Search Results
Abstract
During 1984, five current meter moorings measured velocity and temperature in the Gulf Stream anticyclonic flank at a location approximately 250 km downstream of Cape Hatteras. Here, these data are used to analyze the energy budgets of the Gulf Stream mean flow with a view towards examining gulf stream vertical structure and inertial character.
We find that Gulf Stream dynamics exhibits considerable vertical structure at our experiment site. At 380 m, the kinetic energy flux appears to be convergent, but the eddies augment mean kinetic energy. Order of magnitude estimates of processes involving vertical velocity appear to be too small to account for this mismatch; hence, we conclude that a conversion of mean kinetic to mean potential energy, via flow up a mean pressure gradient, must be occurring, Opposite tendencies are found at 880 m, leading to conclusion that the Gulf Stream is flowing down a mean pressure gradient at this depth. Evidence supporting a situation similar to the observations in terms of northward shifts of the gyre structure with depth. Of course, our observations are undoubtedly influenced by lateral topography, bottom topography and eddies and the effects of these are poorly understood from a theoretical point of view.
We also present evidence that the Deep Western Boundary Current is restoring energy to the deep potential energy field by flow up a mean pressure gradient. The rates are considerably smaller than those in the Gulf Stream but structurally resemble our results at 380 m.
Abstract
During 1984, five current meter moorings measured velocity and temperature in the Gulf Stream anticyclonic flank at a location approximately 250 km downstream of Cape Hatteras. Here, these data are used to analyze the energy budgets of the Gulf Stream mean flow with a view towards examining gulf stream vertical structure and inertial character.
We find that Gulf Stream dynamics exhibits considerable vertical structure at our experiment site. At 380 m, the kinetic energy flux appears to be convergent, but the eddies augment mean kinetic energy. Order of magnitude estimates of processes involving vertical velocity appear to be too small to account for this mismatch; hence, we conclude that a conversion of mean kinetic to mean potential energy, via flow up a mean pressure gradient, must be occurring, Opposite tendencies are found at 880 m, leading to conclusion that the Gulf Stream is flowing down a mean pressure gradient at this depth. Evidence supporting a situation similar to the observations in terms of northward shifts of the gyre structure with depth. Of course, our observations are undoubtedly influenced by lateral topography, bottom topography and eddies and the effects of these are poorly understood from a theoretical point of view.
We also present evidence that the Deep Western Boundary Current is restoring energy to the deep potential energy field by flow up a mean pressure gradient. The rates are considerably smaller than those in the Gulf Stream but structurally resemble our results at 380 m.
Abstract
Pointwise energy balances in the Gulf Stream System at 73°W (downstream of Cape Hatteras) are examined. Five current meter moorings obtained verlocity and temperature data at four different depths for approximately one year. These data were used to estimate eddy kinetic and potential energy flux divergences and the rates of energy conversion between the eddy and mean fields in both the Gulf Stream and the Deep Western Boundary Current.
Based on our results and those of others, we argue that (i) the Gulf Stream is on average baroclinically unstable in this region, although energy release from the horizontal shear dominates near the surface; (ii) eddy pressure work is an important component of Gulf Stream eddy energetics and acts as a sink of eddy energy at 73°W; (iii) the Deep Western Boundary Current appears to be baroclinically unstable; (iv) Gulf Stream eddies are affected by the “far-field"; and (v) the eddy-field at 73°W is significantly different in its effects on the Gulf stream than that upstream on Hatteras or that at 68°W, although the interaction rates are of comparable magnitude. We propose a hypothesis of eddy field dynamics in the vicinity of the Gulf Stream from the Florida Straits through the Gulf Stream extension.
Abstract
Pointwise energy balances in the Gulf Stream System at 73°W (downstream of Cape Hatteras) are examined. Five current meter moorings obtained verlocity and temperature data at four different depths for approximately one year. These data were used to estimate eddy kinetic and potential energy flux divergences and the rates of energy conversion between the eddy and mean fields in both the Gulf Stream and the Deep Western Boundary Current.
Based on our results and those of others, we argue that (i) the Gulf Stream is on average baroclinically unstable in this region, although energy release from the horizontal shear dominates near the surface; (ii) eddy pressure work is an important component of Gulf Stream eddy energetics and acts as a sink of eddy energy at 73°W; (iii) the Deep Western Boundary Current appears to be baroclinically unstable; (iv) Gulf Stream eddies are affected by the “far-field"; and (v) the eddy-field at 73°W is significantly different in its effects on the Gulf stream than that upstream on Hatteras or that at 68°W, although the interaction rates are of comparable magnitude. We propose a hypothesis of eddy field dynamics in the vicinity of the Gulf Stream from the Florida Straits through the Gulf Stream extension.
