Subantarctic Mode Water Formation Research Articles

Dissolved cadmium in the Southern Ocean: Distribution, speciation, and relation to phosphate

We report isotope dilution analyses of dissolved cadmium (Cd) and electrochemical Cd speciation measurements in the Atlantic sector of the Southern Ocean. Bioavailable inorganic Cd is > 100 times higher in near‐surface waters south of the Polar Front compared to the Subantarctic Zone because of upwelling and reduced complexation by organic Cd ligands. To trace local changes in the relation between Cd and P, we examine the deviations from a linear deep‐water Cd vs. P relation (Cd*), and find that changes in Cd* coincide with the position of frontal systems and covary with primary productivity and total dissolved Mn and Fe concentrations. These covariations agree with potential local changes in phytoplankton Cd uptake rates, resulting from differences in the availability of Cd, Zn, Mn, and Fe. A band of negative Cd* values is associated with formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). In contrast to SAMW, which may export low Cd : P ratios from the Southern Ocean, the Cd : P ratios in AAIW increase by mixing with underlying Upper Circumpolar Deep Water before being exported from the Southern Ocean. Deep waters show constant Cd : P ratios, and both elements behave conservatively with end‐member mixing between deep waters of the Weddell Gyre, the Antarctic Circumpolar Current, and inflowing North Atlantic Deep Water. Overall, our results support the hypothesis that the kink in the global Cd vs. P relation is largely caused by high Cd : P uptake ratios in the trace‐nutrient—limited Southern Ocean.

Subantarctic Mode Water Formation, Destruction, and Export in the Eddy-Permitting Southern Ocean State Estimate

Abstract Subantarctic Mode Water (SAMW) is examined using the data-assimilating, eddy-permitting Southern Ocean State Estimate, for 2005 and 2006. Surface formation due to air–sea buoyancy flux is estimated using Walin analysis, and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30°S, accounting for volume change with time. Water in the density range 26.5 < σθ < 27.1 kg m−3 that includes SAMW is exported northward in all three ocean sectors, with a net transport of (18.2, 17.1) Sv (1 Sv ≡ 106 m3 s−1; for years 2005, 2006); air–sea buoyancy fluxes form (13.2, 6.8) Sv, diapycnal mixing removes (−14.5, −12.6) Sv, and there is a volume loss of (−19.3, −22.9) Sv mostly occurring in the strongest SAMW formation locations. The most vigorous SAMW formation is in the Indian Ocean by air–sea buoyancy flux (9.4, 10.9) Sv, where it is partially destroyed by diapycnal mixing (−6.6, −3.1) Sv. There is strong export to the Pacific, where SAMW is destroyed both by air–sea buoyancy flux (−1.1, −4.6) Sv and diapycnal mixing (−5.6, −8.4) Sv. In the South Atlantic, SAMW is formed by air–sea buoyancy flux (5.0, 0.5) Sv and is destroyed by diapycnal mixing (−2.3, −1.1) Sv. Peaks in air–sea flux formation occur at the Southeast Indian and Southeast Pacific SAMWs (SEISAMWs, SEPSAMWs) densities. Formation over the broad SAMW circumpolar outcrop windows is largely from denser water, driven by differential freshwater gain, augmented or decreased by heating or cooling. In the SEISAMW and SEPSAMW source regions, however, formation is from lighter water, driven by differential heat loss.

On the formation, ventilation, and erosion of mode waters in the North Atlantic and Southern Oceans

The mean residence times, subduction rates, and formation rates of Subtropical Mode Water (STMW) and Subpolar Mode Water (SPMW) in the North Atlantic and Subantarctic Mode Water (SAMW) in the Southern Ocean are estimated by combining a model and observations of chlorofluorocarbon‐11 (CFC‐11) via Bayesian Model Averaging (BMA), a statistical technique that weights model estimates according to how close they agree with observations. Subduction rates are estimated in two different ways to investigate the non‐advective contribution to thermocline ventilation, which in turn are compared to formation rate estimates. One subduction rate estimate is based on entrainment/detrainment velocities and the other subduction rate estimate allows ventilation to be both an advective and diffusive process instead of a purely advective one by using transit‐time distributions (TTDs). It is found that the subduction of all three mode waters is mostly an advective process, but up to about one‐third of STMW subduction likely owes to non‐advective processes. Also, while the formation of STMW is mostly due to subduction, the formation of SPMW is mostly due to other processes. About half of the formation of SAMW is due to subduction and half is due to other processes. A combination of air‐sea flux, acting on relatively short timescales, and turbulent mixing, acting on a wide range of timescales, is likely the dominant SPMW erosion mechanism. Air‐sea flux is likely responsible for most STMW erosion, and turbulent mixing is likely responsible for most SAMW erosion.

