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- Author or Editor: Bernadette M. Sloyan x
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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.
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.
Abstract
Estimates of area-averaged diapycnal fluxes for the southern oceans are derived from basin-scale budgets of mass, heat, and salt using a box inverse model. The diapycnal fluxes are found to be significant terms in the isopycnal budgets of mass, heat, and salt. Dense water entering the subtropical Indian and Pacific basins from the south is returned below the thermocline as less dense deep water. In the Southern Ocean deep water is converted to denser deep and bottom water. Water properties at intermediate depth are substantially modified by diapycnal fluxes of heat and salt, but the modification of intermediate water is not solely driven by interior mixing. The inferred fluxes help explain the changes in temperature–salinity curves observed across each basin, and they are consistent with our understanding of the overall three-dimensional circulation of the Southern Ocean.
The fact that area-averaged diapycnal fluxes can be determined from basin-scale budgets using a suitably designed inverse model is encouraging: similar methods applied to the high-quality measurements collected during the World Ocean Circulation Experiment promise to provide the first global maps of diapycnal fluxes derived from ocean observations.
Abstract
Estimates of area-averaged diapycnal fluxes for the southern oceans are derived from basin-scale budgets of mass, heat, and salt using a box inverse model. The diapycnal fluxes are found to be significant terms in the isopycnal budgets of mass, heat, and salt. Dense water entering the subtropical Indian and Pacific basins from the south is returned below the thermocline as less dense deep water. In the Southern Ocean deep water is converted to denser deep and bottom water. Water properties at intermediate depth are substantially modified by diapycnal fluxes of heat and salt, but the modification of intermediate water is not solely driven by interior mixing. The inferred fluxes help explain the changes in temperature–salinity curves observed across each basin, and they are consistent with our understanding of the overall three-dimensional circulation of the Southern Ocean.
The fact that area-averaged diapycnal fluxes can be determined from basin-scale budgets using a suitably designed inverse model is encouraging: similar methods applied to the high-quality measurements collected during the World Ocean Circulation Experiment promise to provide the first global maps of diapycnal fluxes derived from ocean observations.
Abstract
Nine hydrographic sections are combined in an inverse box model of the Southern Ocean south of ∼12°S. The inclusion of independent diapycnal flux unknowns for each property and air–sea (heat, freshwater, and momentum) fluxes make it possible to estimate the three-dimensional deep water circulation. The authors find a vigorous 50 × 106 m3 s−1 deep overturning circulation that is dominated by an equatorward flow of Lower Circumpolar Deep Water and Antarctic Bottom Water and poleward flow of upper deep water including Indian and Pacific Deep Water below 1500 dbar. In the subtropical Indian and Pacific Oceans the deep overturning cell is essentially isolated from the thermocline and intermediate waters of the subtropical gyre. The southward flowing upper deep water shoals south of the Antarctic Circumpolar Current, where air–sea fluxes convert outcropping upper deep water to Antarctic surface water and drive a net air–sea transformation of 34 × 106 m3 s−1 to lighter intermediate water. It is the outcropping of upper deep water and transformation by air–sea fluxes that connects the deep and intermediate circulation cells. The significant poleward transport of relatively light (i.e., above all topography at the latitude of Drake Passage) upper deep water, as required here to balance lateral and diapycnal divergence and air–sea exchange, provides observational evidence that advection by standing and transient eddies carries significant meridional transport in the Southern Ocean.
