Search Results
Abstract
Oceanic rings tend to have length scales larger than the deformation radius and also to he long-lived. This latter characteristic, in view of the former, is particularly curious as many quasigeostrophic and primitive equation simulations suggest such eddies are quite unstable. Large eddies eventually break into smaller deformation scale vortices, with the attendant production of considerable variability.
Here it is argued that the stability characteristics of oceanic eddies and rings are sensitive to the presence of deep flows. In particular, eddies in which the deep flow is counter to the sense of the shallow flows are often more unstable than eddies with no deep flow, while eddies with circulations in the same sense as the shallow circulation can experience an enhanced stability. For a given vertical shear, oceanic eddy stability can vary dramatically. (This is in contrast to quasigeostrophic theory, where stability properties are largely determined by vertical shear.) The onset of these mechanics is quite pronounced for Gaussian oceanic eddies. Linear “f”- plane stability calculations reveal a marked suppression of unstable growth rates for warm corotating eddies with relatively weak deep flows. Cold eddies also experience a suppression of instability in the corotating state, although relatively weak unstable modes have been found. Comparisons of f- and β-plane numerical primitive equation experiments support these results, as well as demonstrate some relevant limitations. Finally, studies of dipolar eddies and non-Gaussian circular eddies are used to examine the generality of the results. We suggest such stability considerations may be partially responsible for the observed long lives of oceanic rings.
An examination of the unstable normal modes from the f-plane model demonstrates an intimate coupling between the suppression of instability and the appearance of multiple critical layers. The normal-mode energetics are used to demonstrate the role of upgradient momentum fluxes at the points of stabilization, and a heuristic argument involving critical layers is given.
Abstract
Oceanic rings tend to have length scales larger than the deformation radius and also to he long-lived. This latter characteristic, in view of the former, is particularly curious as many quasigeostrophic and primitive equation simulations suggest such eddies are quite unstable. Large eddies eventually break into smaller deformation scale vortices, with the attendant production of considerable variability.
Here it is argued that the stability characteristics of oceanic eddies and rings are sensitive to the presence of deep flows. In particular, eddies in which the deep flow is counter to the sense of the shallow flows are often more unstable than eddies with no deep flow, while eddies with circulations in the same sense as the shallow circulation can experience an enhanced stability. For a given vertical shear, oceanic eddy stability can vary dramatically. (This is in contrast to quasigeostrophic theory, where stability properties are largely determined by vertical shear.) The onset of these mechanics is quite pronounced for Gaussian oceanic eddies. Linear “f”- plane stability calculations reveal a marked suppression of unstable growth rates for warm corotating eddies with relatively weak deep flows. Cold eddies also experience a suppression of instability in the corotating state, although relatively weak unstable modes have been found. Comparisons of f- and β-plane numerical primitive equation experiments support these results, as well as demonstrate some relevant limitations. Finally, studies of dipolar eddies and non-Gaussian circular eddies are used to examine the generality of the results. We suggest such stability considerations may be partially responsible for the observed long lives of oceanic rings.
An examination of the unstable normal modes from the f-plane model demonstrates an intimate coupling between the suppression of instability and the appearance of multiple critical layers. The normal-mode energetics are used to demonstrate the role of upgradient momentum fluxes at the points of stabilization, and a heuristic argument involving critical layers is given.
Abstract
A combined analytical and numerical examination of submesoscale coherent vortex (SCV) dynamics and propagation is conducted. This study is prompted by observations of the movement relative to their surroundings of one class of SCVs, that is, Meddies. An asymptotic analysis is performed to study the mechanics governing SCV propagation. It is found that the large-scale flow plays a dominant role in determining the trajectory of SCVs and that the β effect and form drag of neighboring layers are weaker effects. As a result, SCVs propagate at a speed that is a density-weighted average of the flow in the surrounding layers. Meddies may thus move relative to the surrounding water, which is in accordance with observations.
