Exchange in the Southern Ocean in Coupled Climate Models." Journal of Physical Oceanography 41, no. 9 (September 1, 2011): 1756–71. http://dx.doi.org/10.1175/2011jpo4586.1.
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Abstract The authors estimate water mass transformation rates resulting from surface buoyancy fluxes and interior diapycnal fluxes in the region south of 30°S in the Estimating the Circulation and Climate of the Ocean (ECCO) model-based state estimation and three free-running coupled climate models. The meridional transport of deep and intermediate waters across 30°S agrees well between models and observationally based estimates in the Atlantic Ocean but not in the Indian and Pacific, where the model-based estimates are much smaller. Associated with this, in the models about half the southward-flowing deep water is converted into lighter waters and half is converted to denser bottom waters, whereas the observationally based estimates convert most of the inflowing deep water to bottom waters. In the models, both Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW) are formed primarily via an interior diapycnal transformation rather than being transformed at the surface via heat or freshwater fluxes. Given the small vertical diffusivity specified in the models in this region, the authors conclude that other processes such as cabbeling and thermobaricity must be playing an important role in water mass transformation. Finally, in the models, the largest contribution of the surface buoyancy fluxes in the Southern Ocean is to convert Upper Circumpolar Deep Water (UCDW) and AAIW into lighter Subantarctic Mode Water (SAMW).
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Hamilton, LJ. "Temperature inversions at intermediate depths in the Antarctic Intermediate waters of the South-western Pacific." Marine and Freshwater Research 41, no. 3 (1990): 325. http://dx.doi.org/10.1071/mf9900325.
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Deep (about 1000 m) marked temperature inversions and/or salinity reversals found in conductivity- temperature-depth profiles in the south-western Pacific for 1985 to 1987 are shown to arise from confluences of different branches of the Antarctic Intermediate Water (AAIW). Salinity reversals lead to the presence of several intermediate-depth salinity minima instead of the simple broad minimum in the vertical usually described as characterizing the presence of AAIW in this area. The anomalies are found at particular locations, often near ridges and rises. Significantly differing thermohaline properties acquired by the branches from mixing over separate travel paths apparently allows the formation of temperature inversions by isentropic penetrations. Some perturbations are dynamically caused in that at least one branch of AAIW is transported in association with strong surface currents with deep influence. Other confluences are caused by topographic control on AAIW flows moving independently of surface currents. Perturbations south of the Subtropical Convergence are related to the initial formation of the AAIW, but these are not of major interest in this analysis, being well known and simply explained. Locations of perturbations correspond in some areas to flow patterns of intermediate waters inferred by researchers using historical data, and they indicate other areas where the flow patterns need more investigation. Some comments are made on East Australian Current outflows from the Tasman and Coral Seas that have considerable influence on the flow of branches of the AAIW. Other remarks are made concerning the peculiarities of the temperature-salinity regime of the Tasman Front, which inhibits the formation of temperature inversions at depth. In general, the medium- and fine-scale structure in the central Tasman is that of the stepped type, with intrusive type in other areas.
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Iudicone, Daniele, Keith B. Rodgers, Richard Schopp, and Gurvan Madec. "An Exchange Window for the Injection of Antarctic Intermediate Water into the South Pacific." Journal of Physical Oceanography 37, no. 1 (January 1, 2007): 31–49. http://dx.doi.org/10.1175/jpo2985.1.
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Abstract Antarctic Intermediate Water (AAIW) occupies the intermediate horizon of most of the world oceans. Formed in the Southern Ocean, it is characterized by a relative salinity minimum. With a new, denser in situ National Oceanographic Data Center dataset, the authors have reanalyzed the export characteristics of AAIW from the Southern Ocean into the South Pacific Ocean. These new data show that part of the AAIW is exported from the subpolar frontal region by the large-scale circulation through an exchange window of 10° width situated east of 90°W in the southeast corner of the Pacific basin. This suggests the origin of this water to be in the Antarctic Circumpolar Current. A set of numerical modeling experiments has been used to reproduce these observed features and to demonstrate that the dynamics of the exchange window is controlled by the basin-scale meridional pressure gradient. The exchange of AAIW between the Southern and Pacific Oceans must therefore be understood in the context of the large basin-scale dynamical balance rather than simply local effects.
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Tomczak, M. "Variability of Antarctic intermediate Water properties in the South Pacific Ocean." Ocean Science 3, no. 3 (August 3, 2007): 363–77. http://dx.doi.org/10.5194/os-3-363-2007.
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Abstract. Argo float time series data are used to study the salinity field at the depth of the salinity minimum produced by Antarctic Intermediate Water (AAIW). It is found that far from showing the smooth erosion of the minimum that would result from diffusive flow, the salinity field is characterized by features of geostrophic turbulence such as fronts, eddies and intrusions. Comparison of the Argo float observations with the climatology of the World Ocean Atlas (WOA) reveals significant differences between the two data sets. Some of the differences may have their origin in problems with the WOA data density in remote regions of the South Pacific, but most are more likely produced by interannual variations of the AAIW salinity field.
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Tomczak, M. "Variability of Antarctic intermediate water properties in the South Pacific Ocean." Ocean Science Discussions 3, no. 6 (December 1, 2006): 2021–58. http://dx.doi.org/10.5194/osd-3-2021-2006.
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Abstract. Argo float time series data are used to study the salinity field at the depth of the salinity minimum produced by Antarctic Intermediate Water (AAIW). It is found that far from showing the smooth erosion of the minimum that would result from diffusive flow, the salinity field is characterized by features of geostrophic turbulence such as fronts, eddies and intrusions. Comparison of the Argo float observations with the climatology of the World Ocean Atlas (WOA) reveals significant differences between the two data sets. Some of the differences may have their origin in problems with the WOA data density in remote regions of the South Pacific, but most are more likely produced by interannual variations of the AAIW salinity field.
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Yao, Wenjun, Jiuxin Shi, and Xiaolong Zhao. "Freshening of Antarctic Intermediate Water in the South Atlantic Ocean in 2005–2014." Ocean Science 13, no. 4 (July 6, 2017): 521–30. http://dx.doi.org/10.5194/os-13-521-2017.
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Abstract. Basin-scale freshening of Antarctic Intermediate Water (AAIW) is reported to have occurred in the South Atlantic Ocean during the period from 2005 to 2014, as shown by the gridded monthly means of the Array for Real-time Geostrophic Oceanography (Argo) data. This phenomenon was also revealed by two repeated transects along a section at 30° S, performed during the World Ocean Circulation Experiment Hydrographic Program. Freshening of the AAIW was compensated for by a salinity increase of thermocline water, indicating a hydrological cycle intensification. This was supported by the precipitation-minus-evaporation change in the Southern Hemisphere from 2000 to 2014. Freshwater input from atmosphere to ocean surface increased in the subpolar high-precipitation region and vice versa in the subtropical high-evaporation region. Against the background of hydrological cycle changes, a decrease in the transport of Agulhas Leakage (AL), which was revealed by the simulated velocity field, was proposed to be a contributor to the associated freshening of AAIW. Further calculation showed that such a decrease could account for approximately 53 % of the observed freshening (mean salinity reduction of about 0.012 over the AAIW layer). The estimated variability of AL was inferred from a weakening of wind stress over the South Indian Ocean since the beginning of the 2000s, which would facilitate freshwater input from the source region. The mechanical analysis of wind data here was qualitative, but it is contended that this study would be helpful to validate and test predictably coupled sea–air model simulations.
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Pérez-Hernández, M. Dolores, Alonso Hernández-Guerra, Isis Comas-Rodríguez, Verónica M. Benítez-Barrios, Eugenio Fraile-Nuez, Josep L. Pelegrí, and Alberto C. Naveira Garabato. "Differences between 1999 and 2010 across the Falkland Plateau: fronts and water masses." Ocean Science 13, no. 4 (July 7, 2017): 577–87. http://dx.doi.org/10.5194/os-13-577-2017.
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Abstract. Decadal differences in the Falkland Plateau are studied from the two full-depth hydrographic data collected during the ALBATROSS (April 1999) and MOC-Austral (February 2010) cruises. Differences in the upper 100 dbar are due to changes in the seasonal thermocline, as the ALBATROSS cruise took place in the austral fall and the MOC-Austral cruise in summer. The intermediate water masses seem to be very sensitive to the wind conditions existing in their formation area, showing cooling and freshening for the decade as a consequence of a higher Antarctic Intermediate Water (AAIW) contribution and of a decrease in the Subantarctic Mode Water (SAMW) stratum. The deeper layers do not exhibit any significant change in the water mass properties. The Subantarctic Front (SAF) in 1999 is observed at 52.2–54.8° W with a relative mass transport of 32.6 Sv. In contrast, the SAF gets wider in 2010, stretching from 51.1 to 57.2° W (the Falkland Islands), and weakening to 17.9 Sv. Changes in the SAF can be linked with the westerly winds and mainly affect the northward flow of Subantarctic Surface Water (SASW), SAMW and AAIW/Antarctic Surface Water (AASW). The Polar Front (PF) carries 24.9 Sv in 1999 (49.8–44.4° W), while in 2010 (49.9–49.2° W) it narrows and strengthens to 37.3 Sv.
