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Abstract
Two moored arrays deployed in the Romanche Fracture Zone and Chain Fracture Zone in the equatorial Atlantic Ocean provide two-year-long time series of current and temperature in the Lower North Atlantic Deep Water and the Antarctic Bottom Water. Total time-averaged transport of Antarctic Bottom Water (potential temperature θ < 1.9°C) across the Mid-Atlantic Ridge amounts to 1.22 × 106 m3 s−1 eastward with a standard deviation of ±0.25 × 106 m3 s−1. A time-averaged transport of 0.36(± 0.23) × 106 m3 s−1 eastward is found for the Lower North Atlantic Deep Water in the 1.9° < θ < 2.1°C temperature range, but this may represent only a fraction of the total flow of this water mass across the ridge. Contributions of the Romanche Fracture Zone and Chain Fracture Zone to the Antarctic Bottom Water transport are similar, while the Chain Fracture Zone has the greater share of Lower North Atlantic Deep Water transport. Semiannual and annual periods are detected in the transport time series and together explain 24% of the Antarctic Bottom Water transport variance in the Romanche Fracture Zone. In the Chain Fracture Zone, Antarctic Bottom Water transport variance is dominated by fluctuations in the period band 10–20 days.
Abstract
Two moored arrays deployed in the Romanche Fracture Zone and Chain Fracture Zone in the equatorial Atlantic Ocean provide two-year-long time series of current and temperature in the Lower North Atlantic Deep Water and the Antarctic Bottom Water. Total time-averaged transport of Antarctic Bottom Water (potential temperature θ < 1.9°C) across the Mid-Atlantic Ridge amounts to 1.22 × 106 m3 s−1 eastward with a standard deviation of ±0.25 × 106 m3 s−1. A time-averaged transport of 0.36(± 0.23) × 106 m3 s−1 eastward is found for the Lower North Atlantic Deep Water in the 1.9° < θ < 2.1°C temperature range, but this may represent only a fraction of the total flow of this water mass across the ridge. Contributions of the Romanche Fracture Zone and Chain Fracture Zone to the Antarctic Bottom Water transport are similar, while the Chain Fracture Zone has the greater share of Lower North Atlantic Deep Water transport. Semiannual and annual periods are detected in the transport time series and together explain 24% of the Antarctic Bottom Water transport variance in the Romanche Fracture Zone. In the Chain Fracture Zone, Antarctic Bottom Water transport variance is dominated by fluctuations in the period band 10–20 days.
Abstract
Antarctic Bottom Water flows into the western North Atlantic across the equator, shifting from the western side to the eastern side of the trough between the American continents and the Mid-Atlantic Ridge as it continues north. This is puzzling because such large-scale motion is thought to be controlled by dynamics that disallows an eastern boundary current. Previous explanations for the transposition involve a (necessarily small-scale) density current that changes sides because of the change in sign of rotation across the equator, or a topographic effect that changes the sign of the effective mean vorticity gradient and thus requires an eastern boundary current. Here an alternative explanation for the overall structure of bottom flow is given.
A source of mass to a thin bottom layer is assumed to upwell uniformly across its interface into a less dense layer at rest. A simple formula for the magnitude of the upwelling and thickness of the layer is derived that depends on the source strength to the bottom layer. For a strong enough source, the bottom layer thickness is zero along a grounding curve that separates the bottom water from the western boundary and confines it to the east. A band of recirculating interior flow occurs, supplied by an isolated northern and western boundary current. Similar structures appear to exist in the Antarctic Bottom Water of the western North Atlantic.
Abstract
Antarctic Bottom Water flows into the western North Atlantic across the equator, shifting from the western side to the eastern side of the trough between the American continents and the Mid-Atlantic Ridge as it continues north. This is puzzling because such large-scale motion is thought to be controlled by dynamics that disallows an eastern boundary current. Previous explanations for the transposition involve a (necessarily small-scale) density current that changes sides because of the change in sign of rotation across the equator, or a topographic effect that changes the sign of the effective mean vorticity gradient and thus requires an eastern boundary current. Here an alternative explanation for the overall structure of bottom flow is given.