Abstract
Nine hydrographic sections are combined in an inverse box model of the Southern Ocean south of ∼12°S. The inclusion of independent diapycnal flux unknowns for each property and air–sea (heat, freshwater, and momentum) fluxes make it possible to estimate the three-dimensional deep water circulation. The authors find a vigorous 50 × 106 m3 s−1 deep overturning circulation that is dominated by an equatorward flow of Lower Circumpolar Deep Water and Antarctic Bottom Water and poleward flow of upper deep water including Indian and Pacific Deep Water below 1500 dbar. In the subtropical Indian and Pacific Oceans the deep overturning cell is essentially isolated from the thermocline and intermediate waters of the subtropical gyre. The southward flowing upper deep water shoals south of the Antarctic Circumpolar Current, where air–sea fluxes convert outcropping upper deep water to Antarctic surface water and drive a net air–sea transformation of 34 × 106 m3 s−1 to lighter intermediate water. It is the outcropping of upper deep water and transformation by air–sea fluxes that connects the deep and intermediate circulation cells. The significant poleward transport of relatively light (i.e., above all topography at the latitude of Drake Passage) upper deep water, as required here to balance lateral and diapycnal divergence and air–sea exchange, provides observational evidence that advection by standing and transient eddies carries significant meridional transport in the Southern Ocean.
Abstract
Nine hydrographic sections are combined in an inverse box model of the Southern Ocean south of ∼12°S. The inverse model has two novel features: the inclusion of independent diapycnal flux unknowns for each property and the explicit inclusion of air–sea fluxes (heat, freshwater, and momentum) and the water mass transformation they drive. Transformation of 34 × 106 m3 s−1 of Antarctic Surface Water by air–sea buoyancy fluxes, and cooling and freshening where Subantarctic Mode Water outcrops, renews cold, fresh Antarctic Intermediate Water of the southeast Pacific and southwest Atlantic. Relatively cold, fresh mode and intermediate water enter the subtropical gyres, are modified by air–sea fluxes and interior mixing, and return poleward as warmer, saltier mode and intermediate water. While the zonally integrated meridional transport in these layers is small, the gross exchange is approximately 80 × 106 m3 s−1.
The air–sea transformation of Antarctic surface water to intermediate water is compensated in the Southern Ocean by an interior diapycnal flux of 32 × 106 m3 s−1 of intermediate water to upper deep water. The small property differences between slightly warmer, saltier intermediate water and cold, fresh Antarctic Surface Water results in a poleward transfer of heat and salt across the Polar Front zone.
Mode and intermediate water are crucial participants in the North Atlantic Deep Water overturning and Indonesian Throughflow circulation cells. The North Atlantic Deep Water overturning is closed by cold, fresh intermediate water that is modified to warm, salty varieties by air–sea fluxes and interior mixing in the Atlantic and southwest Indian Oceans. The Indonesian Throughflow is part of a circum-Australia circulation. In the Indian Ocean, surface water is converted to denser thermocline and mode water by air–sea fluxes and interior mixing, excess mode water flows eastward south of Australia, and air–sea fluxes convert mode water to thermocline water in the Pacific.
Abstract
Nine hydrographic sections are combined in an inverse box model of the Southern Ocean south of ∼12°S. The inverse model has two novel features: the inclusion of independent diapycnal flux unknowns for each property and the explicit inclusion of air–sea fluxes (heat, freshwater, and momentum) and the water mass transformation they drive. Transformation of 34 × 106 m3 s−1 of Antarctic Surface Water by air–sea buoyancy fluxes, and cooling and freshening where Subantarctic Mode Water outcrops, renews cold, fresh Antarctic Intermediate Water of the southeast Pacific and southwest Atlantic. Relatively cold, fresh mode and intermediate water enter the subtropical gyres, are modified by air–sea fluxes and interior mixing, and return poleward as warmer, saltier mode and intermediate water. While the zonally integrated meridional transport in these layers is small, the gross exchange is approximately 80 × 106 m3 s−1.
The air–sea transformation of Antarctic surface water to intermediate water is compensated in the Southern Ocean by an interior diapycnal flux of 32 × 106 m3 s−1 of intermediate water to upper deep water. The small property differences between slightly warmer, saltier intermediate water and cold, fresh Antarctic Surface Water results in a poleward transfer of heat and salt across the Polar Front zone.