The role of air‐sea fluxes in Subantarctic Mode Water formation

Two hydrographic surveys and a one‐dimensional mixed layer model are used to assess the role of air‐sea fluxes in forming deep Subantarctic Mode Water (SAMW) mixed layers in the southeast Pacific Ocean. Forty‐two SAMW mixed layers deeper than 400 m were observed north of the Subantarctic Front during the 2005 winter cruise, with the deepest mixed layers reaching 550 m. The densest, coldest, and freshest mixed layers were found in the cruise's eastern sections near 77°W. The deep SAMW mixed layers were observed concurrently with surface ocean heat loss of approximately −200 W m−2. The heat, momentum, and precipitation flux fields of five flux products are used to force a one‐dimensional KPP mixed layer model initialized with profiles from the 2006 summer cruise. The simulated winter mixed layers generated by all of the forcing products resemble Argo observations of SAMW; this agreement also validates the flux products. Mixing driven by buoyancy loss and wind forcing is strong enough to deepen the SAMW layers. Wind‐driven mixing is central to SAMW formation, as model runs forced with buoyancy forcing alone produce shallow mixed layers. Air‐sea fluxes indirectly influence winter SAMW properties by controlling how deeply the profiles mix. The stratification and heat content of the initial profiles determine the properties of the SAMW and the likelihood of deep mixing. Summer profiles from just upstream of Drake Passage have less heat stored between 100 and 600 m than upstream profiles, and so, with sufficiently strong winter forcing, form a cold, dense variety of SAMW.

Decadal biogeochemical changes in the subtropical Indian Ocean associated with Subantarctic Mode Water

Within the Subantarctic Mode Water (SAMW) density level, we study temporal changes in salinity, nutrients, oxygen and TTD (Transit Time Distribution) ages in the western (W) and eastern (E) subtropical gyre of the Indian Ocean (IO) from 1987 to 2002. Additionally, changes in Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC) are evaluated between 1995 and 2002. The mechanisms behind the detected changes are discussed along with the results from a hindcast model run (Community Climate System Model). The increasing salinity and decreasing oxygen trends from 1960 to 1987 reversed from 1987 to 2002 along the gyre. In the W-IO a decreasing trend in TTD ages points to a faster delivery of SAMW, thus less biogenic matter remineralization, explaining the oxygen increase and noisier nutrients decrease. In the E-IO SAMW, no change in TTD ages was detected, therefore the trends in oxygen and inorganic nutrients relate to changes in the Antarctic Surface Water transported into the E-IO SAMW formation area. In the W-IO between 1995 and 2002, the DIC increase is equal or even less than the anthropogenic input as the reduction in remineralization contributes to mask the increasing trend. In the E-IO between 1995 and 2002, DIC decreases slightly despite the increase in the anthropogenic input. Differences in the preformed E-IO SAMW conditions would explain this behavior. Trends in the W and E IO SAMW are decoupled and related to different forcing mechanisms in the two main sites of SAMW formation in the IO, at 40°S–70°E and 45°S–90°E, respectively.

Formation rates of Subantarctic mode water and Antarctic intermediate water within the South Pacific

The formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) significantly contributes to the total uptake and storage of anthropogenic gases, such as CO 2 and chlorofluorocarbons (CFCs), within the world's oceans. SAMW and AAIW formation rates in the South Pacific are quantified based on CFC-12 inventories using hydrographic data from WOCE, CLIVAR, and data collected in the austral winter of 2005. This study documents the first wintertime observations of CFC-11 and CFC-12 saturations with respect to the 2005 atmosphere in the formation region of the southeast Pacific for SAMW and AAIW. SAMW is 94% and 95% saturated for CFC-11 and CFC-12, respectively, and AAIW is 60% saturated for both CFC-11 and CFC-12. SAMW is defined from the Subantarctic Front to the equator between potential densities 26.80–27.06 kg m −3, and AAIW is defined from the Polar Front to 20°N between potential densities 27.06–27.40 kg m −3. CFC-12 inventories are 16.0×10 6 moles for SAMW and 8.7×10 6 moles for AAIW, corresponding to formation rates of 7.3±2.1 Sv for SAMW and 5.8±1.7 Sv for AAIW circulating within the South Pacific. Inter-ocean transports of SAMW from the South Pacific to the South Atlantic are estimated to be 4.4±0.6 Sv. Thus, the total formation of SAMW in the South Pacific is approximately 11.7±2.2 Sv. These formation rates represent the average formation rates over the major period of CFC input, from 1970 to 2005. The CFC-12 inventory maps provide direct evidence for two areas of formation of SAMW, one in the southeast Pacific and one in the central Pacific. Furthermore, eddies in the central Pacific containing high CFC concentrations may contribute to SAMW and to a lesser extent AAIW formation. These CFC-derived rates provide a baseline with which to compare past and future formation rates of SAMW and AAIW.