Abstract
Some simple air-sea momentum transfer models, which include sea surface velocity and temperature, are considered for their effects on Gulf Stream rings. Perturbing the stress calculation with sea surface velocity results in a “top drag”, which causes interior motions to decay. Numerical experiments with equivalent barotropic quasi-geostrophic dynamics and reasonable estimates for the top drag suggest that this mechanism can amount for a significant fraction of observed isotherm subsidence rates in rings. Perturbing the stress calculation with a temperature sensitive drag coefficient produces a dipolar Ekman pumping field over a ring. For an eastward directed wind, the result is a tendency for the ring to self-propagate to the south. Integral constraints can be used to estimate the meridional propagation rate precisely, and for reasonable stress and thermal anomaly values, the estimate compares well with observations.
Abstract
Some simple air-sea momentum transfer models, which include sea surface velocity and temperature, are considered for their effects on Gulf Stream rings. Perturbing the stress calculation with sea surface velocity results in a “top drag”, which causes interior motions to decay. Numerical experiments with equivalent barotropic quasi-geostrophic dynamics and reasonable estimates for the top drag suggest that this mechanism can amount for a significant fraction of observed isotherm subsidence rates in rings. Perturbing the stress calculation with a temperature sensitive drag coefficient produces a dipolar Ekman pumping field over a ring. For an eastward directed wind, the result is a tendency for the ring to self-propagate to the south. Integral constraints can be used to estimate the meridional propagation rate precisely, and for reasonable stress and thermal anomaly values, the estimate compares well with observations.
Abstract
A three-layer, wind-driven, general circulation model involving both subtropical and subpolar gyres has been developed to study intergyre exchange. Following some early studies, the present work allows flow to baroclinically cross the intergyre boundary. This model differs from past work by examining a three-layer fluid. Solutions with both southward and northward subsurface flows are obtained. The two principal objectives of this work are to clarify the structure and maintenance of the permanent thermocline and to aid in understanding the distribution of deep water masses.
A class of thermocline structures at the zero Ekman pumping line has been constructed that permits intergyre exchange, or communication. The zones of exchange are called windows. In this study, the windows have several unique properties relative to those computed elsewhere, and exhibit relatively rich structure. Principally, the addition of an active third layer allows a new second baroclinic window to open. This new window is physically and dynamically distinct from the first window (found in previous studies), and most of the intergyre baroclinic transport can occur through it. Its appearance also supports the conjecture that the number of communication windows increases with the number of active layers.
In addition to the model development, observed potential vorticity distributions have been reexamined within the context of this model. Possible explanations for deep potential vorticity contours in the North Atlantic and North Pacific oceans are proposed.
Abstract
A three-layer, wind-driven, general circulation model involving both subtropical and subpolar gyres has been developed to study intergyre exchange. Following some early studies, the present work allows flow to baroclinically cross the intergyre boundary. This model differs from past work by examining a three-layer fluid. Solutions with both southward and northward subsurface flows are obtained. The two principal objectives of this work are to clarify the structure and maintenance of the permanent thermocline and to aid in understanding the distribution of deep water masses.
A class of thermocline structures at the zero Ekman pumping line has been constructed that permits intergyre exchange, or communication. The zones of exchange are called windows. In this study, the windows have several unique properties relative to those computed elsewhere, and exhibit relatively rich structure. Principally, the addition of an active third layer allows a new second baroclinic window to open. This new window is physically and dynamically distinct from the first window (found in previous studies), and most of the intergyre baroclinic transport can occur through it. Its appearance also supports the conjecture that the number of communication windows increases with the number of active layers.
In addition to the model development, observed potential vorticity distributions have been reexamined within the context of this model. Possible explanations for deep potential vorticity contours in the North Atlantic and North Pacific oceans are proposed.