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Servettaz, Aymeric PM, Yusuke Yokoyama, Shoko Hirabayashi, Markus Kienast, Yosuke Miyairi, and Mahyar Mohtadi. "Dissolved inorganic Radiocarbon content of the Western Coral sea: Implications for Intermediate and Deep Water Circulation." Radiocarbon 61, no. 6 (November 21, 2019): 1685–96. http://dx.doi.org/10.1017/rdc.2019.122.
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ABSTRACTThe South Pacific Ocean contributes to the global carbon cycle by exchanging CO2 between the atmosphere and intermediate to deep water masses. The path of the Antarctic Intermediate Water (AAIW) in the South Pacific gyre has been inferred from salinity, oxygen, and nutrient measurements, but radiocarbon (14C) measurements—a direct tracer of the carbon cycle—remain sparse. Here, we present the first radiocarbon profiles in the western Coral Sea and compare our measurements with South Pacific stations from GLODAPv2, a database of ocean hydrochemistry. Surface and subsurface waters in the Coral Sea cannot be attributed to a single source based on their Δ14C signatures, and we observe a penetration of bomb-produced 14C. AAIW in the western Coral Sea shows Δ14C values comparable to those in the South Pacific gyre, consistent with circulation of AAIW in the lower part of the southern equatorial current. The deep waters of the western Coral Sea have significantly higher 14C than the South Pacific at the same isopycnal, consistent with a northward intrusion of Circumpolar Deep Water from the Tasman Sea, along with a westward influx of deep waters from the Central Pacific. In accordance with silicate concentrations published previously, this shows the dual origin of deep waters in the Coral Sea.
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Sévellec, F., A. Colin de Verdiére, and M. Ollitrault. "Evolution of Intermediate Water Masses Based on Argo Float Displacements." Journal of Physical Oceanography 47, no. 7 (July 2017): 1569–86. http://dx.doi.org/10.1175/jpo-d-16-0182.1.
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AbstractThe evolution and dispersion of intermediate water masses in the ocean interior is studied. To this purpose, an empirical statistical model of Lagrangian tracers at a constant depth level is developed. The model follows the transfer operator based on 10-day deep displacements of Argo floats at ~1000 m depth. An asymptotic analysis of the model shows the existence of 10 principal stationary points (the 10 locations attract asymptotically 97% of the tracers). It takes ~1000 years to reach this asymptotic regime relevant for estimating the stationary points. For Lagrangian floats, the concept of attractor needs to be generalized in a statistical sense (versus deterministic), except for a few places in the ocean. In this new framework, a tracer has a likelihood to reach the stationary points, rather than a certainty to reach a single stationary point. The empirical statistical model is used to describe the fate of three intermediate water masses: North Pacific Intermediate Water (NPIW), Mediterranean Water (MW), and Antarctic Intermediate Water (AAIW). These experiments show a dramatic difference in the long-time behavior of NPIW, MW, and AAIW. In the permanent regime, the NPIW concentrates locally (in the Kuroshio recirculation) and the MW remains mainly regional (concentrated in the subtropical gyre of the North Atlantic), whereas the AAIW spreads globally (well mixed throughout the entire ocean).
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Downes, Stephanie M., Clothilde Langlais, Jordan P. Brook, and Paul Spence. "Regional Impacts of the Westerly Winds on Southern Ocean Mode and Intermediate Water Subduction." Journal of Physical Oceanography 47, no. 10 (October 2017): 2521–30. http://dx.doi.org/10.1175/jpo-d-17-0106.1.
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AbstractSubduction processes in the Southern Ocean transfer oxygen, heat, and anthropogenic carbon into the ocean interior. The future response of upper-ocean subduction, in the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) classes, is dependent on the evolution of the combined surface buoyancy forcing and overlying westerly wind stress. Here, the recently observed pattern of a poleward intensification of the westerly winds is divided into its shift and increase components. SAMW and AAIW formation occurs in regional “hot spots” in deep mixed layer zones, primarily in the southeast Indian and Pacific. It is found that the mixed layer depth responds differently to wind stress perturbations across these regional formation zones. An increase only in the westerly winds in the Indian sector steepens isopycnals and increases the local circulation, driving deeper mixed layers and increased subduction. Conversely, in the same region, a poleward shift and poleward intensification of the westerly winds reduces heat loss and increases freshwater input, thus decreasing the mixed layer depth and consequently the associated SAMW and AAIW subduction. In the Pacific sector, all wind stress perturbations lead to increases in heat loss and decreases in freshwater input, resulting in a net increase in SAMW and AAIW subduction. Overall, the poleward shift in the westerly wind stress dominates the SAMW subduction changes, rather than the increase in wind stress. The net decrease in SAMW subduction across all basins would likely decrease anthropogenic carbon sequestration; however, the net AAIW subduction changes across the Southern Ocean are overall minor.
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Carracedo, L. I., P. C. Pardo, S. Flecha, and F. F. Pérez. "On the Mediterranean Water Composition." Journal of Physical Oceanography 46, no. 4 (April 2016): 1339–58. http://dx.doi.org/10.1175/jpo-d-15-0095.1.
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AbstractThe Mediterranean Outflow Water (MOW) spills from the Mediterranean Sea (east North Atlantic basin) west off the Strait of Gibraltar. As MOW outflows, it entrains eastern North Atlantic Central Waters (ENACW) and Intermediate Waters to form the neutrally buoyant Mediterranean Water (MW) that can be traced over the entire North Atlantic basin. Its high salinity content influences the thermohaline properties of the intermediate–deep water column in the North Atlantic and its dynamics. Here, the composition of MW in its source region (the Gulf of Cádiz, west off Strait of Gibraltar) is investigated on the basis of an optimum multiparameter analysis. The results obtained indicate that mixing of MOW (34.1% ± 0.3%) occurs mainly with overlying ENACW (57.1% ± 0.8%) in a process broadly known as central water entrainment. A diluted form (80% of dilution) of the Antarctic Intermediate Water (AAIW) reaches the region and also takes part in MW formation (8.3% ± 0.5%). Finally, the underlying Labrador Sea Water (LSW) also contributes (0.4% ± 0.1%) to the characteristics of MW. From these results and considering 0.74 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) as the mean outflow of MOW, the MW exportation rate was inferred (2.2 Sv), which, decomposing MW, means that the MOW outflow is accompanied by 1.24 Sv of entrained ENACW, 0.18 Sv of AAIW, and <0.01 Sv of LSW.
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Fu, Yao, Johannes Karstensen, and Peter Brandt. "Atlantic Meridional Overturning Circulation at 14.5° N in 1989 and 2013 and 24.5° N in 1992 and 2015: volume, heat, and freshwater transports." Ocean Science 14, no. 4 (July 10, 2018): 589–616. http://dx.doi.org/10.5194/os-14-589-2018.
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Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is analyzed by applying a box inverse model to hydrographic data from transatlantic sections along 14.5∘ N, occupied in 1989 and 2013, and along 24.5∘ N, occupied in 1992 and 2015. Direct comparison of water mass properties among the different realizations at the respective latitudes shows that the Antarctic Intermediate Water (AAIW) became warmer and saltier at 14.5∘ N, and the densest Antarctic Bottom Water became lighter, while the North Atlantic Deep Water freshened at both latitudes. The inverse solution shows that the intermediate layer transport at 14.5∘ N was also markedly weaker in 2013 than in 1989, indicating that the AAIW property changes at this latitude may be related to changes in the circulation. The inverse solution was validated using the RAPID and MOVE array data, and the GECCO2 ocean state estimate. Comparison among these datasets indicates that the AMOC has not significantly weakened over the past 2 decades at both latitudes. Sensitivity tests of the inverse solution suggest that the overturning structure and heat transport across the 14.5∘ N section are sensitive to the Ekman transport, while freshwater transport is sensitive to the transport-weighted salinity at the western boundary.
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Downes, Stephanie M., Nathaniel L. Bindoff, and Stephen R. Rintoul. "Changes in the Subduction of Southern Ocean Water Masses at the End of the Twenty-First Century in Eight IPCC Models." Journal of Climate 23, no. 24 (December 15, 2010): 6526–41. http://dx.doi.org/10.1175/2010jcli3620.1.