Mode and intermediate water are crucial participants in the North Atlantic Deep Water overturning and Indonesian Throughflow circulation cells. The North Atlantic Deep Water overturning is closed by cold, fresh intermediate water that is modified to warm, salty varieties by air–sea fluxes and interior mixing in the Atlantic and southwest Indian Oceans. The Indonesian Throughflow is part of a circum-Australia circulation. In the Indian Ocean, surface water is converted to denser thermocline and mode water by air–sea fluxes and interior mixing, excess mode water flows eastward south of Australia, and air–sea fluxes convert mode water to thermocline water in the Pacific.
ABSTRACT
Observations suggest that enhanced turbulent dissipation and mixing over rough topography are modulated by the transient eddy field through the generation and breaking of lee waves in the Southern Ocean. Idealized simulations also suggest that lee waves are important in the energy pathway from eddies to turbulence. However, the energy loss from eddies due to lee wave generation remains poorly estimated. This study quantifies the relative energy loss from the time-mean and transient eddy flow in the Southern Ocean due to lee wave generation using an eddy-resolving global ocean model and three independent topographic datasets. The authors find that the energy loss from the transient eddy flow (0.12 TW; 1 TW = 1012 W) is larger than that from the time-mean flow (0.04 TW) due to lee wave generation; lee wave generation makes a larger contribution (0.12 TW) to the energy loss from the transient eddy flow than the dissipation in turbulent bottom boundary layer (0.05 TW). This study also shows that the energy loss from the time-mean flow is regulated by the transient eddy flow, and energy loss from the transient eddy flow is sensitive to the representation of anisotropy in small-scale topography. It is implied that lee waves should be parameterized in eddy-resolving global ocean models to improve the energetics of resolved flow.
ABSTRACT
Observations suggest that enhanced turbulent dissipation and mixing over rough topography are modulated by the transient eddy field through the generation and breaking of lee waves in the Southern Ocean. Idealized simulations also suggest that lee waves are important in the energy pathway from eddies to turbulence. However, the energy loss from eddies due to lee wave generation remains poorly estimated. This study quantifies the relative energy loss from the time-mean and transient eddy flow in the Southern Ocean due to lee wave generation using an eddy-resolving global ocean model and three independent topographic datasets. The authors find that the energy loss from the transient eddy flow (0.12 TW; 1 TW = 1012 W) is larger than that from the time-mean flow (0.04 TW) due to lee wave generation; lee wave generation makes a larger contribution (0.12 TW) to the energy loss from the transient eddy flow than the dissipation in turbulent bottom boundary layer (0.05 TW). This study also shows that the energy loss from the time-mean flow is regulated by the transient eddy flow, and energy loss from the transient eddy flow is sensitive to the representation of anisotropy in small-scale topography. It is implied that lee waves should be parameterized in eddy-resolving global ocean models to improve the energetics of resolved flow.
Abstract
In this study, low-frequency variability of the meridional temperature transport in the Indian Ocean is examined using a mesoscale-eddy-resolving global ocean circulation model for the period 1979–2014. The dominant empirical orthogonal function (EOF) of the meridional temperature transport is found to be highly influenced by Pacific El Niño–Southern Oscillation (ENSO) through both oceanic and atmospheric waveguides, with the southward temperature transport being stronger during La Niña and weaker during El Niño. A dynamical decomposition of the meridional streamfunction and temperature transport shows that the relative importance of different dynamic modes varies with latitude; these modes act together to contribute to the coherent ENSO response. The Ekman mode explains a larger part of low-frequency variability in overturning and temperature transport north of the equator. Between 25° and 3°S, variations associated with vertical shear mode are of greater importance. The external mode has an important contribution between 30° and 25°S where the western boundary currents impinge on topography. South of 25°S, the variability of the external mode contribution has significant negative correlations with the vertical shear mode, suggesting that the large variability of external mode depends on the joint effects of baroclinicity and topography, such that hydrographic sections alone may not be suitable for deducing changes in the meridional temperature transport at these latitudes.