Antarctic Intermediate Water and Subantarctic Mode Water Formation in the Southeast Pacific: The Role of Turbulent Mixing

Abstract During the 2005 austral winter (late August–early October) and 2006 austral summer (February–mid-March) two intensive hydrographic surveys of the southeast Pacific sector of the Southern Ocean were completed. In this study the turbulent kinetic energy dissipation rate ε, diapycnal diffusivity κ, and buoyancy flux Jb are estimated from the CTD/O2 and XCTD profiles for each survey. Enhanced κ of O(10−3 to 10−4 m2 s−1) is found near the Subantarctic Front (SAF) during both surveys. During the winter survey, enhanced κ was also observed north of the “subduction front,” the northern boundary of the winter deep mixed layer north of the SAF. In contrast, the summer survey found enhanced κ across the entire region north of the SAF below the shallow seasonal mixed layer. The enhanced κ below the mixed layer decays rapidly with depth. A number of ocean processes are considered that may provide the energy flux necessary to support the observed diffusivity. The observed buoyancy flux (4.0 × 10−8 m2 s−3) surrounding the SAF during the summer survey is comparable to the mean buoyancy flux (0.57 × 10−8 m2 s−3) associated with the change in the interior stratification between austral summer and autumn, determined from Argo profiles. The authors suggest that reduced ocean stratification during austral summer and autumn, by interior mixing, preconditions the water column for the rapid development of deep mixed layers and efficient Antarctic Intermediate Water and Subantarctic Mode Water formation during austral winter and early spring.

Origin and mechanism of Subantarctic Mode Water formation and transformation in the Southern Indian Ocean

The sources and pathways of mode waters and lower thermocline waters entering the subtropical gyre of the Indian Ocean are examined. A Lagrangian analysis is performed on an eddy-admitting simulation of the Global Ocean performed by the DRAKKAR Group (NEMO/OPA), which captures the main observed features. We trace the subducted mode water’s pathways, identify their formation regions and trace whether their source waters come from the Atlantic, Pacific or Indian sectors of the Southern Ocean. Three main sites for mode waters ventilation in the Indian sector are identified with different circulation pathways and source water masses: (a) just north of Kerguelen, where 4.2 Sv of lighter Subantarctic Mode Waters (SAMW); σ 0 ∼ 26.5) are exported—originating in the Atlantic and Agulhas Retroflection regions; (b) SW of Australia, where 6.5 Sv of medium SAMW (σ 0 ∼ 26.6) are ventilated—originating in the southern and denser Agulhas Retroflection region; (c) SW of Tasmania and along the South Australian coast, where 3 Sv of denser SAMW (σ 0 ∼ 26.75) are ventilated—originating from three sources: Leeuwin Current waters, Tasman Sea (Pacific) waters and Antarctic Surface Waters. In all cases, modelled mode waters were last ventilated in the Indian Ocean just north of the deepest winter-mixed layers. For the waters subducted SW of Australia, the last ventilation site extends even further north. Waters ventilated in the deepest mixed layers north of the Subantarctic Front are then re-ventilated 5 years later southwest of Australia. The model results raise new hypotheses that revisit the classical picture of the SAMW formation and transformation, where a large homogeneous mixed layer is subducted and ‘slides’ equatorward, essentially maintaining the T/S characteristics acquired at the surface. Firstly, the last ventilation of the modelled mode waters is not in the region of the deepest mixed layers, as previously thought, but further north in regions of moderate meso-scale eddy activity. Secondly, the model shows for the first time a significant source region for Indian Ocean mode waters coming from deep winter-mixed layers along the south Australian coast. Finally, this analysis shows how the mode water characteristics are modified after subduction, due to internal eddy mixing. The simulation shows that resolved eddies have a strong impact on the mixed layer properties and that isopycnal eddy mixing also contributes to the generation of more homogeneous mode water characteristics in the interior.

