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- Author or Editor: Michel Arhan x
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Abstract
Hydrographic surveys carried out in 1983–84 along both sides of the Mid-Atlantic Ridge between 24° and 53°N provide a detailed description of the well-known North Atlantic water masses with particular emphasis on their meridional distribution and zonal dissymetry. In the upper layers the dense horizontal sampling resolves the several narrow Gulf Stream extensions into the ocean interior, giving the image, in the Central Water density range, of a mosaïc of mode waters separated by fronts. At intermediate depths a vertical shear in the distributions of the Mediterranean Water and Labrador Sea Water stands out, both water masses having their lower part displaced southwards relative to their upper parts. Bottom waters containing nearly 20 percent of pure Antarctic Bottom Water are observed at 50°N in the eastern basin, in contrast with the western basin where proportions greater than 10 percent were found only south of 36°N along our section. This water mass analysis also gives indications that strong mixing occurs at several water mass boundaries: between Subarctic Intermediate Water and (i) North Atlantic Central Water, (ii) Mediterranean Water, and between Antarctic Bottom Water and the overlying waters.
Abstract
Hydrographic surveys carried out in 1983–84 along both sides of the Mid-Atlantic Ridge between 24° and 53°N provide a detailed description of the well-known North Atlantic water masses with particular emphasis on their meridional distribution and zonal dissymetry. In the upper layers the dense horizontal sampling resolves the several narrow Gulf Stream extensions into the ocean interior, giving the image, in the Central Water density range, of a mosaïc of mode waters separated by fronts. At intermediate depths a vertical shear in the distributions of the Mediterranean Water and Labrador Sea Water stands out, both water masses having their lower part displaced southwards relative to their upper parts. Bottom waters containing nearly 20 percent of pure Antarctic Bottom Water are observed at 50°N in the eastern basin, in contrast with the western basin where proportions greater than 10 percent were found only south of 36°N along our section. This water mass analysis also gives indications that strong mixing occurs at several water mass boundaries: between Subarctic Intermediate Water and (i) North Atlantic Central Water, (ii) Mediterranean Water, and between Antarctic Bottom Water and the overlying waters.
Abstract
A mechanism is proposed, based on the assumption of ventilation, to explain the middepth northward flow observed in the North Atlantic. The main feature of the solution is that the outcropping line of an intermediate layer at high latitudes is uniquely specified by the condition that the middepth waters of the subtropical gyre may be “sucked up” by the positive Ekman pumping of the subpolar gyre.
This model is consistent with several characteristic features of the circulation in that region such as its vertical structure, the greater northward extension of the warm waters in the eastern ocean and the shape of the large scale Mediterranean water plume. In a variation of the model, the observed isopycnal slopes along the eastern coast are included in the analysis; this implies the existence of eastern boundary layers.
Abstract
A mechanism is proposed, based on the assumption of ventilation, to explain the middepth northward flow observed in the North Atlantic. The main feature of the solution is that the outcropping line of an intermediate layer at high latitudes is uniquely specified by the condition that the middepth waters of the subtropical gyre may be “sucked up” by the positive Ekman pumping of the subpolar gyre.
This model is consistent with several characteristic features of the circulation in that region such as its vertical structure, the greater northward extension of the warm waters in the eastern ocean and the shape of the large scale Mediterranean water plume. In a variation of the model, the observed isopycnal slopes along the eastern coast are included in the analysis; this implies the existence of eastern boundary layers.
Abstract
A Lagrangian one-dimensional model is used to study the subduction of Subpolar Mode Water in the eastern North Atlantic and to analyze recently observed hydrological features related to this process. Considering a southward moving column of mode water, subduction occurs when the upper part of the column starts gaining buoyancy in an annual budget. The seasonal pycnocline on top of the column can no longer be completely eroded in winter, and it is shown how its remnant forms a shallow “secondary” pycnocline at about 200 m depth, which isolates the lower part of the fluid column from the atmosphere. This mechanism for the subduction of Subpolar Mode Water induces a strong meridional gradient in the winter mixed layer depth.
The various components of the buoyancy input are thoroughly studied. Horizontal advection in the shallow Ekman layer and vertical advection along the column are shown to modify significantly both the thermal and haline contents of the column, with magnitudes comparable to the air–sea exchanges.