Abstract
Based on observations, the proposition is forwarded that some rings involve important deep flow. The work described herein is directed at understanding the consequences on eddy evolution of such structure. An analysis of the equations of motion is conducted that emphasizes the importance of the lower layer evolution. The thermocline responds in a largely passive fashion. This analysis differs considerably from previous theories, which focus on the evolution of surface-intensified rings. The most important practical differences are that the coupled system can be expected to exhibit propagation in any direction (as opposed to predominantly west, as in reduced gravity theories), and that the propagation rates can be an order of magnitude greater than those of reduced gravity systems. These aspects of the present analysis are in accord with many ring observations. A series of primitive equation numerical experiments are conducted to test these ideas, with the result that the experiments support such “barotropically dominated dynamics” as a useful qualitative and quantitative tool for the study of eddies and rings. The asymptotic analysis also suggests that initial conditions with closed regions of potential vorticity should differ significantly from those with no closed potential vorticity zones. This hypothesis is supported by primitive equation runs; approximately compensated lower-layer experiments (with no closed potential vorticity contours) exhibit qualitatively and quantitatively different behavior than experiments with initially energetic lower layers (which have closed potential vorticity contours).
Abstract
Based on observations, the proposition is forwarded that some rings involve important deep flow. The work described herein is directed at understanding the consequences on eddy evolution of such structure. An analysis of the equations of motion is conducted that emphasizes the importance of the lower layer evolution. The thermocline responds in a largely passive fashion. This analysis differs considerably from previous theories, which focus on the evolution of surface-intensified rings. The most important practical differences are that the coupled system can be expected to exhibit propagation in any direction (as opposed to predominantly west, as in reduced gravity theories), and that the propagation rates can be an order of magnitude greater than those of reduced gravity systems. These aspects of the present analysis are in accord with many ring observations. A series of primitive equation numerical experiments are conducted to test these ideas, with the result that the experiments support such “barotropically dominated dynamics” as a useful qualitative and quantitative tool for the study of eddies and rings. The asymptotic analysis also suggests that initial conditions with closed regions of potential vorticity should differ significantly from those with no closed potential vorticity zones. This hypothesis is supported by primitive equation runs; approximately compensated lower-layer experiments (with no closed potential vorticity contours) exhibit qualitatively and quantitatively different behavior than experiments with initially energetic lower layers (which have closed potential vorticity contours).
Abstract
Many recent observations have described fronts in the interior of the ocean at locations far away from any lateral boundaries. Some of these fronts are observed to be associated with considerable mass transports, which suggests that they participate importantly in setting the water mass structure of the ocean interior, and represent considerable local departures from linear Sverdrup dynamics. In this paper, a simple analytic theory of interior fronts is developed. The main features of this theory are that the fronts are highly inertial and anisotropic, and reside on the edge of a somewhat larger scale interior inertial recirculation. The recirculation is taken to be modonlike; the dynamic height difference across the edge of the recirculation supports an interior jet, which is clockwise around the edge of the recirculation and carries water from the subpolar into the subtropical gyre. Unlike in previous theories of interior fronts, all of the transports, both in the large-scale and the fronts, are “anomalous” and in excess of any wind-driven transport. The fronts themselves represent interior, deformation-scale boundary layers, which are necessary to smoothly join the baroclinic parts of the inertial recirculation and the sluggish Sverdrup zones. The authors speculate on the role of these dynamics in the LDE jet.
Abstract
Many recent observations have described fronts in the interior of the ocean at locations far away from any lateral boundaries. Some of these fronts are observed to be associated with considerable mass transports, which suggests that they participate importantly in setting the water mass structure of the ocean interior, and represent considerable local departures from linear Sverdrup dynamics. In this paper, a simple analytic theory of interior fronts is developed. The main features of this theory are that the fronts are highly inertial and anisotropic, and reside on the edge of a somewhat larger scale interior inertial recirculation. The recirculation is taken to be modonlike; the dynamic height difference across the edge of the recirculation supports an interior jet, which is clockwise around the edge of the recirculation and carries water from the subpolar into the subtropical gyre. Unlike in previous theories of interior fronts, all of the transports, both in the large-scale and the fronts, are “anomalous” and in excess of any wind-driven transport. The fronts themselves represent interior, deformation-scale boundary layers, which are necessary to smoothly join the baroclinic parts of the inertial recirculation and the sluggish Sverdrup zones. The authors speculate on the role of these dynamics in the LDE jet.
Abstract
The Rossby adjustment of an initially circular column of water, the so-called collapse of a cylinder, continues to be a widely used method for forming lenslike eddies in the laboratory. Here, we consider the structure of an eddy so formed as well as some ramifications of that formation. We demonstrate that the calculation of the eddy structure can be reduced to the extraction of the roots of two nonlinear, coupled algebraic equations. Analytical solutions in the limit of the collapse of a needle are given and roots are obtained numerically otherwise. It is concluded that in the collapse of a cylinder initially spanning the entire column of water, the eddy always maintains contact with both surfaces. (This is not the case in the seemingly equivalent two-dimensional case with no variation in one Cartesian direction.) In the event the initial cold column is separated only slightly from the surface, the above solution acts as the lowest order solution in a regular perturbation sequence.