This theory extends previous studies on eddy propagation by considering more general situations. For example, a lenslike eddy embedded in a nonzonal, vertically and horizontally sheared flow is studied. A significant difference between this study and most previous related work is that the submesoscale nature of SCVs is exploited. It is this nature that leads to our conclusions about SCV drift.
The theory is tested both by solving the asymptotic equations and through experiments with a primitive equation model. Agreement is found between the results of our numerical experiments and the analytical predictions, thus suggesting that the asymptotic analysis has captured the leading order behavior of SCV propagation.
Abstract
A combined analytical and numerical examination of submesoscale coherent vortex (SCV) dynamics and propagation is conducted. This study is prompted by observations of the movement relative to their surroundings of one class of SCVs, that is, Meddies. An asymptotic analysis is performed to study the mechanics governing SCV propagation. It is found that the large-scale flow plays a dominant role in determining the trajectory of SCVs and that the β effect and form drag of neighboring layers are weaker effects. As a result, SCVs propagate at a speed that is a density-weighted average of the flow in the surrounding layers. Meddies may thus move relative to the surrounding water, which is in accordance with observations.
This theory extends previous studies on eddy propagation by considering more general situations. For example, a lenslike eddy embedded in a nonzonal, vertically and horizontally sheared flow is studied. A significant difference between this study and most previous related work is that the submesoscale nature of SCVs is exploited. It is this nature that leads to our conclusions about SCV drift.
The theory is tested both by solving the asymptotic equations and through experiments with a primitive equation model. Agreement is found between the results of our numerical experiments and the analytical predictions, thus suggesting that the asymptotic analysis has captured the leading order behavior of SCV propagation.
Abstract
A consistent scheme for vertical mixing in layered numerical models is derived in this paper. The fact that the vertical coordinate (density) depends on the properties being transported (namely salinity and potential temperature) renders the inclusion of vertical mixing in layered models a subtle problem. The approach the authors have taken is based upon the entrainment into a layer being proportional to the turbulent activity in that layer. Across each interface there are then two entrainment velocities, one upward velocity that is the entrainment of fluid into the layer above the interface, and one downward velocity, being the entrainment velocity into the layer below the interface. This double entrainment accounts for both the diffusive and the advective consequences of turbulent mixing. The proposed scheme works without approximation for a nonlinear equation of state and can readily handle the production of density caused by cabbeling. Several examples are given.
Abstract
A consistent scheme for vertical mixing in layered numerical models is derived in this paper. The fact that the vertical coordinate (density) depends on the properties being transported (namely salinity and potential temperature) renders the inclusion of vertical mixing in layered models a subtle problem. The approach the authors have taken is based upon the entrainment into a layer being proportional to the turbulent activity in that layer. Across each interface there are then two entrainment velocities, one upward velocity that is the entrainment of fluid into the layer above the interface, and one downward velocity, being the entrainment velocity into the layer below the interface. This double entrainment accounts for both the diffusive and the advective consequences of turbulent mixing. The proposed scheme works without approximation for a nonlinear equation of state and can readily handle the production of density caused by cabbeling. Several examples are given.
Abstract
The propagation of long, first mode, baroclinic planetary waves in eddy-resolving quasigeostrophic general circulation models is studied. Recent TOPEX/Poseidon observations argue oceanic first-mode planetary waves move with speeds other than those predicted by simple theory. These data have prompted theoretical analyses of wave propagation in a mean flow, with the results suggesting mean shear can have a controlling effect on the planetary wave guide. Some of the predicted effects appear to be relevant to the observations, while others are less obvious. This, coupled with other explanations for the observations, motivates the calculations.
Based on these experiments, the authors suggest that the predicted effects of mean shear on wave propagation are consistent with those computed in a fully geostrophically turbulent ocean. These are that a two-layer model misses the dominant component of long-wave interaction with a mean flow, a three-layer model captures this interaction qualitatively, and the correction to wave propagation is in the direction opposite to the mean flow. Quantitative comparisons between the theory and the numerical experiments are good in the northern latitudes and questionable in the southern latitudes. Reasons for the southern discrepancy are offered.