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Abstract A multimodel comparison method is used to assess the sensitivity of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) formation to climate change. For the Intergovernmental Panel on Climate Change A2 emissions scenario (where atmospheric CO2 is 860 ppm at 2100), the models show cooling and freshening on density surfaces less than about 27.4 kg m−3, a pattern that has been observed in the late twentieth century. SAMW (defined by the low potential vorticity layer) and AAIW (defined by the salinity minimum layer) warm and freshen as they shift to lighter density classes. Heat and freshwater fluxes at the ocean surface dominate the projected buoyancy gain at outcrop regions of SAMW and AAIW, whereas the net increase in the Ekman flux of heat and freshwater contributes to a lesser extent. This buoyancy gain, combined with shoaling of the winter mixed layer, reduces the volume of SAMW subducted into the ocean interior by a mean of 8 Sv (12%), and the subduction of AAIW decreases by a mean of 14 Sv (23%; 1 Sv ≡ 106 m3 s−1). Decreases in the projected subduction of the key Southern Ocean upper-water masses imply a slow down in the Southern Ocean circulation in the future, driven by surface warming and freshening. A reduction in the subduction of intermediate waters implies a likely future decrease in the capacity of the Southern Ocean to sequester CO2.
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Jullion, Loic, Karen J. Heywood, Alberto C. Naveira Garabato, and David P. Stevens. "Circulation and Water Mass Modification in the Brazil–Malvinas Confluence." Journal of Physical Oceanography 40, no. 5 (May 1, 2010): 845–64. http://dx.doi.org/10.1175/2009jpo4174.1.
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Abstract The confluence between the Brazil Current and the Malvinas Current [the Brazil–Malvinas Confluence (BMC)] in the Argentine Basin is characterized by a complicated thermohaline structure favoring the exchanges of mass, heat, and salt between the subtropical gyre and the Antarctic Circumpolar Current (ACC). Analysis of thermohaline properties of hydrographic sections in the BMC reveals strong interactions between the ACC and subtropical fronts. In the Subantarctic Front, Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Circumpolar Deep Water (CDW) warm (become saltier) by 0.4° (0.08), 0.3° (0.02), and 0.6°C (0.1), respectively. In the subtropical gyre, AAIW and North Atlantic Deep Water have cooled (freshened) by 0.4° (0.07) and 0.7°C (0.11), respectively. To quantify those ACC–subtropical gyre interactions, a box inverse model surrounding the confluence is built. The model diagnoses a subduction of 16 ± 4 Sv (1 Sv ≡ 106 m3 s−1) of newly formed SAMW and AAIW under the subtropical gyre corresponding to about half of the total subduction rate of the South Atlantic found in previous studies. Cross-frontal heat (0.06 PW) and salt (2.4 × 1012 kg s−1) gains by the ACC in the BMC contribute to the meridional poleward heat and salt fluxes across the ACC. These estimates correspond to perhaps half of the total cross-ACC poleward heat flux. The authors’ results highlight the BMC as a key region in the subtropical–ACC exchanges.
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Leboucher, Viviane, James Orr, Philippe Jean-Baptiste, Maurice Arnold, Patrick Monfray, Nadine Tisnerat-Laborde, Alain Poisson, and Jean-Claude Duplessy. "Oceanic Radiocarbon Between Antarctica and South Africa Along Woce Section 16 at 30°E." Radiocarbon 41, no. 1 (1999): 51–73. http://dx.doi.org/10.1017/s0033822200019330.
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Accelerator mass spectrometry (AMS) radiocarbon measurements were made on 120 samples collected between Antarctica and South Africa along 30°E during the WOCE-France CIVA1 campaign in February 1993. Our principal objective was to complement the Southern Ocean's sparse existing data set in order to improve the 14C benchmark used for validating ocean carbon-cycle models, which disagree considerably in this region. Measured 14C is consistent with the θ-S characteristics of CIVA1. Antarctic Intermediate Water (AAIW) forming north of the Polar Front (PF) is rich in 14C, whereas surface waters south of the PF are depleted in 14C. A distinct old 14C signal was found for the contribution of the Pacific Deep Water (PDW) to the return flow of Circumpolar Deep Waters (CDW). Comparison to previous measurements shows a 14C decrease in surface waters, consistent with northward displacement of surface waters, replacement by old deep waters upwelled at the Antarctic Divergence, and atmospheric decline in 14C. Conversely, an increase was found in deeper layers, in the AAIW. Large uncertainties, associated with previous methods for separating natural and bomb 14C when in the Southern Ocean south of 45°S, motivated us to develop a new approach that relies on a simple mixing model and on chlorofluorocarbon (CFC) measurements also taken during CIVA1. This approach leads to inventories for CIVA1 that are equal to or higher than those calculated with previous methods. Differences between old and new methods are especially high south of approximately 55°S, where bomb 14C inventories are relatively modest.
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Downes, Stephanie M., Nathaniel L. Bindoff, and Stephen R. Rintoul. "Impacts of Climate Change on the Subduction of Mode and Intermediate Water Masses in the Southern Ocean." Journal of Climate 22, no. 12 (June 15, 2009): 3289–302. http://dx.doi.org/10.1175/2008jcli2653.1.
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Abstract Changes in the temperature, salinity, and subduction of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) between the 1950s and 2090s are diagnosed using the CSIRO Mark version 3.5 (Mk3.5) climate system model Caps under a CO2 forcing that reaches 860 ppm by the year 2100. These Southern Ocean upper-limb water masses ventilate the ocean interior, and changes in their properties have been related to climate change in numerous studies. Over time, the authors follow the low potential vorticity and salinity minimum layers describing SAMW and AAIW and find that the water column in the 2090s shifts to lighter densities by approximately 0.2 kg m−3. The model projects a reduction in the SAMW and AAIW annual mean subduction rates as a result of a combination of a shallower mixed layer, increased potential vorticity at the base of the mixed layer, and a net buoyancy gain. There is little change in the projected total volume of SAMW transported into the ocean interior via the subduction process; however, the authors find a significant decrease in the subduction of AAIW. The authors find overall that increases in the air–sea surface heat and freshwater fluxes mainly control the reduction in the mean loss of the SAMW and AAIW surface buoyancy flux when compared with the effect of changes supplied by Ekman transport because of increased zonal wind stress. In the A2 scenario, there are cooling and freshening on neutral density surfaces less than 27.3 kg m−3 in response to the warming and freshening observed at the ocean’s surface. The model projects deepening of density surfaces due to southward shifts in the outcrop regions and the downward displacement of these surfaces north of 45°S. The volume transport across 32°S is predicted to decrease in all three basins, with southward transport of SAMW and AAIW decreasing by up to 1.2 and 2.0 Sv (1 Sv ≡ 106 m3 s−1), respectively, in the Indian Ocean. These projected reductions in the subduction and transport of mode and intermediate water masses in the CSIRO Mk3.5 model could potentially decrease the absorption and storage of CO2 in the Southern Ocean.
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Liu, Mian, and Toste Tanhua. "Water masses in the Atlantic Ocean: characteristics and distributions." Ocean Science 17, no. 2 (March 15, 2021): 463–86. http://dx.doi.org/10.5194/os-17-463-2021.
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Abstract. A large number of water masses are presented in the Atlantic Ocean, and knowledge of their distributions and properties is important for understanding and monitoring of a range of oceanographic phenomena. The characteristics and distributions of water masses in biogeochemical space are useful for, in particular, chemical and biological oceanography to understand the origin and mixing history of water samples. Here, we define the characteristics of the major water masses in the Atlantic Ocean as source water types (SWTs) from their formation areas, and map out their distributions. The SWTs are described by six properties taken from the biased-adjusted Global Ocean Data Analysis Project version 2 (GLODAPv2) data product, including both conservative (conservative temperature and absolute salinity) and non-conservative (oxygen, silicate, phosphate and nitrate) properties. The distributions of these water masses are investigated with the use of the optimum multi-parameter (OMP) method and mapped out. The Atlantic Ocean is divided into four vertical layers by distinct neutral densities and four zonal layers to guide the identification and characterization. The water masses in the upper layer originate from wintertime subduction and are defined as central waters. Below the upper layer, the intermediate layer consists of three main water masses: Antarctic Intermediate Water (AAIW), Subarctic Intermediate Water (SAIW) and Mediterranean Water (MW). The North Atlantic Deep Water (NADW, divided into its upper and lower components) is the dominating water mass in the deep and overflow layer. The origin of both the upper and lower NADW is the Labrador Sea Water (LSW), the Iceland–Scotland Overflow Water (ISOW) and the Denmark Strait Overflow Water (DSOW). The Antarctic Bottom Water (AABW) is the only natural water mass in the bottom layer, and this water mass is redefined as Northeast Atlantic Bottom Water (NEABW) in the north of the Equator due to the change of key properties, especially silicate. Similar with NADW, two additional water masses, Circumpolar Deep Water (CDW) and Weddell Sea Bottom Water (WSBW), are defined in the Weddell Sea region in order to understand the origin of AABW.