The processes that control the temperature–salinity relation of the model fluid column are studied. It appears that if horizontal baroclinic advection can be held as negligible, a good correlation between the annual inputs of heat and salt to the winter mixed layer has to exist to account for the quasi-linear T–S relation prevailing in the Central Water. Vertical mixing by turbulent processes, including double diffusion, is seen to cause only a limited rearrangement of the relation defined at the surface.
Finally, comparing the location of the different types of mode water in the subtropical North Atlantic, that of a climatological line of zero buoyancy flux assumed to drive subduction, and the general circulation pattern in the upper layers, shows a good consistency and supports the conclusions of this study.
Abstract
A Lagrangian one-dimensional model is used to study the subduction of Subpolar Mode Water in the eastern North Atlantic and to analyze recently observed hydrological features related to this process. Considering a southward moving column of mode water, subduction occurs when the upper part of the column starts gaining buoyancy in an annual budget. The seasonal pycnocline on top of the column can no longer be completely eroded in winter, and it is shown how its remnant forms a shallow “secondary” pycnocline at about 200 m depth, which isolates the lower part of the fluid column from the atmosphere. This mechanism for the subduction of Subpolar Mode Water induces a strong meridional gradient in the winter mixed layer depth.
The various components of the buoyancy input are thoroughly studied. Horizontal advection in the shallow Ekman layer and vertical advection along the column are shown to modify significantly both the thermal and haline contents of the column, with magnitudes comparable to the air–sea exchanges.
The processes that control the temperature–salinity relation of the model fluid column are studied. It appears that if horizontal baroclinic advection can be held as negligible, a good correlation between the annual inputs of heat and salt to the winter mixed layer has to exist to account for the quasi-linear T–S relation prevailing in the Central Water. Vertical mixing by turbulent processes, including double diffusion, is seen to cause only a limited rearrangement of the relation defined at the surface.
Finally, comparing the location of the different types of mode water in the subtropical North Atlantic, that of a climatological line of zero buoyancy flux assumed to drive subduction, and the general circulation pattern in the upper layers, shows a good consistency and supports the conclusions of this study.
Abstract
This paper investigates the behavior of a surface-intensified anticyclone encountering a seamount on the β plane in a stratified ocean. The eddy may be strongly eroded, and sometimes subdivided, provided that it gets close enough to the seamount. In case of subdivision, the detached part has a vertical structure different from that of the initial eddy, and a subsurface vortex may result. The basic erosion mechanism previously observed with f-plane experiments is still active on the β plane. Deep fluid motions induced by the initial vortex across the isobaths generate topographic vortices whose upper parts exert a shear/strain on the initial eddy, causing its filamentation. On the β plane, this process is further complicated by the presence of additional eddies created by fluid motion across the planetary vorticity gradient. Experiments without any topography show that these eddies by themselves can erode the initial vortex. In particular, a deep positive potential vorticity pole influences the near-bottom signature of the original vortex with a strong temporal variability. This reflects on the manner in which the surface eddy feels an underlying seamount. Sensitivity experiments show that the eddy erosion rate after encountering a seamount can no longer be related to basic parameters such as the minimum eddy–seamount distance, as it was on the f plane. The additional vorticity poles influencing the eddy on the β plane make the result of the eddy–seamount encounter very sensitive to small variations of the initial conditions, and impossible to predict.
Abstract
This paper investigates the behavior of a surface-intensified anticyclone encountering a seamount on the β plane in a stratified ocean. The eddy may be strongly eroded, and sometimes subdivided, provided that it gets close enough to the seamount. In case of subdivision, the detached part has a vertical structure different from that of the initial eddy, and a subsurface vortex may result. The basic erosion mechanism previously observed with f-plane experiments is still active on the β plane. Deep fluid motions induced by the initial vortex across the isobaths generate topographic vortices whose upper parts exert a shear/strain on the initial eddy, causing its filamentation. On the β plane, this process is further complicated by the presence of additional eddies created by fluid motion across the planetary vorticity gradient. Experiments without any topography show that these eddies by themselves can erode the initial vortex. In particular, a deep positive potential vorticity pole influences the near-bottom signature of the original vortex with a strong temporal variability. This reflects on the manner in which the surface eddy feels an underlying seamount. Sensitivity experiments show that the eddy erosion rate after encountering a seamount can no longer be related to basic parameters such as the minimum eddy–seamount distance, as it was on the f plane. The additional vorticity poles influencing the eddy on the β plane make the result of the eddy–seamount encounter very sensitive to small variations of the initial conditions, and impossible to predict.