A source of mass to a thin bottom layer is assumed to upwell uniformly across its interface into a less dense layer at rest. A simple formula for the magnitude of the upwelling and thickness of the layer is derived that depends on the source strength to the bottom layer. For a strong enough source, the bottom layer thickness is zero along a grounding curve that separates the bottom water from the western boundary and confines it to the east. A band of recirculating interior flow occurs, supplied by an isolated northern and western boundary current. Similar structures appear to exist in the Antarctic Bottom Water of the western North Atlantic.
Abstract
The total transport of Antarctic Bottom Water across the Rio Grande Rise, including the western boundary, the Vema Channel, and the Hunter Channel is estimated from hydrographic measurements across these pathways. The contribution of the Vema Channel is greatest at 3.9 × 106 m3 s−1, which is very close to earlier estimates. The western boundary current contribution is 2.0 × 106 m3 s−1 and that of the Hunter Channel 0.7 × 106 m3 s−1. The lower values outside the Vema Channel are offset by the important source of mass they form to the lower density classes of bottom water. About 40% of the flow is concentrated in the highest density class representing the source of Weddell Sea Deep Water to the Brazil Basin. The flow structure is characterized by horizontal and vertical recirculation.
Abstract
The total transport of Antarctic Bottom Water across the Rio Grande Rise, including the western boundary, the Vema Channel, and the Hunter Channel is estimated from hydrographic measurements across these pathways. The contribution of the Vema Channel is greatest at 3.9 × 106 m3 s−1, which is very close to earlier estimates. The western boundary current contribution is 2.0 × 106 m3 s−1 and that of the Hunter Channel 0.7 × 106 m3 s−1. The lower values outside the Vema Channel are offset by the important source of mass they form to the lower density classes of bottom water. About 40% of the flow is concentrated in the highest density class representing the source of Weddell Sea Deep Water to the Brazil Basin. The flow structure is characterized by horizontal and vertical recirculation.
Abstract
Surface heat and freshwater fluxes from the Comprehensive 0cean-Atmosphere Data Set are revised and used diagnostically to compute air-sea transformation rates on density, temperature, and salinity classes over the domain of the data. Maximum rates occur over the warmest water and over mode waters, which are the dominant result of air-sea interaction. Transformation in different is accordingly distinguished by temperature and salinity, just as water masses in different oceans are so distinguished. Over the entire domain, to about 30°S, approximately 80×106 m3 s−1 of warm cool water are transformed by air-sea fluxes, on annual average. Calculations for several seas in the North Atlantic, where deep water is thought to originate, we also presented.
Abstract
Surface heat and freshwater fluxes from the Comprehensive 0cean-Atmosphere Data Set are revised and used diagnostically to compute air-sea transformation rates on density, temperature, and salinity classes over the domain of the data. Maximum rates occur over the warmest water and over mode waters, which are the dominant result of air-sea interaction. Transformation in different is accordingly distinguished by temperature and salinity, just as water masses in different oceans are so distinguished. Over the entire domain, to about 30°S, approximately 80×106 m3 s−1 of warm cool water are transformed by air-sea fluxes, on annual average. Calculations for several seas in the North Atlantic, where deep water is thought to originate, we also presented.
Abstract
A laboratory experiment of multiple baroclinic zonal jets is described, thought to be dynamically similar to flow observed in the Antarctic Circumpolar Current. Differential heating sets the overall temperature difference and drives unstable baroclinic flow, but the circulation is free to determine its own structure and local stratification; experiments were run to a stationary state and extend the dynamical regime of previous experiments. A topographic analog to the planetary β effect is imposed by the gradient of fluid depth with radius supplied by a sloping bottom and a parabolic free surface. New regimes of a low thermal Rossby number (Ro T ~ 10−3) and high Taylor number (Ta ~ 1011) are explored such that the deformation radius L ρ is much smaller than the annulus gap width L and similar to the Rhines length. Multiple jets emerge in rough proportion to the smallness of the Rhines scale, relatively insensitive to the Taylor number; a regime diagram taking the β effect into account better reflects the emergence of the jets. Eddy momentum fluxes are consistent with an active role in maintaining the jets, and jet development appears to follow the Vallis and Maltrud phenomenology of anisotropic wave–turbulence interaction on a β plane. Intermittency and episodes of coherent meridional jet migration occur, especially during spinup.