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Fripiat, F., A. J. Cavagna, F. Dehairs, S. Speich, L. André, and D. Cardinal. "Silicon pool dynamics and biogenic silica export in the Southern Ocean, inferred from Si-isotopes." Ocean Science Discussions 8, no. 2 (March 30, 2011): 639–74. http://dx.doi.org/10.5194/osd-8-639-2011.
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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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Salt, L. A., S. M. A. C. van Heuven, M. E. Claus, E. M. Jones, and H. J. W. de Baar. "Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean." Biogeosciences 12, no. 5 (March 5, 2015): 1387–401. http://dx.doi.org/10.5194/bg-12-1387-2015.
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Abstract. Observations along the southwestern Atlantic WOCE A17 line made during the Dutch GEOTRACES-NL programme (2010–2011) were compared with historical data from 1994 to quantify the changes in the anthropogenic component of the total pool of dissolved inorganic carbon (ΔCant). Application of the extended multi-linear regression (eMLR) method shows that the ΔCant from 1994 to 2011 has largely remained confined to the upper 1000 dbar. The greatest changes occur in the upper 200 dbar in the Subantarctic Zone (SAZ), where a maximum increase of 37 μmol kg−1 is found. South Atlantic Central Water (SACW) experienced the highest rate of increase in Cant, at 0.99 ± 0.14 μmol kg−1 yr−1, resulting in a maximum rate of decrease in pH of 0.0016 yr−1. The highest rates of acidification relative to ΔCant, however, were found in Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The low buffering capacity of SAMW and AAIW combined with their relatively high rates of Cant, increase of 0.53 ± 0.11 and 0.36 ± 0.06 μmol kg−1 yr−1, respectively, has lead to rapid acidification in the SAZ, and will continue to do so whilst simultaneously reducing the chemical buffering capacity of this significant CO2 sink.
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Salt, L. A., S. M. A. C. van Heuven, M. E. Claus, E. M. Jones, and H. J. W. de Baar. "Rapid acidification of mode and intermediate waters in the southwest Atlantic Ocean." Biogeosciences Discussions 11, no. 5 (May 12, 2014): 6755–92. http://dx.doi.org/10.5194/bgd-11-6755-2014.
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Abstract. Observations along the southwest Atlantic WOCE A17 line made during the Dutch GEOTRACES-NL program (2010–2011) were compared with historical data from 1994 to quantify the changes in the anthropogenic component of the total pool of dissolved inorganic carbon (ΔCant). Application of the extended Multi Linear Regression (eMLR) method shows that the ΔCant from 1994 to 2011 has largely remained confined to the upper 1000 dbar. The greatest changes occur in the upper 200 dbar in the SubAntarctic Zone (SAZ), where a maximum increase of 37 μmol kg−1 is found. South Atlantic Central Water (SACW) experienced the highest rate of increase in Cant, at 0.99 ± 0.14 μmol kg−1 yr−1, resulting in a rate of decrease in pH of −0.0016 yr−1. The highest rates of acidification relative to ΔCant, however, were found in SubAntarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The low buffering capacity of SAMW and AAIW combined with their relatively high rates of Cant increase of 0.53 ± 0.11 μmol kg−1 yr−1 and 0.36 ± 0.06 μmol kg−1 yr−1, respectively, will lead to rapid acidification in the SAZ and simultaneously reduce the chemical buffering capacity of this significant CO2 sink.
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Fripiat, F., A. J. Cavagna, F. Dehairs, S. Speich, L. André, and D. Cardinal. "Silicon pool dynamics and biogenic silica export in the Southern Ocean inferred from Si-isotopes." Ocean Science 7, no. 5 (September 6, 2011): 533–47. http://dx.doi.org/10.5194/os-7-533-2011.
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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 &pm; 1.3 and 1.1 &pm; 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 &pm; 0.4 and 0.1 &pm; 0.5 mol Si m−2, respectively.
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Sijp, Willem P., and Matthew H. England. "Sensitivity of the Atlantic Thermohaline Circulation and Its Stability to Basin-Scale Variations in Vertical Mixing." Journal of Climate 19, no. 21 (November 1, 2006): 5467–78. http://dx.doi.org/10.1175/jcli3909.1.
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Abstract This study shows that a reduction in vertical mixing applied inside the Atlantic basin can drastically increase North Atlantic Deep Water (NADW) stability with respect to freshwater perturbations applied to the North Atlantic. This is contrary to the notion that the stability of the ocean’s thermohaline circulation simply scales with vertical mixing rates. An Antarctic Intermediate Water (AAIW) reverse cell, reliant upon upwelling of cold AAIW into the Atlantic thermocline, is found to be associated with stable states where NADW is collapsed. Transitions between NADW “on” and “off” states are characterized by interhemispheric competition between this AAIW cell and the NADW cell. In contrast to the AAIW reverse cell, NADW eventually upwells outside the Atlantic basin and is thus not as sensitive to changes in vertical mixing within the Atlantic. A reduction in vertical mixing in the Atlantic weakens the AAIW reverse cell, resulting in an enhanced stability of NADW formation. The results also suggest that the AAIW reverse cell is responsible for the stability of NADW collapsed states, and thereby plays a key role in maintaining multiple equilibria in the climate system. A global increase of vertical mixing in the model results in significantly enhanced NADW stability, as found in previous studies. However, an enhancement of vertical mixing applied only inside the Atlantic Ocean results in a reduction of NADW stability. It is concluded that the stability of NADW formation to freshwater perturbations depends critically on the basin-scale distribution of vertical mixing in the world’s oceans.
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Pardo, Paula Conde, Bronte Tilbrook, Clothilde Langlais, Thomas William Trull, and Stephen Rich Rintoul. "Carbon uptake and biogeochemical change in the Southern Ocean, south of Tasmania." Biogeosciences 14, no. 22 (November 21, 2017): 5217–37. http://dx.doi.org/10.5194/bg-14-5217-2017.
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Abstract. Biogeochemical change in the water masses of the Southern Ocean, south of Tasmania, was assessed for the 16-year period between 1995 and 2011 using data from four summer repeats of the WOCE–JGOFS–CLIVAR–GO-SHIP (Key et al., 2015; Olsen et al., 2016) SR03 hydrographic section (at ∼ 140° E). Changes in temperature, salinity, oxygen, and nutrients were used to disentangle the effect of solubility, biology, circulation and anthropogenic carbon (CANT) uptake on the variability of dissolved inorganic carbon (DIC) for eight water mass layers defined by neutral surfaces (γn). CANT was estimated using an improved back-calculation method. Warming (∼ 0.0352 ± 0.0170 °C yr−1) of Subtropical Central Water (STCW) and Antarctic Surface Water (AASW) layers decreased their gas solubility, and accordingly DIC concentrations increased less rapidly than expected from equilibration with rising atmospheric CO2 (∼ 0.86 ± 0.16 µmol kg−1 yr−1 versus ∼ 1 ± 0.12 µmol kg−1 yr−1). An increase in apparent oxygen utilisation (AOU) occurred in these layers due to either remineralisation of organic matter or intensification of upwelling. The range of estimates for the increases in CANT were 0.71 ± 0.08 to 0.93 ± 0.08 µmol kg−1 yr−1 for STCW and 0.35 ± 0.14 to 0.65 ± 0.21 µmol kg−1 yr−1 for AASW, with the lower values in each water mass obtained by assigning all the AOU change to remineralisation. DIC increases in the Sub-Antarctic Mode Water (SAMW, 1.10 ± 0.14 µmol kg−1 yr−1) and Antarctic Intermediate Water (AAIW, 0.40 ± 0.15 µmol kg−1 yr−1) layers were similar to the calculated CANT trends. For SAMW, the CANT increase tracked rising atmospheric CO2. As a consequence of the general DIC increase, decreases in total pH (pHT) and aragonite saturation (ΩAr) were found in most water masses, with the upper ocean and the SAMW layer presenting the largest trends for pHT decrease (∼ −0.0031 ± 0.0004 yr−1). DIC increases in deep and bottom layers (∼ 0.24 ± 0.04 µmol kg−1 yr−1) resulted from the advection of old deep waters to resupply increased upwelling, as corroborated by increasing silicate (∼ 0.21 ± 0.07 µmol kg−1 yr−1), which also reached the upper layers near the Antarctic Divergence (∼ 0.36 ± 0.06 µmol kg−1 yr−1) and was accompanied by an increase in salinity. The observed changes in DIC over the 16-year span caused a shoaling (∼ 340 m) of the aragonite saturation depth (ASD, ΩAr = 1) within Upper Circumpolar Deep Water that followed the upwelling path of this layer. From all our results, we conclude a scenario of increased transport of deep waters into the section and enhanced upwelling at high latitudes for the period between 1995 and 2011 linked to strong westerly winds. Although enhanced upwelling lowered the capacity of the AASW layer to uptake atmospheric CO2, it did not limit that of the newly forming SAMW and AAIW, which exhibited CANT storage rates (∼ 0.41 ± 0.20 mol m−2 yr−1) twice that of the upper layers.