Next, these “collapse eddy” solutions, which possess motions in both layers and finite energies, are used to examine lens merger. Two collapse eddies of equal volume jointly possess less energy than one collapse eddy of twice the volume. However, we argue that two collapse eddies of equal volume can have more energy than the circularly symmetric end-state eddy formed from them if the two initial eddies “mix.” We also offer evidence that the energy budgets may be balanced exactly if the end-state eddy is slightly asymmetric. Comparisons with some previous laboratory experiments are made.
Abstract
The Rossby adjustment of an initially circular column of water, the so-called collapse of a cylinder, continues to be a widely used method for forming lenslike eddies in the laboratory. Here, we consider the structure of an eddy so formed as well as some ramifications of that formation. We demonstrate that the calculation of the eddy structure can be reduced to the extraction of the roots of two nonlinear, coupled algebraic equations. Analytical solutions in the limit of the collapse of a needle are given and roots are obtained numerically otherwise. It is concluded that in the collapse of a cylinder initially spanning the entire column of water, the eddy always maintains contact with both surfaces. (This is not the case in the seemingly equivalent two-dimensional case with no variation in one Cartesian direction.) In the event the initial cold column is separated only slightly from the surface, the above solution acts as the lowest order solution in a regular perturbation sequence.
Next, these “collapse eddy” solutions, which possess motions in both layers and finite energies, are used to examine lens merger. Two collapse eddies of equal volume jointly possess less energy than one collapse eddy of twice the volume. However, we argue that two collapse eddies of equal volume can have more energy than the circularly symmetric end-state eddy formed from them if the two initial eddies “mix.” We also offer evidence that the energy budgets may be balanced exactly if the end-state eddy is slightly asymmetric. Comparisons with some previous laboratory experiments are made.
Abstract
A quasigeostrophic point vortex numerical model is used to explore interactions of eddies and seamounts. The ultimate objective of this study is to assess the role of meddy–seamount interaction as an input to Mediterranean salt tongue maintenance. Secondary objectives are to clarify the dynamics of meddy–seamount interaction. The results suggest that meddies survive seamount collisions with 60%–70% of their initial cores remaining intact as coherent vortices. Given observed formation rates, it appears meddies, in their interactions with seamounts, inject between one-quarter and one-half of the salt anomaly necessary to sustain the Mediterranean salt tongue. Other considerations suggest the anomalous mass flux by meddies is comparable to that due to the mean flow. In summary, meddies are important to the maintenance of the salt tongue, although other mechanisms are needed. Coherent vortex transport, of which meddies are one example, is a mesoscale process not well described by the downgradient mixing algorithms normally employed in general circulation models. More sophisticated mesoscale models are thus suggested by this study. In particular, survival by meddies of collisions with seamounts emerges as a potentially important limiting effect on the Mediterranean salt tongue. This effect has climatically significant implications for ocean simulations.
Abstract
A quasigeostrophic point vortex numerical model is used to explore interactions of eddies and seamounts. The ultimate objective of this study is to assess the role of meddy–seamount interaction as an input to Mediterranean salt tongue maintenance. Secondary objectives are to clarify the dynamics of meddy–seamount interaction. The results suggest that meddies survive seamount collisions with 60%–70% of their initial cores remaining intact as coherent vortices. Given observed formation rates, it appears meddies, in their interactions with seamounts, inject between one-quarter and one-half of the salt anomaly necessary to sustain the Mediterranean salt tongue. Other considerations suggest the anomalous mass flux by meddies is comparable to that due to the mean flow. In summary, meddies are important to the maintenance of the salt tongue, although other mechanisms are needed. Coherent vortex transport, of which meddies are one example, is a mesoscale process not well described by the downgradient mixing algorithms normally employed in general circulation models. More sophisticated mesoscale models are thus suggested by this study. In particular, survival by meddies of collisions with seamounts emerges as a potentially important limiting effect on the Mediterranean salt tongue. This effect has climatically significant implications for ocean simulations.
Abstract
The time dependence of the ventilated thermocline is examined via analytical and numerical means. The original Henderschott model is modified such that the outcrops all occur on the same geopotential surface, rather than at staggered geopotential surfaces. This model has the advantage that the ocean interior can be ventilated directly by the Sverdrup flow, rather than by western boundary processes. The propagation of disturbances governed by linearized forms of the three-layer or four-layer modified Henderschott model, nonlinear solutions of the full modified Henderschott model, and numerical solutions of the planetary geostrophic equations are considered.