Abstract
A laboratory experiment of multiple baroclinic zonal jets is described, thought to be dynamically similar to flow observed in the Antarctic Circumpolar Current. Differential heating sets the overall temperature difference and drives unstable baroclinic flow, but the circulation is free to determine its own structure and local stratification; experiments were run to a stationary state and extend the dynamical regime of previous experiments. A topographic analog to the planetary β effect is imposed by the gradient of fluid depth with radius supplied by a sloping bottom and a parabolic free surface. New regimes of a low thermal Rossby number (Ro T ~ 10−3) and high Taylor number (Ta ~ 1011) are explored such that the deformation radius L ρ is much smaller than the annulus gap width L and similar to the Rhines length. Multiple jets emerge in rough proportion to the smallness of the Rhines scale, relatively insensitive to the Taylor number; a regime diagram taking the β effect into account better reflects the emergence of the jets. Eddy momentum fluxes are consistent with an active role in maintaining the jets, and jet development appears to follow the Vallis and Maltrud phenomenology of anisotropic wave–turbulence interaction on a β plane. Intermittency and episodes of coherent meridional jet migration occur, especially during spinup.
Abstract
This paper describes a quasi-3D Bayesian inversion of oceanographic tracer data from the South Atlantic Ocean. Initially, one active neutral-density layer is considered with an upper and lower boundary. The available hydrographic data are linked to model parameters (water velocities, diffusion coefficients) via a 3D advection–diffusion equation. A robust solution to the inverse problem can be obtained by introducing prior information about parameters and modeling the observation error. This approach estimates both horizontal and vertical flow as well as diffusion coefficients. A system of alternating zonal jets is found at the depths of the North Atlantic Deep Water, consistent with direct measurements of flow and concentration maps. A uniqueness analysis of the model is performed in terms of the oxygen consumption rate. The vertical mixing coefficient bears some relation to the bottom topography even though the authors do not incorporate topography into their model. The method is extended to a multilayer model, using thermal wind relations weakly in a local fashion (as opposed to integrating the entire water column) to connect layers vertically. Results suggest that the estimated deep zonal jets extend vertically, with a clear depth-dependent structure. The vertical structure of the flow field is modified by the tracer fields relative to the a priori flow field defined by thermal wind. The velocity estimates are consistent with independent observed flow at the depths of the Antarctic Intermediate Water; at still shallower depths, above the layers considered here, the subtropical gyre is a significant feature of the horizontal flow.
Abstract
This paper describes a quasi-3D Bayesian inversion of oceanographic tracer data from the South Atlantic Ocean. Initially, one active neutral-density layer is considered with an upper and lower boundary. The available hydrographic data are linked to model parameters (water velocities, diffusion coefficients) via a 3D advection–diffusion equation. A robust solution to the inverse problem can be obtained by introducing prior information about parameters and modeling the observation error. This approach estimates both horizontal and vertical flow as well as diffusion coefficients. A system of alternating zonal jets is found at the depths of the North Atlantic Deep Water, consistent with direct measurements of flow and concentration maps. A uniqueness analysis of the model is performed in terms of the oxygen consumption rate. The vertical mixing coefficient bears some relation to the bottom topography even though the authors do not incorporate topography into their model. The method is extended to a multilayer model, using thermal wind relations weakly in a local fashion (as opposed to integrating the entire water column) to connect layers vertically. Results suggest that the estimated deep zonal jets extend vertically, with a clear depth-dependent structure. The vertical structure of the flow field is modified by the tracer fields relative to the a priori flow field defined by thermal wind. The velocity estimates are consistent with independent observed flow at the depths of the Antarctic Intermediate Water; at still shallower depths, above the layers considered here, the subtropical gyre is a significant feature of the horizontal flow.