Abstract
The possibility for a preexisting surface-intensified anticyclone to subduct beneath a surface front is investigated using an isopycnal numerical model. Subduction occurs for strong coherent vortices and is usually accompanied by strong dissipation. Two main mechanisms cause the erosion of the vortex core. The first one is induced by the velocity shear associated with the front. It results in the peeling of the vortex potential vorticity (PV) core, sometimes leading to its complete disappearance. The second mechanism occurs after the vortex has subducted. Entrainment of high positive PV fluid parcels from the front above the vortex low negative PV core modifies the stability properties of the latter. Meanders are observed to grow at the rim of the structure, which favors the formation of PV filaments. The erosion rates caused by each mechanism are discussed in relation to the jet and vortex characteristics, and to the background stratification. The trapping of high PV fluid parcels above the vortex is also shown to be partly responsible for the decrease and eventual loss of the eddy altimetric signal, after it has subducted.
Abstract
The possibility for a preexisting surface-intensified anticyclone to subduct beneath a surface front is investigated using an isopycnal numerical model. Subduction occurs for strong coherent vortices and is usually accompanied by strong dissipation. Two main mechanisms cause the erosion of the vortex core. The first one is induced by the velocity shear associated with the front. It results in the peeling of the vortex potential vorticity (PV) core, sometimes leading to its complete disappearance. The second mechanism occurs after the vortex has subducted. Entrainment of high positive PV fluid parcels from the front above the vortex low negative PV core modifies the stability properties of the latter. Meanders are observed to grow at the rim of the structure, which favors the formation of PV filaments. The erosion rates caused by each mechanism are discussed in relation to the jet and vortex characteristics, and to the background stratification. The trapping of high PV fluid parcels above the vortex is also shown to be partly responsible for the decrease and eventual loss of the eddy altimetric signal, after it has subducted.
Abstract
Numerical experiments are carried out on the f plane, using a shallow-water isopycnal model, to analyze the behavior of a surface-intensified anticyclonic vortex when it encounters an isolated seamount. The advection by the vortex of deep fluid parcels across the isobaths is known to generate deep anticyclonic and cyclonic circulations above and near the bathymetry, respectively. These circulations are shown to exert a strong shear on the upper layers, which causes an erosion of the initial vortex by filamentation. The erosion often results in a subdivision of the eddy. While the eroded original structure forms a dipole with the deep cyclone and is advected away, the filaments torn off from the original core aggregate into a new eddy above the seamount. Splitting in more than two structures is sometimes observed. The erosion process is quantified by the bulk volume integral of the eddy potential vorticity anomaly. A sensitivity study to different parameters of the configuration (distance between vortex and seamount, vortex radius, seamount radius, seamount height, or stratification) shows that the intensities of the deep anticyclonic and cyclonic circulations and the vortex erosion are governed both by the reservoir of positive potential vorticity associated with the seamount and by the strength of the cross-isobath flow induced by the eddy.
Abstract
Numerical experiments are carried out on the f plane, using a shallow-water isopycnal model, to analyze the behavior of a surface-intensified anticyclonic vortex when it encounters an isolated seamount. The advection by the vortex of deep fluid parcels across the isobaths is known to generate deep anticyclonic and cyclonic circulations above and near the bathymetry, respectively. These circulations are shown to exert a strong shear on the upper layers, which causes an erosion of the initial vortex by filamentation. The erosion often results in a subdivision of the eddy. While the eroded original structure forms a dipole with the deep cyclone and is advected away, the filaments torn off from the original core aggregate into a new eddy above the seamount. Splitting in more than two structures is sometimes observed. The erosion process is quantified by the bulk volume integral of the eddy potential vorticity anomaly. A sensitivity study to different parameters of the configuration (distance between vortex and seamount, vortex radius, seamount radius, seamount height, or stratification) shows that the intensities of the deep anticyclonic and cyclonic circulations and the vortex erosion are governed both by the reservoir of positive potential vorticity associated with the seamount and by the strength of the cross-isobath flow induced by the eddy.