Low-frequency disturbances are predicted by the linear models to move on characteristics jointly set by advection and wave dynamics. It is shown that perturbations due to wind stress anomalies project strongly onto the first mode and propagate westward similarly to the classical first baroclinic Rossby mode. They do not experience much interaction with the mean flow (the so-called non-Doppler effect). On the other hand, perturbations generated by buoyancy anomalies have strong projections onto the second or third modes, and propagate along pathways very close to the mean circulation. Their speed is somewhat slower than the current speed, however. These properties appear in the linearized and simplified nonlinear models and their occurrence in planetary geostrophic results argues the relevance of the Henderschott model. Also, these properties are consistent with results from other studies.
Abstract
The time dependence of the ventilated thermocline is examined via analytical and numerical means. The original Henderschott model is modified such that the outcrops all occur on the same geopotential surface, rather than at staggered geopotential surfaces. This model has the advantage that the ocean interior can be ventilated directly by the Sverdrup flow, rather than by western boundary processes. The propagation of disturbances governed by linearized forms of the three-layer or four-layer modified Henderschott model, nonlinear solutions of the full modified Henderschott model, and numerical solutions of the planetary geostrophic equations are considered.
Low-frequency disturbances are predicted by the linear models to move on characteristics jointly set by advection and wave dynamics. It is shown that perturbations due to wind stress anomalies project strongly onto the first mode and propagate westward similarly to the classical first baroclinic Rossby mode. They do not experience much interaction with the mean flow (the so-called non-Doppler effect). On the other hand, perturbations generated by buoyancy anomalies have strong projections onto the second or third modes, and propagate along pathways very close to the mean circulation. Their speed is somewhat slower than the current speed, however. These properties appear in the linearized and simplified nonlinear models and their occurrence in planetary geostrophic results argues the relevance of the Henderschott model. Also, these properties are consistent with results from other studies.
Abstract
A highly idealized model for the oceanic haline circulation is studied. Specifically, loops filled with salty water and subjected to either the natural boundary condition, the virtual salt flux condition, or salinity relaxation are considered. It is shown that the characteristics of the solutions, especially the transition between steady and unsteady convection, depend critically on the applied boundary conditions. It is found that the relaxation condition generally modifies the location of the Hopf bifurcation so highly that models based on it should always remain in the regime of steady convection. On the other hand, the location of the Hopf bifurcation for models based on flux conditions is much less extreme. Thus, in these models, limit cycles or chaotic behavior can easily be excited.
Further, the nature of the Hopf bifurcation depends sensitively on the boundary condition. For example, if the frictional parameter is gradually reduced, the model based on the natural boundary condition goes through a supercritical Hopf bifurcation, while the model based on virtual salt flux goes through a subcritical Hopf bifurcation. Similar dependencies are found when other parameters are varied. Beyond the Hopf bifurcations, windows of limit cycle solutions alternate with windows of chaos. In addition, for a given set of parameters, the system can have multiple solutions, such as a limit cycle and a chaotic solution, or limit cycles which have distinctively different structure.
These results comment on the types of behavior that more complicated three-dimensional models may exhibit.
Abstract
A highly idealized model for the oceanic haline circulation is studied. Specifically, loops filled with salty water and subjected to either the natural boundary condition, the virtual salt flux condition, or salinity relaxation are considered. It is shown that the characteristics of the solutions, especially the transition between steady and unsteady convection, depend critically on the applied boundary conditions. It is found that the relaxation condition generally modifies the location of the Hopf bifurcation so highly that models based on it should always remain in the regime of steady convection. On the other hand, the location of the Hopf bifurcation for models based on flux conditions is much less extreme. Thus, in these models, limit cycles or chaotic behavior can easily be excited.
Further, the nature of the Hopf bifurcation depends sensitively on the boundary condition. For example, if the frictional parameter is gradually reduced, the model based on the natural boundary condition goes through a supercritical Hopf bifurcation, while the model based on virtual salt flux goes through a subcritical Hopf bifurcation. Similar dependencies are found when other parameters are varied. Beyond the Hopf bifurcations, windows of limit cycle solutions alternate with windows of chaos. In addition, for a given set of parameters, the system can have multiple solutions, such as a limit cycle and a chaotic solution, or limit cycles which have distinctively different structure.
These results comment on the types of behavior that more complicated three-dimensional models may exhibit.