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England, Matthew H., and Véronique C. Garçon. "South Atlantic circulation in a world ocean model." Annales Geophysicae 12, no. 9 (August 31, 1994): 812–25. http://dx.doi.org/10.1007/s00585-994-0812-y.
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Abstract. The circulation in the South Atlantic Ocean has been simulated within a global ocean general circulation model. Preliminary analysis of the modelled ocean circulation in the region indicates a rather close agreement of the simulated upper ocean flows with conventional notions of the large-scale geostrophic currents in the region. The modelled South Atlantic Ocean witnesses the return flow and export of North Atlantic Deep Water (NADW) at its northern boundary, the inflow of a rather barotropic Antarctic Circumpolar Current (ACC) through the Drake Passage, and the inflow of warm saline Agulhas water around the Cape of Good Hope. The Agulhas leakage amounts to 8.7 Sv, within recent estimates of the mass transport shed westward at the Agulhas retroflection. Topographic steering of the ACC dominates the structure of flow in the circumpolar ocean. The Benguela Current is seen to be fed by a mixture of saline Indian Ocean water (originating from the Agulhas Current) and fresher Subantarctic surface water (originating in the ACC). The Benguela Current is seen to modify its flow and fate with depth; near the surface it flows north-westwards bifurcating most of its transport northward into the North Atlantic Ocean (for ultimate replacement of North Atlantic surface waters lost to the NADW conveyor). Deeper in the water column, more of the Benguela Current is destined to return with the Brazil Current, though northward flows are still generated where the Benguela Current extension encounters the coast of South America. At intermediate levels, these northward currents trace the flow of Antarctic Intermediate Water (AAIW) equatorward, though even more AAIW is seen to recirculate poleward in the subtropical gyre. In spite of the model's rather coarse resolution, some subtle features of the Brazil-Malvinas Confluence are simulated rather well, including the latitude at which the two currents meet. Conceptual diagrams of the recirculation and interocean exchange of thermocline, intermediate and deep waters are constructed from an analysis of flows bound between isothermal and isobaric surfaces. This analysis shows how the return path of NADW is partitioned between a cold water route through the Drake Passage (6.5 Sv), a warm water route involving the Agulhas Current sheeding thermocline water westward (2.5 Sv), and a recirculation of intermediate water originating in the Indian Ocean (1.6 Sv).
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de Souza, G. F., B. C. Reynolds, G. C. Johnson, J. L. Bullister, and B. Bourdon. "Silicon stable isotope distribution traces Southern Ocean export of Si to the eastern South Pacific thermocline." Biogeosciences Discussions 9, no. 6 (June 5, 2012): 6409–43. http://dx.doi.org/10.5194/bgd-9-6409-2012.
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Abstract. The cycling and transport of dissolved silicon (Si) in the ocean may be traced by its stable isotope composition, δ30Si. We present a dataset of δ30Si values along 103° W in the eastern South Pacific Ocean, ranging from the Antarctic Zone of the Southern Ocean (62° S) to the equatorial Pacific (12° S). At high southern latitudes, the uptake and associated isotope fractionation of Si by diatoms results in highly elevated δ30Si values (up to +3.2 ‰) in the summer mixed layer. The efficient export of diatom opal to depths inaccessible to annual winter convection is reflected by high δ30Si values (+2 ‰) preserved in high-latitude winter mixed layers. These elevated δ30Si values are introduced into the ocean interior by the subduction of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW), whose northward spreading results in a strong isopycnal control on lower-thermocline and intermediate δ30Si values in the well-ventilated eastern South Pacific. Values of δ30Si are strongly conserved along SAMW and AAIW density levels as far north as 26° S, documenting the importance of the export of preformed Si from the surface Southern Ocean to lower latitudes. In contrast, in the equatorial Pacific, depressed δ30Si values in the mesopelagic ocean are observed, most likely documenting the combined influence of a North Pacific Si source as well as the accumulation of remineralized Si within the eastern equatorial Pacific shadow zone. At depth, δ30Si values in the South Pacific remain indistinguishable from deep Southern Ocean values of +1.25 ‰, even within Si-rich and oxygen-poor deep waters returning from the North Pacific. This homogeneity implies that the dissolution of opal plays a negligible role in altering the δ30Si value of deep waters as they traverse the deep Pacific Ocean.
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de Souza, G. F., B. C. Reynolds, G. C. Johnson, J. L. Bullister, and B. Bourdon. "Silicon stable isotope distribution traces Southern Ocean export of Si to the eastern South Pacific thermocline." Biogeosciences 9, no. 11 (November 1, 2012): 4199–213. http://dx.doi.org/10.5194/bg-9-4199-2012.
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Abstract. The cycling and transport of dissolved silicon (Si) in the ocean may be traced by its stable isotope composition, δ30Si. We present a dataset of δ30Si values along 103° W in the eastern South Pacific Ocean, ranging from the Antarctic Zone of the Southern Ocean (62° S) to the equatorial Pacific (12° S). At high southern latitudes, the uptake and associated isotope fractionation of Si by diatoms results in highly elevated δ30Si values (up to +3.2‰) in the summer mixed layer. High δ30Si values (+2‰) are also preserved in the high-latitude fossil winter mixed layer, documenting the efficient export of diatom opal beyond the maximum depth of winter convection. This elevated winter mixed layer δ30Si signature is introduced into the ocean interior by the subduction of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW), whose northward spreading results in a strong isopycnal control on lower-thermocline and intermediate δ30Si values in the well-ventilated eastern South Pacific. Values of δ30Si are strongly conserved along SAMW and AAIW density levels as far north as 26° S, documenting the importance of the export of preformed Si from the surface Southern Ocean to lower latitudes. In contrast, in the equatorial Pacific, depressed δ30Si values in the mesopelagic ocean are observed, most likely documenting the combined influence of a North Pacific Si source as well as the accumulation of remineralized Si within the eastern equatorial Pacific shadow zone. At depth, δ30Si values in the South Pacific remain indistinguishable from deep Southern Ocean values of +1.25‰, even within Si-rich and oxygen-poor deep waters returning from the North Pacific. This homogeneity implies that the dissolution of opal plays a negligible role in altering the δ30Si value of deep waters as they traverse the deep Pacific Ocean.
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Olbers, Dirk, and Martin Visbeck. "A Model of the Zonally Averaged Stratification and Overturning in the Southern Ocean." Journal of Physical Oceanography 35, no. 7 (July 1, 2005): 1190–205. http://dx.doi.org/10.1175/jpo2750.1.
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Abstract The ocean area south of the Antarctic Circumpolar Current (ACC) frontal system is a region of major watermass modification. Influx of North Atlantic Deep Water (NADW), small-scale mixing, eddy transport and diffusion, as well as the fluxes of momentum and buoyancy at the sea surface combine in a complex array of processes to generate the unique stratification of the Southern Ocean with its southward uprising isopycnals and northward flux of Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water. Comprehensive analytical models of this scenario are rare. The authors develop and apply a model based on zonally and temporally averaged theory to explain the conversion of NADW into AAIW with all of the aforementioned processes contained in an extremely simplified way. Eddies appear via a transformed Eulerian mean (TEM) approach with a conventional downgradient parameterization of the meridional density flux. The structure of the eddy coefficient is estimated from hydrographic and wind stress data by a simple inverse approach. Mixing is limited to a near-surface layer and is treated in a most simple entrainment form. The model determines the zonal mean density stratification in the Southern Ocean and the baroclinic transport of the ACC from the applied wind stress and the surface density flux and unravels the role and importance of the different processes responsible for shaping the stratification (Ekman and eddy-induced advection and pumping, mixing, surface buoyancy flux, and eddy-induced diffusion). All of these processes must be present to yield an agreement between the simulated stratification and the observed one, but details of their parameterization might not be too critical. The ACC transport is shown to have a contribution forced by the local wind stress as well as another contribution relating to the nonlocal forcing by wind stress and density flux over the entire Antarctic zone.