Abstract
The behavior of the intense anticyclonic eddy observed during the Tourbillon Experiment (September-November 1979) is studied within the framework of quasi-geostrophic dynamics. The nondivergent part of the pressure held is estimated through objective analysis at the times of four quasi-synoptic CTD arrays, making use as well of velocity data from the current-meter mooring array and subsurface acoustic floats. Maps of relative vorticity, vortex stretching and potential vorticity allow identification of the signature of the eddy. Nearby companion anomalies caused by an intrusion of Mediterranean Water (MW) can be singled out and are comparable to those of the main eddy around 1200 m. Eulerian and Lagrangian tests of the conservation of potential vorticity are presented and the close similarities of salinity and potential vorticity as tracers of mesoscale motions are vividly demonstrated. Computations of advection of vortex stretching and relative vorticity show that the anticyclonic path of the main eddy above 1000 m is controlled by a genuine interaction with the MW intrusion whose ultimate cited is to sheer the vertical axis of the eddy. It is argued that such interactions must be rather common in the eastern North Atlantic for warm salty patches of Mediterranean origin are often associated with low density anomalies.
Abstract
The behavior of the intense anticyclonic eddy observed during the Tourbillon Experiment (September-November 1979) is studied within the framework of quasi-geostrophic dynamics. The nondivergent part of the pressure held is estimated through objective analysis at the times of four quasi-synoptic CTD arrays, making use as well of velocity data from the current-meter mooring array and subsurface acoustic floats. Maps of relative vorticity, vortex stretching and potential vorticity allow identification of the signature of the eddy. Nearby companion anomalies caused by an intrusion of Mediterranean Water (MW) can be singled out and are comparable to those of the main eddy around 1200 m. Eulerian and Lagrangian tests of the conservation of potential vorticity are presented and the close similarities of salinity and potential vorticity as tracers of mesoscale motions are vividly demonstrated. Computations of advection of vortex stretching and relative vorticity show that the anticyclonic path of the main eddy above 1000 m is controlled by a genuine interaction with the MW intrusion whose ultimate cited is to sheer the vertical axis of the eddy. It is argued that such interactions must be rather common in the eastern North Atlantic for warm salty patches of Mediterranean origin are often associated with low density anomalies.
Abstract
Numerical experiments are carried out on the f plane, using a shallow-water isopycnal model, to analyze the behavior of a surface-intensified anticyclonic vortex when it encounters an isolated seamount. The advection by the vortex of deep fluid parcels across the isobaths is known to generate deep anticyclonic and cyclonic circulations above and near the bathymetry, respectively. These circulations are shown to exert a strong shear on the upper layers, which causes an erosion of the initial vortex by filamentation. The erosion often results in a subdivision of the eddy. While the eroded original structure forms a dipole with the deep cyclone and is advected away, the filaments torn off from the original core aggregate into a new eddy above the seamount. Splitting in more than two structures is sometimes observed. The erosion process is quantified by the bulk volume integral of the eddy potential vorticity anomaly. A sensitivity study to different parameters of the configuration (distance between vortex and seamount, vortex radius, seamount radius, seamount height, or stratification) shows that the intensities of the deep anticyclonic and cyclonic circulations and the vortex erosion are governed both by the reservoir of positive potential vorticity associated with the seamount and by the strength of the cross-isobath flow induced by the eddy.
Abstract
Numerical experiments are carried out on the f plane, using a shallow-water isopycnal model, to analyze the behavior of a surface-intensified anticyclonic vortex when it encounters an isolated seamount. The advection by the vortex of deep fluid parcels across the isobaths is known to generate deep anticyclonic and cyclonic circulations above and near the bathymetry, respectively. These circulations are shown to exert a strong shear on the upper layers, which causes an erosion of the initial vortex by filamentation. The erosion often results in a subdivision of the eddy. While the eroded original structure forms a dipole with the deep cyclone and is advected away, the filaments torn off from the original core aggregate into a new eddy above the seamount. Splitting in more than two structures is sometimes observed. The erosion process is quantified by the bulk volume integral of the eddy potential vorticity anomaly. A sensitivity study to different parameters of the configuration (distance between vortex and seamount, vortex radius, seamount radius, seamount height, or stratification) shows that the intensities of the deep anticyclonic and cyclonic circulations and the vortex erosion are governed both by the reservoir of positive potential vorticity associated with the seamount and by the strength of the cross-isobath flow induced by the eddy.