Abstract
In this study, low-frequency variability of the meridional temperature transport in the Indian Ocean is examined using a mesoscale-eddy-resolving global ocean circulation model for the period 1979–2014. The dominant empirical orthogonal function (EOF) of the meridional temperature transport is found to be highly influenced by Pacific El Niño–Southern Oscillation (ENSO) through both oceanic and atmospheric waveguides, with the southward temperature transport being stronger during La Niña and weaker during El Niño. A dynamical decomposition of the meridional streamfunction and temperature transport shows that the relative importance of different dynamic modes varies with latitude; these modes act together to contribute to the coherent ENSO response. The Ekman mode explains a larger part of low-frequency variability in overturning and temperature transport north of the equator. Between 25° and 3°S, variations associated with vertical shear mode are of greater importance. The external mode has an important contribution between 30° and 25°S where the western boundary currents impinge on topography. South of 25°S, the variability of the external mode contribution has significant negative correlations with the vertical shear mode, suggesting that the large variability of external mode depends on the joint effects of baroclinicity and topography, such that hydrographic sections alone may not be suitable for deducing changes in the meridional temperature transport at these latitudes.
Abstract
Lee waves play an important role in transferring energy from the geostrophic eddy field to turbulent mixing in the Southern Ocean. As such, lee waves can impact the Southern Ocean circulation and modulate its response to changing climate through their regulation on the eddy field and turbulent mixing. The drag effect of lee waves on the eddy field and the mixing effect of lee waves on the tracer field have been studied separately to show their importance. However, it remains unclear how the drag and mixing effects act together to modify the Southern Ocean circulation. In this study, a lee-wave parameterization that includes both lee-wave drag and its associated lee-wave-driven mixing is developed and implemented in an eddy-resolving idealized model of the Southern Ocean to simulate and quantify the impacts of lee waves on the Southern Ocean circulation. The results show that lee waves enhance the baroclinic transport of the Antarctic Circumpolar Current (ACC) and strengthen the lower overturning circulation. The impact of lee waves on the large-scale circulation are explained by the control of lee-wave drag on isopycnal slopes through their effect on eddies, and by the control of lee-wave-driven mixing on deep stratification and water mass transformation. The results also show that the drag and mixing effects are coupled such that they act to weaken one another. The implication is that the future parameterization of lee waves in global ocean and climate models should take both drag and mixing effects into consideration for a more accurate representation of their impact on the ocean circulation.
Abstract
Lee waves play an important role in transferring energy from the geostrophic eddy field to turbulent mixing in the Southern Ocean. As such, lee waves can impact the Southern Ocean circulation and modulate its response to changing climate through their regulation on the eddy field and turbulent mixing. The drag effect of lee waves on the eddy field and the mixing effect of lee waves on the tracer field have been studied separately to show their importance. However, it remains unclear how the drag and mixing effects act together to modify the Southern Ocean circulation. In this study, a lee-wave parameterization that includes both lee-wave drag and its associated lee-wave-driven mixing is developed and implemented in an eddy-resolving idealized model of the Southern Ocean to simulate and quantify the impacts of lee waves on the Southern Ocean circulation. The results show that lee waves enhance the baroclinic transport of the Antarctic Circumpolar Current (ACC) and strengthen the lower overturning circulation. The impact of lee waves on the large-scale circulation are explained by the control of lee-wave drag on isopycnal slopes through their effect on eddies, and by the control of lee-wave-driven mixing on deep stratification and water mass transformation. The results also show that the drag and mixing effects are coupled such that they act to weaken one another. The implication is that the future parameterization of lee waves in global ocean and climate models should take both drag and mixing effects into consideration for a more accurate representation of their impact on the ocean circulation.