Abstract
The propagation of long, first mode, baroclinic planetary waves in eddy-resolving quasigeostrophic general circulation models is studied. Recent TOPEX/Poseidon observations argue oceanic first-mode planetary waves move with speeds other than those predicted by simple theory. These data have prompted theoretical analyses of wave propagation in a mean flow, with the results suggesting mean shear can have a controlling effect on the planetary wave guide. Some of the predicted effects appear to be relevant to the observations, while others are less obvious. This, coupled with other explanations for the observations, motivates the calculations.
Based on these experiments, the authors suggest that the predicted effects of mean shear on wave propagation are consistent with those computed in a fully geostrophically turbulent ocean. These are that a two-layer model misses the dominant component of long-wave interaction with a mean flow, a three-layer model captures this interaction qualitatively, and the correction to wave propagation is in the direction opposite to the mean flow. Quantitative comparisons between the theory and the numerical experiments are good in the northern latitudes and questionable in the southern latitudes. Reasons for the southern discrepancy are offered.
Abstract
The energy budgets of the eddies and the mean flow in the Gulf Stream near a topographic feature known as the Charleston bump are computed. First, we consider these results in the context of the amplification hypothesis for the development of Gulf Stream meanders. According to this hypothesis, the finite amplitude Gulf Stream fluctuations observed offshore of Onslow Bay are the result of the destabilizing effect of the bump on the Stream. The present dataset was obtained both immediately upstream and downstream of the bump, and the results of our analysis suggest: 1) Immediately south of the Charleston bump, the eddies perform net work on the Gulf Stream at a rate of (1.02 ± .66) × 10−2 ergs cm−3 s−1 by transporting momentum offshore; 2) The net work performed by the eddies south of the bump is not used locally to accelerate the mean; rather, it is exported to the rest of the ocean at a rate of (1.58 ± 1.39) × 10−2 ergs cm−3 s−1; 3) In spite of the net work performed by the eddies south of the bump, eddy kinetic energy apparently does not decrease; 4) Immediately north of the Charleston bump, the flow appears to be both barotropically and baroclinically unstable. These results support the amplification hypothesis by demonstrating the destabilizing effect of the bump on the eddies (points 1 and 4) and that upstream perturbations may survive to encounter the bump topography (point 3). Other results of our analysis are that the mean of mean kinetic energy by the eddies constitutes the dominant form of energy conversion and that eddy pressure work may be an important factor in the fluctuation energy budget.
The second application of our calculations is to a characterization of the mean Gulf Stream in the South Atlantic Bight (SAB). The results of this analysis indicate the following: 1) The mean Gulf Stream kinetic energy flux increases downstream at a rate of (2.17 ± .98) × 10−2 ergs cm−3 s−1; 2) The eddies tend to decelerate the mean flow at a rate of (-0.57 ± 1.3) × 10−2 ergs cm−3 s−1; 3) In order that the mean energy equation be balanced, the Gulf Stream in the SAB must be releasing mean potential energy by flowing down a mean pressure gradient. Thus we have evidence suggesting the existence of a component of the pressure gradient associated with the Gulf Stream which is not geostrophically balanced. The downstream pressure gradient inferred at our array site is consistent with published estimates of mean alongshore pressure gradients in the SAB; however, the partitioning of the pressure force between mean acceleration and eddy Reynolds stress most likely holds only near the bump. We also estimate the net loss from the mean potential energy in the SAB using our measured conversion rate and demonstrate that it compares in magnitude but is opposite in sign to that thought to occur downstream of Cape Hatteras. Thus we argue that the Gulf Stream in the SAB is exhibiting some of the properties of the inflow regions of western boundary layers in inviscid inertial models of the general ocean circulation. Our measurements, however, also indicate the presence of vigorous eddies whose effects in the mean energy equation are potentially sizeable. Such eddies are, of course, not contained in strictly inviscid, inertial models of the western boundary layer.