Abstract
Transports across 48°N in the Atlantic Ocean are estimated from five repeat World Ocean Circulation Experiment (WOCE) hydrographic lines collected in this region in 1993–2000, from time-varying air–sea heat and freshwater fluxes north of 48°N, and from a synthesis of these two data sources using inverse box model methods.
Results from hydrography and air–sea fluxes treated separately are analogous to recently published transport variation studies and demonstrate the sensitivity of the results to either the choice of reference level and reference velocities for thermal wind calculations or the specific flux dataset chosen. In addition, flux-based calculations do not include the effects of subsurface mixing on overturning and transports of specific water masses. The inverse model approach was used to find unknown depth-independent velocities, interior diapycnal fluxes, and adjustments to air–sea fluxes subject to various constraints on the system. Various model choices were made to focus on annually averaged results, as opposed to instantaneous values during the occupation of the hydrographic lines. The results reflect the constraints and choices made in the construction of the model.
The inverse model solutions show only marginal, not significantly different temporal changes in the net overturning cell strength and heat transport across 48°N. These small changes are similar to seasonally or annually averaged numerical model simulations of overturning. Significant variability is found for deep transports and air–sea flux quantities in density layers. Put another way, if one ignores the details of layer exchanges, the model can be constrained to produce the same net overturning for each repeat line; however, constraining individual layers to have the same transport for each line fails.
Diapycnal fluxes are found to be important in the mean but are relatively constant from one repeat line to the next. Mean air–sea fluxes are modified slightly but are still essentially consistent with either the NCEP data or the National Oceanography Centre, Southampton (NOC) Comprehensive Ocean–Atmosphere Data Set (COADS) within error. Modest reductions in air–sea flux uncertainties would give these constraints a much greater impact. Direct transport estimates over broader regions than the western boundary North Atlantic Current are needed to help resolve circulation structure that is important for variability in net overturning.
Abstract
Transports across 48°N in the Atlantic Ocean are estimated from five repeat World Ocean Circulation Experiment (WOCE) hydrographic lines collected in this region in 1993–2000, from time-varying air–sea heat and freshwater fluxes north of 48°N, and from a synthesis of these two data sources using inverse box model methods.
Results from hydrography and air–sea fluxes treated separately are analogous to recently published transport variation studies and demonstrate the sensitivity of the results to either the choice of reference level and reference velocities for thermal wind calculations or the specific flux dataset chosen. In addition, flux-based calculations do not include the effects of subsurface mixing on overturning and transports of specific water masses. The inverse model approach was used to find unknown depth-independent velocities, interior diapycnal fluxes, and adjustments to air–sea fluxes subject to various constraints on the system. Various model choices were made to focus on annually averaged results, as opposed to instantaneous values during the occupation of the hydrographic lines. The results reflect the constraints and choices made in the construction of the model.
The inverse model solutions show only marginal, not significantly different temporal changes in the net overturning cell strength and heat transport across 48°N. These small changes are similar to seasonally or annually averaged numerical model simulations of overturning. Significant variability is found for deep transports and air–sea flux quantities in density layers. Put another way, if one ignores the details of layer exchanges, the model can be constrained to produce the same net overturning for each repeat line; however, constraining individual layers to have the same transport for each line fails.
Diapycnal fluxes are found to be important in the mean but are relatively constant from one repeat line to the next. Mean air–sea fluxes are modified slightly but are still essentially consistent with either the NCEP data or the National Oceanography Centre, Southampton (NOC) Comprehensive Ocean–Atmosphere Data Set (COADS) within error. Modest reductions in air–sea flux uncertainties would give these constraints a much greater impact. Direct transport estimates over broader regions than the western boundary North Atlantic Current are needed to help resolve circulation structure that is important for variability in net overturning.