A New Algorithm for Finding Mixed Layer Depths with Applications to Argo Data and Subantarctic Mode Water Formation*

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.

Variability of Subantarctic Mode Water and Antarctic Intermediate Water in the Drake Passage during the Late-Twentieth and Early-Twenty-First Centuries

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.

Southern Ocean mixed‐layer depth from Argo float profiles

Argo float profiles of temperature, salinity, and pressure are used to derive the mixed‐layer depth (MLD) in the Southern Ocean. MLD is determined from individual profiles using both potential density and potential temperature criteria, and a monthly climatology is derived from individual MLDs using an objective mapping method. Quantitative data are available in the auxiliary material. The spatial structures of MLDs are similar in each month, with deep mixed layers within and just north of the Antarctic Circumpolar Current (ACC) in the Pacific and Indian oceans. The deepest mixed layers are found from June to October and are located just north of the ACC where Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are formed. Examination of individual MLDs indicates that deep mixed layers (MLD ≥ 400 m) from both the density and temperature criteria are concentrated in a narrow surface density band which is within the density range of SAMW. The surface salinity for these deep mixed layers associated with the SAMW formation are slightly fresher compared to historical estimates. Differences in air‐sea heat exchanges, wind stress, and wind stress curl in the Pacific and Indian oceans suggest that the mode water formation in each ocean basin may be preconditioned by different processes. Wind mixing and Ekman transport of cold water from the south may assist the SAMW formation in the Indian Ocean. In the eastern Pacific, the formation of mode water is potentially preconditioned by the relative strong cooling and weak stratification from upwelling.

Formation regions of Subantarctic Mode Water detected by OFES and Argo profiling floats

We investigated the distribution of the deep mixed layer in the Southern Ocean, where Subantarctic Mode Water (SAMW) forms, by using Argo profiling floats and the ocean general circulation model for the Earth simulator (OFES). The mixed‐layer depth (MLD) generally increased eastward from 50°–180°E and from 180°–80°W, with distinct local maxima near major bathymetric features. A sudden transition from deep to shallow mixed layer was found along the equatorward limit of the deep mixed layer. This diagnosed MLD front coincides with the line of ug · ∇σs = 0, indicating the distributions of the general deepening and local maxima of MLD were affected by near‐surface geostrophic flow. The flow is steered by the major bottom topographies, and the local MLD maxima occurred where the flow direction was deflected southward. This implies a link between the bathymetric features and formation of thick SAMW.

Observations of small-scale disturbances of the Subantarctic Front south of Australia

High-resolution XCTD and XBT observations were conducted to study eddy variability in the Subantarctic Front (SAF) and its possible impact on the properties of the adjacent Subantarctic Mode Water (SAMW) in the region south of Australia. The stations were occupied along cross-frontal transects in February 2004 and in February and March 2002. Coarse-resolution data from previous WOCE observations were also examined to reveal the water mass properties, including dissolved oxygen (DO). Small-scale (10–20 km) features were detected in the SAF for all high-resolution sections. Fluctuation of the 7 ∘ C isotherm was clearly revealed at a depth of 300–600 m for the σ θ = 26.8 – 26.9 kg m - 3 density range. Temperature and salinity inversions of a similar spatial scale were also found on the same density surface. The spatial scale and depth range of the small-scale anomalies could be consistent with those of the baroclinic instability in the top several hundred-meter layer. In the potential temperature–salinity ( θ – S) diagram, the small-scale fluctuations constitute cold/fresh intrusions from the warm/saline SAMW curve. The intrusions were found on the θ – S line connecting the deeper SAMW and the shallower Antarctic Surface Water (AASW) of the higher latitudes. From the WOCE observations, the presence of SAMW with high DO was indicated north of the SAF, as previously reported. Within the SAF, intrusions in the θ – S diagram with relatively high DO were found at around the 7 ∘ C isotherm for 26.8 – 26.9 kg m - 3 , the same densities for the small-scale anomalies and the high-DO SAMW. Analysis of heat and salt deficit suggests that the small-scale disturbances have a comparable or larger role than those of meso-scale anomalies. These imply that the small-scale anomalies contribute to the cross-frontal water exchange and to the formation of high-DO SAMW.