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Llanillo, P. J., J. Karstensen, J. L. Pelegrí, and L. Stramma. "Physical and biogeochemical forcing of oxygen and nitrate changes during El Niño/El Viejo and La Niña/La Vieja upper-ocean phases in the tropical eastern South Pacific along 86° W." Biogeosciences 10, no. 10 (October 9, 2013): 6339–55. http://dx.doi.org/10.5194/bg-10-6339-2013.
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Abstract. Temporal changes in the water mass distribution and biogeochemical signals in the tropical eastern South Pacific are investigated with the help of an extended optimum multi-parameter (OMP) analysis, a technique for inverse modeling of mixing and biogeochemical processes through a multidimensional least-square fit. Two ship occupations of a meridional section along 85°50' W from 14° S to 1° N are analysed during relatively warm (El Niño/El Viejo, March 1993) and cold (La Niña/La Vieja, February 2009) upper-ocean phases. The largest El Niño–Southern Oscillation (ENSO) impact was found in the water properties and water mass distribution in the upper 200 m north of 10° S. ENSO promotes the vertical motion of the oxygen minimum zone (OMZ) associated with the hypoxic equatorial subsurface water (ESSW). During a cold phase the core of the ESSW is found at shallower layers, replacing shallow (top 200 m) subtropical surface water (STW). The heave of isopycnals due to ENSO partially explains the intrusion of oxygen-rich and nutrient-poor antarctic intermediate water (AAIW) into the depth range of 150–500 m. The other cause of the AAIW increase at shallower depths is that this water mass flowed along shallower isopycnals in 2009. The shift in the vertical location of AAIW reaching the OMZ induces changes in the amount of oxygen advected and respired inside the OMZ: the larger the oxygen supply, the greater the respiration and the lower the nitrate loss through denitrification. Variations in the intensity of the zonal currents in the equatorial current system, which ventilates the OMZ from the west, are used to explain the patchy latitudinal changes of seawater properties observed along the repeated section. Significant changes reach down to 800 m, suggesting that decadal variability (Pacific decadal oscillation) is also a potential driver in the observed variability.
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Bonecker, Ana C. T., Cristina De O. Dias, Marcia S. De Castro, Pedro F. De Carvalho, Adriana V. Araujo, Rodolfo Paranhos, Anderson S. Cabral, and Sergio L. C. Bonecker. "Vertical distribution of mesozooplankton and ichthyoplankton communities in the South-western Atlantic Ocean (23°14′1″S 40°42′19″W)." Journal of the Marine Biological Association of the United Kingdom 99, no. 1 (January 9, 2018): 51–65. http://dx.doi.org/10.1017/s0025315417001989.
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A study was conducted over eight consecutive days in February 2010 in which daily variations in the vertical distributions of heterotrophic bacteria, mesozooplankton and ichthyoplankton at 1–1200 m in the South-western Atlantic Ocean were investigated. Diurnal and nocturnal samples were collected at an oceanographic station at four regional depths: Tropical Water (TW) (1 m), South Atlantic Central Water (SACW) (250 m), Antarctic Intermediate Water (AAIW) (800 m) and Upper Circumpolar Deep Water (UCDW) (1200 m). Bacterial, mesozooplankton and larval fish densities significantly differed between sample depths but not between sampling tow times. In total, 154 zooplankton species and 18 larval fish species were identified. The highest number of taxa was obtained from the night-time TW trawls. This depth zone had the highest densities of mesozooplankton, larval fish and bacterioplankton (auto and heterotrophic), associated with the highest temperature and salinity and the lowest inorganic nutrient concentrations. Two sample groups were identified based on their mesozooplankton and larval fish compositions: night-time TW and other water masses (daytime TW, SACW, AAIW and UCDW). Thirty-two indicator species were detected in night-time TW. The copepod Nullosetigera impar was, to the best of our knowledge, identified for the first time on the Brazilian coast. Our results showed significant variability in the abundance and vertical distribution of mesozooplankton, bacterioplankton and larval fish along the water column in an oceanic area. We have provided new data and insights on the composition and vertical distribution of mesozooplankton, larval fish and bacterioplankton in deep waters in the South-western Atlantic Ocean.
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Sijp, Willem P., Matthew H. England, and Jonathan M. Gregory. "Precise Calculations of the Existence of Multiple AMOC Equilibria in Coupled Climate Models. Part I: Equilibrium States." Journal of Climate 25, no. 1 (January 1, 2012): 282–98. http://dx.doi.org/10.1175/2011jcli4245.1.
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Abstract This study examines criteria for the existence of two stable states of the Atlantic Meridional Overturning Circulation (AMOC) using a combination of theory and simulations from a numerical coupled atmosphere–ocean climate model. By formulating a simple collection of state parameters and their relationships, the authors reconstruct the North Atlantic Deep Water (NADW) OFF state behavior under a varying external salt-flux forcing. This part (Part I) of the paper examines the steady-state solution, which gives insight into the mechanisms that sustain the NADW OFF state in this coupled model; Part II deals with the transient behavior predicted by the evolution equation. The nonlinear behavior of the Antarctic Intermediate Water (AAIW) reverse cell is critical to the OFF state. Higher Atlantic salinity leads both to a reduced AAIW reverse cell and to a greater vertical salinity gradient in the South Atlantic. The former tends to reduce Atlantic salt export to the Southern Ocean, while the latter tends to increases it. These competing effects produce a nonlinear response of Atlantic salinity and salt export to salt forcing, and the existence of maxima in these quantities. Thus the authors obtain a natural and accurate analytical saddle-node condition for the maximal surface salt flux for which a NADW OFF state exists. By contrast, the bistability indicator proposed by De Vries and Weber does not generally work in this model. It is applicable only when the effect of the AAIW reverse cell on the Atlantic salt budget is weak.
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Kawabe, Masaki, Yuji Kashino, and Yoshifumi Kuroda. "Variability and Linkages of New Guinea Coastal Undercurrent and Lower Equatorial Intermediate Current." Journal of Physical Oceanography 38, no. 8 (August 1, 2008): 1780–93. http://dx.doi.org/10.1175/2008jpo3916.1.
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Abstract Velocity at depths of 700–800 m was measured between September 1998 and October 2002 at 2.5°S, 142°E off the New Guinea coast and at 0°, 138°E to examine the New Guinea Coastal Undercurrent (NGCUC) and the current on the equator carrying Antarctic Intermediate Water (AAIW). Velocity characteristics before November 1999 were markedly different from those after November 1999. The typical state occurred during the second period: the intermediate NGCUC and the Lower Equatorial Intermediate Current (LEIC) varied markedly with an annual cycle in opposite phases. In austral winter, the NGCUC flowed west-northwestward strongly (14 cm s−1, 285°T), especially in May–July during which the LEIC disappeared and eddylike equatorial variations with periods of 20–60 days were significant. In austral summer, the LEIC flowed westward strongly (12 cm s−1, 270°T), especially in October–December, whereas the NGCUC reversed its direction repeatedly to flow east-southeastward in November–February. Thus, the intermediate NGCUC and LEIC are present stably in austral winter and summer, respectively. These variations of the currents must change the pathway of AAIW seasonally. The state during the first period was atypical: the current on the equator flowed eastward strongly (13.0 cm s−1, 81°T), that is, no LEIC was present, and the NGCUC flowed west-northwestward strongly (14.8 cm s−1, 280°T) without changing direction. The atypical state may be related to the 1998–99 La Niña. In addition, power spectral peaks at periods of 14–35 days of meridional velocity at the equator suggest that intermediate tropical instability waves are generated in October–December in the typical state.
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Iudicone, D., K. B. Rodgers, I. Stendardo, O. Aumont, G. Madec, L. Bopp, O. Mangoni, and M. Ribera d'Alcala'. "Water masses as a unifying framework for understanding the Southern Ocean Carbon Cycle." Biogeosciences 8, no. 5 (May 4, 2011): 1031–52. http://dx.doi.org/10.5194/bg-8-1031-2011.