Abstract
The energy budgets of the eddies and the mean flow in the Gulf Stream near a topographic feature known as the Charleston bump are computed. First, we consider these results in the context of the amplification hypothesis for the development of Gulf Stream meanders. According to this hypothesis, the finite amplitude Gulf Stream fluctuations observed offshore of Onslow Bay are the result of the destabilizing effect of the bump on the Stream. The present dataset was obtained both immediately upstream and downstream of the bump, and the results of our analysis suggest: 1) Immediately south of the Charleston bump, the eddies perform net work on the Gulf Stream at a rate of (1.02 ± .66) × 10−2 ergs cm−3 s−1 by transporting momentum offshore; 2) The net work performed by the eddies south of the bump is not used locally to accelerate the mean; rather, it is exported to the rest of the ocean at a rate of (1.58 ± 1.39) × 10−2 ergs cm−3 s−1; 3) In spite of the net work performed by the eddies south of the bump, eddy kinetic energy apparently does not decrease; 4) Immediately north of the Charleston bump, the flow appears to be both barotropically and baroclinically unstable. These results support the amplification hypothesis by demonstrating the destabilizing effect of the bump on the eddies (points 1 and 4) and that upstream perturbations may survive to encounter the bump topography (point 3). Other results of our analysis are that the mean of mean kinetic energy by the eddies constitutes the dominant form of energy conversion and that eddy pressure work may be an important factor in the fluctuation energy budget.
The second application of our calculations is to a characterization of the mean Gulf Stream in the South Atlantic Bight (SAB). The results of this analysis indicate the following: 1) The mean Gulf Stream kinetic energy flux increases downstream at a rate of (2.17 ± .98) × 10−2 ergs cm−3 s−1; 2) The eddies tend to decelerate the mean flow at a rate of (-0.57 ± 1.3) × 10−2 ergs cm−3 s−1; 3) In order that the mean energy equation be balanced, the Gulf Stream in the SAB must be releasing mean potential energy by flowing down a mean pressure gradient. Thus we have evidence suggesting the existence of a component of the pressure gradient associated with the Gulf Stream which is not geostrophically balanced. The downstream pressure gradient inferred at our array site is consistent with published estimates of mean alongshore pressure gradients in the SAB; however, the partitioning of the pressure force between mean acceleration and eddy Reynolds stress most likely holds only near the bump. We also estimate the net loss from the mean potential energy in the SAB using our measured conversion rate and demonstrate that it compares in magnitude but is opposite in sign to that thought to occur downstream of Cape Hatteras. Thus we argue that the Gulf Stream in the SAB is exhibiting some of the properties of the inflow regions of western boundary layers in inviscid inertial models of the general ocean circulation. Our measurements, however, also indicate the presence of vigorous eddies whose effects in the mean energy equation are potentially sizeable. Such eddies are, of course, not contained in strictly inviscid, inertial models of the western boundary layer.
Abstract
The linear stability of two-layer primitive equation ocean rings is considered in the case when the rings are wide compared with a deformation radius, as is usually observed. Asymptotic theory is developed to show the existence of solutions for arbitrarily wide rings, and these solutions can be followed as the rings are made successively narrower. An exponential cubic radial dependence is used for the mean flow, rather than the more usual Gaussian structure. There are two reasons: a Gaussian shape was fully discussed in a previous paper, and a Gaussian has exceptional properties, unlike other power laws. The specific cases of warm and cold Gulf Stream rings are considered in detail. The theory provides an accurate prediction of phase velocity and growth rate for cold rings and a reasonable prediction for warm rings. Solutions in the asymptotic regime have a larger growth rate than other (nonasymptotic) solutions for cold rings, but not for warm rings. Attention is given to the role of the mean barotropic circulation, which had been found in earlier work to have a strong effect on ring stability. There is still evidence for stabilization when the mean flow in the lower layer vanishes, but other features are also involved. In particular, the linear stability of a ring appears to be as sensitive to subtle shape details as it is to the sense of the deep flow. The authors generally find warm co-rotating rings with a cubic exponential form to be unstable, although somewhat less so than counterrotating rings.