Formation of subantarctic mode water in the southeastern Indian Ocean

Subantarctic Mode Water (SAMW) is the name given to the relatively deep surface mixed layers found directly north of the Subantarctic Front in the Southern Ocean, and their extension into the thermocline as weakly stratified or low potential vorticity water masses. The objective of this study is to begin an investigation into the mechanisms controlling SAMW formation, through a heat budget calculation. ARGO profiling floats provide estimates of temperature and salinity typically in the upper 2,000 m and the horizontal velocity at various parking depths. These data are used to estimate terms in the mode water heat budget; in addition, mode water circulation is determined with ARGO data and earlier ALACE float data, and climatological hydrography. We find a rapid transition to thicker layers in the central South Indian Ocean, at about 70°S, associated with a reversal of the horizontal eddy heat diffusion in the surface layer and the meridional expansion of the ACC as it rounds the Kerguelen Plateau. These effects are ultimately related to the bathymetry of the region, leading to the seat of formation in the region southwest of Australia. Upstream of this region, the dominant terms in the heat budget are the air–sea flux, eddy diffusion, and Ekman heat transport, all having approximately equal importance. Within the formation area, the Ekman contribution dominates and leads to a downstream evolution of mode water properties.

Seasonal evolution of the mixed layer in the Subantarctic zone south of Australia

The circumpolar Subantarctic Zone (SAZ) is a globally significant region of water mass formation and carbon dioxide uptake from the atmosphere. Here we synthesize the results of nine voyages over 8 years to describe the seasonal variation in mixed layer properties in the SAZ south of Australia for comparison with biogeochemical process studies carried out in late summer (March 1998) as part of the SAZ Project. Winter mixing extends to depths >400 m, resulting in the formation of Subantarctic Mode Water. In summer the mixed layer shoals to 75–100 m, depths which are still sufficiently deep that phytoplankton growth may be light limited. Nitrate and phosphate concentrations are reduced in summer (e.g., nitrate decreases from >15 to <5 μmol kg−1) but remain well above limiting levels. Silicate, in contrast, is low throughout the year (4 μmol kg−1 in winter and <2 μmol kg−1 in summer). Water mass properties along a north‐south hydrographic section in March 1998 suggest that near‐surface waters spread from south to north across the Subantarctic Front (SAF), supplying cool, fresh, nutrient‐rich water to the SAZ. As a consequence, the properties of the southern SAZ differ from those farther north: the mixed layer in the south is cooler, fresher, deeper, higher in nutrients, and bounded below by a halocline (rather than by a seasonal thermocline, as in the northern SAZ). The contrast between the northern and southern SAZ persists throughout the year, suggesting the cross‐front exchange occurs year‐round and likely contributes to the differences in seasonal thermal amplitude and algal biomass accumulation seen in satellite images. Density‐compensated horizontal gradients of temperature and salinity are common in the mixed layer of the SAZ and the northern SAF, consistent with the hypothesis that the vigor of lateral mixing in the mixed layer is a strong function of the magnitude of the lateral density gradient.

Accumulation and uptake of anthropogenic CO2 in the Southern Ocean, south of Australia between 1968 and 1996

Increases in the anthropogenic CO2 inventory for Southern Ocean waters to the south of Australia were estimated by comparing measurements made 28 years apart in 1968 and 1996. For this period, the deepest penetration of anthropogenic CO2 is consistent with the depth of 28 year old water from CFC‐11 age estimates. Significant accumulation of anthropogenic CO2 (13±10 μmol kg−1) was found in Antarctic Bottom Water (AABW) from 500–4500 m south of 58°S suggesting AABW to be an important water mass for the storage of anthropogenic CO2 in the deep ocean. Deep penetration of anthropogenic CO2 (<1900 m) was found in the sub‐Antarctic zone (SAZ) between 48°S and 51°S while little accumulation (<400 m) was observed south of 53°S. A simple one dimensional box model was used to investigate the anthropogenic CO2 uptake potential in the SAZ based on observed CFC‐11 concentrations and the calculated anthropogenic CO2 inventory. Formation and export of sub‐Antarctic Mode Water (SAMW) was estimated by reproducing the observed present day SAMW CFC‐11 concentration using time histories of atmospheric CFC‐11 concentrations. The anthropogenic CO2 uptake for the SAZ (45°–50°S) ranged from 0.73 to 0.86 μmol kg yr−1. The calculated range of SAMW formation and export rates only increased the anthropogenic CO2 uptake potential for the SAZ by as much as 18% (<0.13 μmol kg−1 yr−1). Extrapolating our estimate to the circumpolar SAZ gives an anthropogenic CO2 uptake of 0.07–0.08 GTC yr−1. Even though SAMW formation does not considerably enhance anthropogenic CO2 uptake in the SAZ, it provides an important mechanism for transporting anthropogenic CO2 into the ocean interior. SAMW therefore may be important for anthropogenic CO2 uptake on a larger regional scale.