Abstract
Mean meridional upper-ocean temperature, salinity, and zonal velocity sections across the Pacific Ocean between 8°S and 8°N are combined with other oceanographic and air–sea flux data in an inverse model. The tropical Pacific Ocean can be divided into three regions with distinct circulation patterns: western (143°E–170°W), central (170°–125°W), and eastern (125°W–eastern boundary). In the central and eastern Pacific the downward limbs of the shallow tropical cells are 15(±13) × 106 m3 s−1 in the north and 20(±11) × 106 m3 s−1 in the south. The Pacific cold tongue in the eastern region results from diapycnal upwelling through all layers of the Equatorial Undercurrent, which preferentially exhausts the lightest (warmer) layers of the Equatorial Undercurrent [10(±6) × 106 m3 s−1] between 125° and 95°W, allowing the denser (cooler) layers to upwell [9(±4) × 106 m3 s−1] east of 95°W and adjacent to the American coast. An interhemispheric exchange of 13(±13) × 106 m3 s−1 between the southern and northern Pacific Ocean forms the Pacific branch of the Pacific–Indian interbasin exchange. Southern Hemisphere water enters the tropical Pacific Ocean via the direct route at the western boundary and via an interior (basin) pathway. However, this water moves irreversibly into the North Pacific by upwelling in the eastern equatorial Pacific and air–sea transformation that drives poleward interior transport across 2°N.
Abstract
Mean meridional upper-ocean temperature, salinity, and zonal velocity sections across the Pacific Ocean between 8°S and 8°N are combined with other oceanographic and air–sea flux data in an inverse model. The tropical Pacific Ocean can be divided into three regions with distinct circulation patterns: western (143°E–170°W), central (170°–125°W), and eastern (125°W–eastern boundary). In the central and eastern Pacific the downward limbs of the shallow tropical cells are 15(±13) × 106 m3 s−1 in the north and 20(±11) × 106 m3 s−1 in the south. The Pacific cold tongue in the eastern region results from diapycnal upwelling through all layers of the Equatorial Undercurrent, which preferentially exhausts the lightest (warmer) layers of the Equatorial Undercurrent [10(±6) × 106 m3 s−1] between 125° and 95°W, allowing the denser (cooler) layers to upwell [9(±4) × 106 m3 s−1] east of 95°W and adjacent to the American coast. An interhemispheric exchange of 13(±13) × 106 m3 s−1 between the southern and northern Pacific Ocean forms the Pacific branch of the Pacific–Indian interbasin exchange. Southern Hemisphere water enters the tropical Pacific Ocean via the direct route at the western boundary and via an interior (basin) pathway. However, this water moves irreversibly into the North Pacific by upwelling in the eastern equatorial Pacific and air–sea transformation that drives poleward interior transport across 2°N.
Abstract
The East Australian Current (EAC) is the complex and highly energetic poleward western boundary current of the South Pacific Ocean. A full-depth current meter and property (temperature and salinity) mooring array was deployed from the continental shelf to the abyssal waters off Brisbane Australia (27°S) for 18 months from April 2012 to August 2013. The EAC mooring array is an essential component of the Australian Integrated Marine Observing System (IMOS). During this period the EAC was coherent with an eddy kinetic to mean kinetic energy ratio of less than 1. The 18-month, mean, poleward-only mass transport above 2000 m is 22.1 ± 7.5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1). The mean, poleward-only heat transport and flow-weighted temperature above 2000 m are −1.35 ± 0.42 PW and 15.33°C, respectively. A difference in the poleward-only and net poleward mass and heat transports above 2000 m of 6.3 Sv and 0.24 PW reflects the presence of an equatorward EAC retroflection at the eastern (offshore) end of the mooring array. A complex empirical orthogonal function (EOF) analysis of the along-slope velocity anomalies finds that the first two modes explain 72.1% of the velocity variance. Mode 1 is dominant at periods of approximately 60 days, and mode 2 is dominant at periods of 120 days. These dominant periods agree with previous studies in the Tasman Sea south of 27°S and suggest that variability of the EAC in the Tasman Sea may be linked to variability north of 27°S.