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Abstract. The scientific motivation for this study is to understand the processes in the ocean interior controlling carbon transfer across 30° S. To address this, we have developed a unified framework for understanding the interplay between physical drivers such as buoyancy fluxes and ocean mixing, and carbon-specific processes such as biology, gas exchange and carbon mixing. Given the importance of density in determining the ocean interior structure and circulation, the framework is one that is organized by density and water masses, and it makes combined use of Eulerian and Lagrangian diagnostics. This is achieved through application to a global ice-ocean circulation model and an ocean biogeochemistry model, with both components being part of the widely-used IPSL coupled ocean/atmosphere/carbon cycle model. Our main new result is the dominance of the overturning circulation (identified by water masses) in setting the vertical distribution of carbon transport from the Southern Ocean towards the global ocean. A net contrast emerges between the role of Subantarctic Mode Water (SAMW), associated with large northward transport and ingassing, and Antarctic Intermediate Water (AAIW), associated with a much smaller export and outgassing. The differences in their export rate reflects differences in their water mass formation processes. For SAMW, two-thirds of the surface waters are provided as a result of the densification of thermocline water (TW), and upon densification this water carries with it a substantial diapycnal flux of dissolved inorganic carbon (DIC). For AAIW, principal formatin processes include buoyancy forcing and mixing, with these serving to lighten CDW. An additional important formation pathway of AAIW is through the effect of interior processing (mixing, including cabelling) that serve to densify SAMW. A quantitative evaluation of the contribution of mixing, biology and gas exchange to the DIC evolution per water mass reveals that mixing and, secondarily, gas exchange, effectively nearly balance biology on annual scales (while the latter process can be dominant at seasonal scale). The distribution of DIC in the northward flowing water at 30° S is thus primarily set by the DIC values of the water masses that are involved in the formation processes.
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Bunzel, Dorothea, Gerhard Schmiedl, Sebastian Lindhorst, Andreas Mackensen, Jesús Reolid, Sarah Romahn, and Christian Betzler. "A multi-proxy analysis of Late Quaternary ocean and climate variability for the Maldives, Inner Sea." Climate of the Past 13, no. 12 (December 13, 2017): 1791–813. http://dx.doi.org/10.5194/cp-13-1791-2017.
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Abstract. As a natural sediment trap, the marine sediments of the sheltered central part of the Maldives Inner Sea represent an exceptional archive for paleoenvironmental and climate changes in the equatorial Indian Ocean. To evaluate the complex interplay between high-latitude and monsoonal climate variability, related dust fluxes, and regional oceanographic responses, we focused on Fe ∕ Al, Ti ∕ Al and Si ∕ Ca ratios as proxies for terrigenous sediment delivery and total organic carbon (TOC) and Br XRF counts as proxies for marine productivity. Benthic foraminiferal fauna distributions, grain size and stable δ18O and δ13C data were used for evaluating changes in the benthic ecosystem and changes in the intermediate water circulation, bottom water current velocity and oxygenation. Our multi-proxy data record reveals an enhanced dust supply during the glacial intervals, causing elevated Fe ∕ Al and Si ∕ Ca ratios, an overall coarsening of the sediment and an increasing amount of agglutinated benthic foraminifera. The enhanced dust fluxes can be attributed to higher dust availability in the Asian desert and loess areas and its transport by intensified winter monsoon winds during glacial conditions. These combined effects of wind-induced mixing of surface waters and dust fertilization during the cold phases resulted in an increased surface water productivity and related organic carbon fluxes. Thus, the development of highly diverse benthic foraminiferal faunas with certain detritus and suspension feeders was fostered. The difference in the δ13C signal between epifaunal and deep infaunal benthic foraminifera reveals intermediate water oxygen concentrations between approximately 40 and 100 µmol kg−1 during this time. The precessional fluctuation pattern of oxygen changes resembles that from the deep Arabian Sea, suggesting an expansion of the oxygen minimum zone (OMZ) from the Arabian Sea into the tropical Indian Ocean with a probable regional signal of strengthened winter-monsoon-induced organic matter fluxes and oxygen consumption further controlled by the varying inflow intensity of the Antarctic Intermediate Water (AAIW). In addition, the bottom water oxygenation pattern of the Maldives Inner Sea reveals a long phase of reduced ventilation during the last glacial period. This process is likely linked to the combined effects of generally enhanced oxygen consumption rates during high-productivity phases, reduced AAIW production and the restriction of upper bathyal environments in the Inner Sea during sea-level lowstands. Thus, our multi-proxy record reflects a close linkage between the Indian monsoon oscillation, intermediate water circulation, productivity and sea-level changes on orbital timescale.
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Thompson, Lu Anne, and Wei Cheng. "Water Masses in the Pacific in CCSM3." Journal of Climate 21, no. 17 (September 1, 2008): 4514–28. http://dx.doi.org/10.1175/2008jcli2280.1.
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Abstract An examination of model water masses in the North Pacific Ocean is performed in the Community Climate System version 3 (CCSM3) and its ocean-only counterpart. While the surface properties of the ocean are well represented in both simulations, biases in thermocline and intermediate-water masses exist that point to errors in both ocean model physics and the atmospheric component of the coupled model. The lack of North Pacific Intermediate Water (NPIW) in both simulations as well as the overexpression of a too-fresh Antarctic Intermediate Water (AAIW) is indicative of ocean model deficiencies. These properties reflect the difficulty of low-resolution ocean models to represent processes that control deep-water formation both in the Southern Ocean and in the Okhotsk Sea. In addition, as is typical of low-resolution ocean models, errors in the position of the Kuroshio, the North Pacific subtropical gyre western boundary current (WBC), impact the formation of the water masses that form the bulk of the thermocline as well as the properties of the NPIW. Biases that arise only in the coupled simulation include too-salty surface water in the subtropical North Pacific and too deep a thermocline, the source of which is the too-strong westerlies at midlatitudes. Biases in the location of the intertropical convergence zone (ITCZ) and the southern Pacific convergence zone (SPCZ) lead to the opposite hemispheric asymmetry in water mass structure when compared to observations. The atmospheric component of the coupled model acts to compound most ocean model biases.
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Holte, James, and Lynne Talley. "A New Algorithm for Finding Mixed Layer Depths with Applications to Argo Data and Subantarctic Mode Water Formation*." Journal of Atmospheric and Oceanic Technology 26, no. 9 (September 1, 2009): 1920–39. http://dx.doi.org/10.1175/2009jtecho543.1.
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Abstract A new hybrid method for finding the mixed layer depth (MLD) of individual ocean profiles models the general shape of each profile, searches for physical features in the profile, and calculates threshold and gradient MLDs to assemble a suite of possible MLD values. It then analyzes the patterns in the suite to select a final MLD estimate. The new algorithm is provided in online supplemental materials. Developed using profiles from all oceans, the algorithm is compared to threshold methods that use the C. de Boyer Montégut et al. criteria and to gradient methods using 13 601 Argo profiles from the southeast Pacific and southwest Atlantic Oceans. In general, the threshold methods find deeper MLDs than the new algorithm and the gradient methods produce more anomalous MLDs than the new algorithm. When constrained to using only temperature profiles, the algorithm offers a clear improvement over the temperature threshold and gradient methods; the new temperature algorithm MLDs more closely approximate the density algorithm MLDs than the temperature threshold and gradient MLDs. The algorithm is applied to profiles from a formation region of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The density algorithm finds that the deepest MLDs in this region routinely reach 500 dbar and occur north of the A. H. Orsi et al. mean Subantarctic Front in the southeastern Pacific Ocean. The deepest MLDs typically occur in August and September and are congruent with the subsurface salinity minimum, a signature of AAIW.
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Bonecker, Ana Cristina Teixeira, Mário Katsuragawa, Márcia Salustiano de Castro, Eduardo De Araújo Pinto Gomes, Cláudia Akemi Pereira Namiki, and Maria De Lourdes Zani-Teixeira. "Larval fish of the Campos Basin, southeastern Brazil." Check List 8, no. 6 (December 1, 2012): 1280. http://dx.doi.org/10.15560/8.6.1280.
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Studies on the vertical distribution of larval fish in water masses along the Brazilian coast are very rare. The present study aimed to identify larval fish occurring in the surface (1 m) layer and at depth in four water masses of the Campos Basin, southeastern Brazil: South Atlantic Central Water (SACW) (250 m), Antarctic Intermediate Water (AAIW) (800 m), Upper Circumpolar Deep Water (UCDW) (1,200 m) and North Atlantic Deep Water (NADW) (2,300 m). Material used in this study was obtained in 2009 through nocturnal horizontal stratified hauls using a Multinet (500 μm mesh size) during both rainy (February to April) and dry periods (August to September). A total of 10,978 fish larvae comprising 169 taxa were identified during the rainy (n = 6,015) and dry (n = 4,963) periods. The number of taxa decreased as the sampling depth increased. Larvae of Clupeidae, Engraulidae and Scombridae dominated in samples collected in the surface layer, while Sternoptychidae and Myctophidae were the most representative families in SACW. The other three water masses were dominated by Gonostomatidae larvae.
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Portela, Esther, Nicolas Kolodziejczyk, Christophe Maes, and Virginie Thierry. "Interior Water-Mass Variability in the Southern Hemisphere Oceans during the Last Decade." Journal of Physical Oceanography 50, no. 2 (February 2020): 361–81. http://dx.doi.org/10.1175/jpo-d-19-0128.1.