On wind-driven mid-latitude convection in ocean general circulation models

Several computational experiments were carried out with a state-of-the-art ocean general circulationmodel to identify the northward directed Ekman transport of antarctic and sub-antarcticorigin surface water as a mechanism which forces mid-latitude convection. The results indicatethat the wind-driven transport of water in the surface layer is most efficient at increasing thedepth of convection in the southeast region of the ocean basin investigated. To the north of thelatitude of maximum wind stress at about 50°S, a deepening of the convectively-mixed layer ofmore than 300 m is simulated if the wind forcing is doubled. In the real ocean, this identifiedmechanism of Ekman transport of cold and fresh water across the path of the antarctic circumpolarcurrent may contribute to the formation of sub-antarctic mode water observed to thenorth of the sub-antarctic front of the southern ocean.

Age determination of mixed water masses using CFC and oxygen data

We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section 15 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48°–55°E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55°E after 15–20 years. It takes ICW some 25–30 years to reach 110°E.

On convection and the formation of Subantarctic Mode Water in the Fine Resolution Antarctic Model (FRAM)

We investigate the formation of Subantarctic Mode Water (SAMW) in the Fine Resolution Antarctic Model (FRAM). FRAM velocity fields are applied to advect an ideal tracer in an off-line diffusion and advection model of the Southern Ocean, and the results from two computational experiments are reported. In the first of our experiments, the tracer was released to the south of the Antarctic Polar Front (APF), in the second experiment to the north of the front. In this manner, we obtain insight into the SAMW formation process and the relative importance of convection, downwelling, vertical mixing and subduction in FRAM. The modelled tracer distribution was not in a steady state; however, good overall agreement was found between the modelled and the observed climatological distribution of ventilated water. The formation of SAMW in the model is found to be only weak for the southeast Pacific Ocean. In contrast, Indian Ocean SAMW is ventilating the Indian Ocean thermocline and is adverted eastward into the Pacific Ocean. We give a quantitative estimate for the amount of Antarctic and Subantarctic surface water found north of the APF and find that its contribution to ventilated water in the upper thermocline is significant.

The vertical thermohaline structure in the Argentine basin

The vertical thermohaline structure in the central Argentine basin is investigated on the basis of high‐resolution hydrographic data. Emphasis is placed on fronts, their horizontal gradients, vertical stability and associated fine structure. Horizontal thermohaline gradients vary strongly with depth, regions of strong gradients alternating with those of weak gradients. The strongest gradients occur often at depths below the upper mixed layer. The salinity gradient maximum occurs above the temperature gradient maximum. Enhanced gradients are found where warm, salty water moves over cooler, fresher water such as between the subtropical water (STW) and antarctic intermediate water (AAIW) and between the North Atlantic deep water (NADW) and the antarctic bottom water (AABW). Temperature, salinity, and density fronts do not always coexist. Density‐compensating temperature and salinity fronts occur in the surface layer of the subantarctic and at abyssal depth below 3500 m. Vertical profiles of the Väisälä frequency indicate much reduced hydrostatic stabilities in the Brazil current and subantarctic frontal zones. Potential vorticity minima indicate the formation of subantarctic mode water just north of the subantarctic front and its subsequent northward and upward spread across isopycnals. The thermohaline fine structure is vertically intermittent. The amplitude variances between the enhanced and quiet regions vary by almost 2 orders of magnitude. Enhanced fine structure amplitudes are found near the STW‐AAIW and NADW‐AABW interfaces, where large horizontal thermohaline gradients occur and the hydrostatic stability is low. The temperature and salinity fine structures are largely density compensating. The coherence between the temperature and salinity fine structure is strongly depth dependent and is highest in regions where warm, salty water moves over cooler, fresher water.