Abstract
The East Australian Current (EAC) is the complex and highly energetic poleward western boundary current of the South Pacific Ocean. A full-depth current meter and property (temperature and salinity) mooring array was deployed from the continental shelf to the abyssal waters off Brisbane Australia (27°S) for 18 months from April 2012 to August 2013. The EAC mooring array is an essential component of the Australian Integrated Marine Observing System (IMOS). During this period the EAC was coherent with an eddy kinetic to mean kinetic energy ratio of less than 1. The 18-month, mean, poleward-only mass transport above 2000 m is 22.1 ± 7.5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1). The mean, poleward-only heat transport and flow-weighted temperature above 2000 m are −1.35 ± 0.42 PW and 15.33°C, respectively. A difference in the poleward-only and net poleward mass and heat transports above 2000 m of 6.3 Sv and 0.24 PW reflects the presence of an equatorward EAC retroflection at the eastern (offshore) end of the mooring array. A complex empirical orthogonal function (EOF) analysis of the along-slope velocity anomalies finds that the first two modes explain 72.1% of the velocity variance. Mode 1 is dominant at periods of approximately 60 days, and mode 2 is dominant at periods of 120 days. These dominant periods agree with previous studies in the Tasman Sea south of 27°S and suggest that variability of the EAC in the Tasman Sea may be linked to variability north of 27°S.
Abstract
The strength and structure of the Southern Hemisphere meridional overturning circulation (SMOC) is related to the along-isopycnal and vertical mixing coefficients by analyzing tracer and density fields from a hydrographic climatology. The meridional transport of Upper Circumpolar Deep Water (UCDW) across the Antarctic Circumpolar Current (ACC) is expressed in terms of the along-isopycnal (K) and diapycnal (D) tracer diffusivities and in terms of the along-isopycnal potential vorticity mixing coefficient (K PV). Uniform along-isopycnal (<600 m2 s−1) and low vertical mixing (10−5 m2 s−1) can maintain a southward transport of less than 60 Sv (Sv = 106 m2 s−1) of UCDW across the ACC, which is distributed largely across the South Pacific and east Indian Ocean basins. For vertical mixing rates of O(10−4 m2 s−1) or greater, the inferred transport is significantly enhanced. The transports inferred from both tracer and density distributions suggest a ratio K to D of O(2 × 106) particularly on deeper layers of UCDW. Given the range of observed southward transports of UCDW, it is found that K = 300 ± 150 m2 s−1 and D = 10−4 ± 0.5 × 10−4 m2 s−1 in the Southern Ocean interior. A view of the SMOC is revealed where dense waters are converted to lighter waters not only at the ocean surface, but also on depths below that of the mixed layer with vertical mixing playing an important role.
Abstract
The strength and structure of the Southern Hemisphere meridional overturning circulation (SMOC) is related to the along-isopycnal and vertical mixing coefficients by analyzing tracer and density fields from a hydrographic climatology. The meridional transport of Upper Circumpolar Deep Water (UCDW) across the Antarctic Circumpolar Current (ACC) is expressed in terms of the along-isopycnal (K) and diapycnal (D) tracer diffusivities and in terms of the along-isopycnal potential vorticity mixing coefficient (K PV). Uniform along-isopycnal (<600 m2 s−1) and low vertical mixing (10−5 m2 s−1) can maintain a southward transport of less than 60 Sv (Sv = 106 m2 s−1) of UCDW across the ACC, which is distributed largely across the South Pacific and east Indian Ocean basins. For vertical mixing rates of O(10−4 m2 s−1) or greater, the inferred transport is significantly enhanced. The transports inferred from both tracer and density distributions suggest a ratio K to D of O(2 × 106) particularly on deeper layers of UCDW. Given the range of observed southward transports of UCDW, it is found that K = 300 ± 150 m2 s−1 and D = 10−4 ± 0.5 × 10−4 m2 s−1 in the Southern Ocean interior. A view of the SMOC is revealed where dense waters are converted to lighter waters not only at the ocean surface, but also on depths below that of the mixed layer with vertical mixing playing an important role.