Abstract
Stirring in the subsurface Southern Ocean is examined using RAFOS float trajectories, collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), along with particle trajectories from a regional eddy permitting model. A central question is the extent to which the stirring is local, by eddies comparable in size to the pair separation, or nonlocal, by eddies at larger scales. To test this, we examine metrics based on averaging in time and in space. The model particles exhibit nonlocal dispersion, as expected for a limited resolution numerical model that does not resolve flows at scales smaller than ~10 days or ~20–30 km. The different metrics are less consistent for the RAFOS floats; relative dispersion, kurtosis, and relative diffusivity suggest nonlocal dispersion as they are consistent with the model within error, while finite-size Lyapunov exponents (FSLE) suggests local dispersion. This occurs for two reasons: (i) limited sampling of the inertial length scales and a relatively small number of pairs hinder statistical robustness in time-based metrics, and (ii) some space-based metrics (FSLE, second-order structure functions), which do not average over wave motions and are reflective of the kinetic energy distribution, are probably unsuitable to infer dispersion characteristics if the flow field includes energetic wave motions that do not disperse particles. The relative diffusivity, which is also a space-based metric, allows averaging over waves to infer the dispersion characteristics. Hence, given the error characteristics of the metrics and data used here, the stirring in the DIMES region is likely to be nonlocal at scales of 5–100 km.
Abstract
Stirring in the subsurface Southern Ocean is examined using RAFOS float trajectories, collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), along with particle trajectories from a regional eddy permitting model. A central question is the extent to which the stirring is local, by eddies comparable in size to the pair separation, or nonlocal, by eddies at larger scales. To test this, we examine metrics based on averaging in time and in space. The model particles exhibit nonlocal dispersion, as expected for a limited resolution numerical model that does not resolve flows at scales smaller than ~10 days or ~20–30 km. The different metrics are less consistent for the RAFOS floats; relative dispersion, kurtosis, and relative diffusivity suggest nonlocal dispersion as they are consistent with the model within error, while finite-size Lyapunov exponents (FSLE) suggests local dispersion. This occurs for two reasons: (i) limited sampling of the inertial length scales and a relatively small number of pairs hinder statistical robustness in time-based metrics, and (ii) some space-based metrics (FSLE, second-order structure functions), which do not average over wave motions and are reflective of the kinetic energy distribution, are probably unsuitable to infer dispersion characteristics if the flow field includes energetic wave motions that do not disperse particles. The relative diffusivity, which is also a space-based metric, allows averaging over waves to infer the dispersion characteristics. Hence, given the error characteristics of the metrics and data used here, the stirring in the DIMES region is likely to be nonlocal at scales of 5–100 km.
Abstract
Float trajectories are compared with the distribution of climatological potential vorticity, Q, on approximate isentropic surfaces for intermediate waters in the North Atlantic. The time-mean displacement and eddy dispersion are calculated for clusters of floats in terms of their movement along and across Q contours. For float clusters with significant mean velocities, the mean flow crosses Q contours at an angle of typically less than 20°–30° in magnitude in the ocean interior. The implied Peclet number in the ocean interior ranges from 1 to 19 with a weighted-mean value of 4.4. This mean Peclet number suggests that there is significant eddy mixing in the ocean interior: tracers should only be quasi-conserved along mean streamlines over a subbasin scale, rather than over an entire basin. The mean flow also strongly crosses Q contours near the western boundary in the Tropics, where the implied Peclet number is 0.7; this value may be a lower bound as Q contours are assumed to be zonal and relative vorticity is ignored. Float clusters with a lifetime greater than 200 days show anisotropic dispersion with greater dispersion along Q contours, than across them; float clusters with shorter lifetimes are ambiguous. This anisotropic dispersion along Q contours cannot generally be distinguished from enhanced dispersion along latitude circles since Q contours are generally zonal for these cases. However, for the null case of uniform Q for the Gulf Stream at 2000 m, there is strong isotropic dispersion, rather than enhanced zonal dispersion. In summary, diagnostics suggest that floats preferentially spread along Q contours over a subbasin scale and imply that passive tracers should likewise preferentially spread along Q contours in the ocean interior.