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AbstractUsing an Argo dataset and the ECCOv4 reanalysis, a volume budget was performed to address the main mechanisms driving the volume change of the interior water masses in the Southern Hemisphere oceans between 2006 and 2015. The subduction rates and the isopycnal and diapycnal water-mass transformation were estimated in a density–spiciness (σ–τ) framework. Spiciness, defined as thermohaline variations along isopycnals, was added to the potential density coordinates to discriminate between water masses spreading on isopycnal layers. The main positive volume trends were found to be associated with the Subantarctic Mode Waters (SAMW) in the South Pacific and South Indian Ocean basins, revealing a lightening of the upper waters in the Southern Hemisphere. The SAMW exhibits a two-layer density structure in which subduction and diapycnal transformation from the lower to the upper layers accounted for most of the upper-layer volume gain and lower-layer volume loss, respectively. The Antarctic Intermediate Waters, defined here between the 27.2 and 27.5 kg m−3 isopycnals, showed the strongest negative volume trends. This volume loss can be explained by their negative isopyncal transformation southward of the Antarctic Circumpolar Current into the fresher and colder Antarctic Winter Waters (AAWW) and northward into spicier tropical/subtropical Intermediate Waters. The AAWW is destroyed by obduction back into the mixed layer so that its net volume change remains nearly zero. The proposed mechanisms to explain the transformation within the Intermediate Waters are discussed in the context of Southern Ocean dynamics. The σ–τ decomposition provided new insight on the spatial and temporal water-mass variability and driving mechanisms over the last decade.
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Llanillo, P. J., J. Karstensen, J. L. Pelegrí, and L. Stramma. "Physical and biogeochemical forcing of oxygen changes in the tropical eastern South Pacific along 86° W: 1993 versus 2009." Biogeosciences Discussions 9, no. 12 (December 11, 2012): 17583–618. http://dx.doi.org/10.5194/bgd-9-17583-2012.
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Abstract. Temporal changes of the water mass distribution and biogeochemical cycling in the tropical eastern South Pacific are investigated based on the extended Optimum Multi-Parameter (OMP) method. Two ship occupations of a meridional section along 85°50´ W, from 14° S to 1° N, are analysed, one during a relatively warm (El Niño/El Viejo, March 1993) and the other during a cold (La Niña/La Vieja, February 2009) upper-ocean phase. The largest El Niño – Southern Oscillation (ENSO) impact was found in the water properties and water mass distribution in the upper 250 m. The most prominent change is the vertical motion of the Oxygen Minimum Zone (OMZ) associated to the hypoxic Equatorial Subsurface Water (ESSW). During a cold phase the core of the ESSW is found at shallower layers, replacing the shallow (top 250 m) Subtropical Surface Water (STW) and allowing an intrusion of oxygen-rich and nutrient-poor Antarctic Intermediate Water (AAIW) in the depth range of 300 to 600 m. The shift in the vertical location of the intrusion of AAIW in the OMZ induces changes in oxygen advection and respiration, the largest the oxygen supply the greatest the respiration and the lowest the nitrate loss by denitrification. Changes in the intensity of the zonal currents in the Equatorial Current System, that ventilate the OMZ from the west, are used to explain the patchy latitudinal changes of seawater properties observed along the repeated section. Given that changes down to 800 m depth are observed, not only interannual (ENSO) but also decadal variability (Pacific Decadal Oscillation) is a potential driver for the observed changes.
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Jenkins, William J., Kathryn L. Elder, Ann P. McNichol, and Karl von Reden. "The Passage of the Bomb Radiocarbon Pulse into the Pacific Ocean." Radiocarbon 52, no. 3 (2010): 1182–90. http://dx.doi.org/10.1017/s0033822200046257.
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We report and compare radiocarbon observations made on 2 meridional oceanographic sections along 150°W in the South Pacific in 1991 and 2005. The distributions reflect the progressive penetration of nuclear weapons-produced 14C into the oceanic thermocline. The changes over the 14 yr between occupations are demonstrably large relative to any possible drift in our analytical standardization. The computed difference field based on the gridded data in the upper 1600 m of the section exhibits a significant decrease over time (approaching 40 to 50‰ in Δ14C) in the upper 200–300 m, consistent with the decadal post-bomb decline in atmospheric 14C levels. A strong positive anomaly (increase with time), centered on the low salinity core of the Antarctic Intermediate Water (AAIW), approaches 50–60‰ in Δ14C, a clear signature of the downstream evolution of the 14C transient in this water mass. We use this observation to estimate the transit time of AAIW from its “source region” in the southeast South Pacific and to compute the effective reservoir age of this water mass. The 2 sections show small but significant changes in the abyssal 14C distributions. Between 1991 and 2005, Δ14C has increased by 9‰ below 2000 m north of 55°S. This change is accompanied overall by a modest increase in salinity and dissolved oxygen, as well as a slight decrease in dissolved silica. Such changes are indicative of greater ventilation. Calculation of “phosphate star” also indicates that this may be due to a shift from the Southern Ocean toward North Atlantic Deep Water as the ventilation source of the abyssal South Pacific.
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Sallée, Jean-Baptiste, Kevin Speer, Steve Rintoul, and S. Wijffels. "Southern Ocean Thermocline Ventilation." Journal of Physical Oceanography 40, no. 3 (March 1, 2010): 509–29. http://dx.doi.org/10.1175/2009jpo4291.1.
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Abstract An approximate mass (volume) budget in the surface layer of the Southern Ocean is used to investigate the intensity and regional variability of the ventilation process, discussed here in terms of subduction and upwelling. Ventilation resulting from Ekman pumping is estimated from satellite winds, the geostrophic mean component is assessed from a climatology strengthened with Argo data, and the eddy-induced advection is included via the parameterization of Gent and McWilliams, together with eddy mixing estimates. All three components contribute significantly to ventilation. Finally, the seasonal cycle of the upper ocean is resolved using Argo data. The circumpolar-averaged circulation shows an upwelling in the Antarctic Intermediate Water (AAIW) density classes, which is carried north into a zone of dense Subantarctic Mode Water (SAMW) subduction. Although no consistent net production is found in the light SAMW density classes, a large subduction of Subtropical Mode Water (STMW) is observed. The STMW area is fed by convergence of a southward and a northward residual meridional circulation. The eddy-induced contribution is important for the water mass transport in the vicinity of the Antartic Circumpolar Current. It balances the horizontal northward Ekman transport as well as the vertical Ekman pumping. While the circumpolar-averaged upper cell structure is consistent with the average surface fluxes, it hides strong longitudinal regional variations and does not represent any local regime. Subduction shows strong regional variability with bathymetrically constrained hotspots of large subduction. These hotspots are consistent with the interior potential vorticity structure and circulation in the thermocline. Pools of SAMW and AAIW of different densities are found along the circumpolar belt in association with the regional pattern of subduction and interior circulation.
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McCarthy, Gerard, Elaine McDonagh, and Brian King. "Decadal Variability of Thermocline and Intermediate Waters at 24°S in the South Atlantic." Journal of Physical Oceanography 41, no. 1 (January 1, 2011): 157–65. http://dx.doi.org/10.1175/2010jpo4467.1.
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Abstract New data are presented from 24°S in the South Atlantic in an investigation of the decadal variability of the intermediate and thermocline water masses at this latitude. Variation of salinity on neutral density surfaces is investigated with three transatlantic, full-depth hydrographic sections from 1958, 1983, and 2009. The thermocline is seen to freshen by 0.05 between 1983 and 2009. The freshening is coherent, basinwide, and of a larger magnitude than any errors associated with the datasets. This freshening reverses a basinwide, coherent increase in salinity of 0.03 in the thermocline between 1958 and 1983. Changes in apparent oxygen utilization (AOU) are investigated to support the salinity changes. In the thermocline of the eastern basin, a correlated relationship exists between local AOU and salinity anomalies, which is consistent with the influence of Indian Ocean Water. This correlated relationship is utilized to estimate the magnitude of Indian Ocean influence on the salinity changes in the thermocline. Indian Ocean influence explains half of the salinity changes in the eastern thermocline from 1958 to 1983 but less of the salinity change in the eastern thermocline from 1983 to 2009. Antarctic Intermediate Water properties significantly warm from 1958 through 1983 to 2009. A significant salinification and increase in AOU is evident from 1958 to 1983. Changes in the salinity of AAIW are shown to be linked with Indian Ocean influence rather than changes in the hydrological cycle. Upper Circumpolar Deep Water is seen to be progressively more saline from 1958 through 1983 to 2009. Increased Agulhas leakage and the intensification of the hydrological cycle are conflicting influences on the salinity of thermocline and intermediate waters in the South Atlantic as the former acts to increase the salinity of these water masses and the latter acts to decrease the salinity of these water masses. The results presented here offer an interpretation of the salinity changes, which considers both of these conflicting influences.