Abstract
Ten-year-long output series from a general circulation model forced by daily realistic winds are used to analyze the annual cycle of the Equatorial Undercurrent (EUC) in the Atlantic Ocean. Two well-defined transport maxima are found: One, present during boreal summer and autumn in the central part of the basin, is generally recognized and regarded as a near-equilibrium response to the equatorial easterly trades that culminate in this period. Another one, most pronounced near the western boundary, occurs in April–May when the trades relax. This second maximum is less patent in the observations, but concomitant signals in previously published analyses of the North Brazil Current and surface velocity seasonal variations might be indirect manifestations of its reality. Because this intensification appears at periods when the boundary between the tropical and equatorial gyres nears the equator, the authors relate its existence to wind stress curl variations at subequatorial latitudes. A link between the interannual variability of the spring transport maximum and that of the low-latitude wind stress curl is, indeed, found in the model. This diagnostic approach suggests that two different dynamical regimes shape up the EUC seasonal cycle: in summer and autumn, local forcing by the equatorial zonal wind component and main supply from the ocean interior; in winter and spring, remote forcing by the low-latitude rotational wind component and supply from the western boundary currents.
Abstract
Ten-year-long output series from a general circulation model forced by daily realistic winds are used to analyze the annual cycle of the Equatorial Undercurrent (EUC) in the Atlantic Ocean. Two well-defined transport maxima are found: One, present during boreal summer and autumn in the central part of the basin, is generally recognized and regarded as a near-equilibrium response to the equatorial easterly trades that culminate in this period. Another one, most pronounced near the western boundary, occurs in April–May when the trades relax. This second maximum is less patent in the observations, but concomitant signals in previously published analyses of the North Brazil Current and surface velocity seasonal variations might be indirect manifestations of its reality. Because this intensification appears at periods when the boundary between the tropical and equatorial gyres nears the equator, the authors relate its existence to wind stress curl variations at subequatorial latitudes. A link between the interannual variability of the spring transport maximum and that of the low-latitude wind stress curl is, indeed, found in the model. This diagnostic approach suggests that two different dynamical regimes shape up the EUC seasonal cycle: in summer and autumn, local forcing by the equatorial zonal wind component and main supply from the ocean interior; in winter and spring, remote forcing by the low-latitude rotational wind component and supply from the western boundary currents.
Abstract
Eulerian velocity measurements carried out along 48°N in the Atlantic Ocean Provide averaged velocities with definite large-scale structure. This warrants an analysis of these mean velocity vectors within the framework of steady and large-scale dynamics: the dominant part played by the bottom forcing in the planetary vorticity balance is demonstrated. Combined use of vertical velocities derived from fills balance and the mean velocity spirals allows us to estimate advection in the heat equation and residual cros-isopycnal velocities. At the westernmost mooring site (35°W) heat losses to the atmosphere account for these residuals, while at the other sites (20°, 25° and 30°W) lateral heat fluxes induced by mesoscale eddues must be invoked.
The hydrology of the region provides elements of comparison with the set of averaged velocities: qualitative through our knowledge of the motions of watermasses at this latitude, then more quantitative using geostrophic calculations. Beta-spiral inversions are carried out on climatological hydrographic data and the same dynamical analysis applied to the resulting velocity profiles. Some incompatibilities with the direct measurements are observed and possible masons for these differences discussed.
Abstract
Eulerian velocity measurements carried out along 48°N in the Atlantic Ocean Provide averaged velocities with definite large-scale structure. This warrants an analysis of these mean velocity vectors within the framework of steady and large-scale dynamics: the dominant part played by the bottom forcing in the planetary vorticity balance is demonstrated. Combined use of vertical velocities derived from fills balance and the mean velocity spirals allows us to estimate advection in the heat equation and residual cros-isopycnal velocities. At the westernmost mooring site (35°W) heat losses to the atmosphere account for these residuals, while at the other sites (20°, 25° and 30°W) lateral heat fluxes induced by mesoscale eddues must be invoked.
The hydrology of the region provides elements of comparison with the set of averaged velocities: qualitative through our knowledge of the motions of watermasses at this latitude, then more quantitative using geostrophic calculations. Beta-spiral inversions are carried out on climatological hydrographic data and the same dynamical analysis applied to the resulting velocity profiles. Some incompatibilities with the direct measurements are observed and possible masons for these differences discussed.