Abstract
Float trajectories are compared with the distribution of climatological potential vorticity, Q, on approximate isentropic surfaces for intermediate waters in the North Atlantic. The time-mean displacement and eddy dispersion are calculated for clusters of floats in terms of their movement along and across Q contours. For float clusters with significant mean velocities, the mean flow crosses Q contours at an angle of typically less than 20°–30° in magnitude in the ocean interior. The implied Peclet number in the ocean interior ranges from 1 to 19 with a weighted-mean value of 4.4. This mean Peclet number suggests that there is significant eddy mixing in the ocean interior: tracers should only be quasi-conserved along mean streamlines over a subbasin scale, rather than over an entire basin. The mean flow also strongly crosses Q contours near the western boundary in the Tropics, where the implied Peclet number is 0.7; this value may be a lower bound as Q contours are assumed to be zonal and relative vorticity is ignored. Float clusters with a lifetime greater than 200 days show anisotropic dispersion with greater dispersion along Q contours, than across them; float clusters with shorter lifetimes are ambiguous. This anisotropic dispersion along Q contours cannot generally be distinguished from enhanced dispersion along latitude circles since Q contours are generally zonal for these cases. However, for the null case of uniform Q for the Gulf Stream at 2000 m, there is strong isotropic dispersion, rather than enhanced zonal dispersion. In summary, diagnostics suggest that floats preferentially spread along Q contours over a subbasin scale and imply that passive tracers should likewise preferentially spread along Q contours in the ocean interior.
Abstract
This study uses potential vorticity and other tracers to identify the pathways of the densest form of Circumpolar Deep Water in the South Pacific, termed “Southwest Pacific Bottom Water” (SPBW), along the 28.2 kg m−3 surface. This study focuses on the potential vorticity signals associated with three major dynamical processes occurring in the vicinity of the Pacific–Antarctic Ridge: 1) the strong flow of the Antarctic Circumpolar Current (ACC), 2) lateral eddy stirring, and 3) heat and stratification changes in bottom waters induced by hydrothermal vents. These processes result in southward and downstream advection of low potential vorticity along rising isopycnal surfaces. Using δ 3He released from the hydrothermal vents, the influence of volcanic activity on the SPBW may be traced across the South Pacific along the path of the ACC to Drake Passage. SPBW also flows within the southern limb of the Ross Gyre, reaching the Antarctic Slope in places and contributes via entrainment to the formation of Antarctic Bottom Water. Finally, it is shown that the magnitude and location of the potential vorticity signals associated with SPBW have endured over at least the last two decades, and that they are unique to the South Pacific sector.
Abstract
This study uses potential vorticity and other tracers to identify the pathways of the densest form of Circumpolar Deep Water in the South Pacific, termed “Southwest Pacific Bottom Water” (SPBW), along the 28.2 kg m−3 surface. This study focuses on the potential vorticity signals associated with three major dynamical processes occurring in the vicinity of the Pacific–Antarctic Ridge: 1) the strong flow of the Antarctic Circumpolar Current (ACC), 2) lateral eddy stirring, and 3) heat and stratification changes in bottom waters induced by hydrothermal vents. These processes result in southward and downstream advection of low potential vorticity along rising isopycnal surfaces. Using δ 3He released from the hydrothermal vents, the influence of volcanic activity on the SPBW may be traced across the South Pacific along the path of the ACC to Drake Passage. SPBW also flows within the southern limb of the Ross Gyre, reaching the Antarctic Slope in places and contributes via entrainment to the formation of Antarctic Bottom Water. Finally, it is shown that the magnitude and location of the potential vorticity signals associated with SPBW have endured over at least the last two decades, and that they are unique to the South Pacific sector.