Abstract
The linear stability of two-layer primitive equation ocean rings is considered in the case when the rings are wide compared with a deformation radius, as is usually observed. Asymptotic theory is developed to show the existence of solutions for arbitrarily wide rings, and these solutions can be followed as the rings are made successively narrower. An exponential cubic radial dependence is used for the mean flow, rather than the more usual Gaussian structure. There are two reasons: a Gaussian shape was fully discussed in a previous paper, and a Gaussian has exceptional properties, unlike other power laws. The specific cases of warm and cold Gulf Stream rings are considered in detail. The theory provides an accurate prediction of phase velocity and growth rate for cold rings and a reasonable prediction for warm rings. Solutions in the asymptotic regime have a larger growth rate than other (nonasymptotic) solutions for cold rings, but not for warm rings. Attention is given to the role of the mean barotropic circulation, which had been found in earlier work to have a strong effect on ring stability. There is still evidence for stabilization when the mean flow in the lower layer vanishes, but other features are also involved. In particular, the linear stability of a ring appears to be as sensitive to subtle shape details as it is to the sense of the deep flow. The authors generally find warm co-rotating rings with a cubic exponential form to be unstable, although somewhat less so than counterrotating rings.
Abstract
An analytical and numerical study of isolated coherent vortices and topography is presented. The motivation for this work comes from many observations of vortices influenced in trajectory, propagation, and decay by encounters with midocean ridges, seamounts, and bottom slopes. In particular, analytical predictions relevant to vortex propagation and evolution are compared with numerical results for lenses on bottom slopes and mixed barotropic–baroclinic eddies over a variety of topographies. The latter case includes examination of short-term and long-term behavior. Analytical theories are found to work well for the bottom lenses, and short-term behavior is captured well by a simple theory that emphasizes barotropic dynamics for mixed vortices. The exception for the latter case occurs for counterrotating eddies (i.e., eddies with opposing upper- and lower-layer swirl), for which the evolution is dominated by vortex instability. Long-term evolution has no comparable theory, and the various possibilities for vortex behavior are delineated by means of exploratory numerical work. A specific application to the case of North Brazil current rings, which are observed to move at anomalous rates, is presented.
Abstract
An analytical and numerical study of isolated coherent vortices and topography is presented. The motivation for this work comes from many observations of vortices influenced in trajectory, propagation, and decay by encounters with midocean ridges, seamounts, and bottom slopes. In particular, analytical predictions relevant to vortex propagation and evolution are compared with numerical results for lenses on bottom slopes and mixed barotropic–baroclinic eddies over a variety of topographies. The latter case includes examination of short-term and long-term behavior. Analytical theories are found to work well for the bottom lenses, and short-term behavior is captured well by a simple theory that emphasizes barotropic dynamics for mixed vortices. The exception for the latter case occurs for counterrotating eddies (i.e., eddies with opposing upper- and lower-layer swirl), for which the evolution is dominated by vortex instability. Long-term evolution has no comparable theory, and the various possibilities for vortex behavior are delineated by means of exploratory numerical work. A specific application to the case of North Brazil current rings, which are observed to move at anomalous rates, is presented.
Abstract
Many state-of-the-art numerical ocean models calculate pressure using the hydrostatic balance, or an equation derived from it. The proper form of this deceptively simple-looking equation, ∂p/∂z = −gρ(S, T, p) (where notation is standard), is nonlinear in the pressure p. In contrast, most numerical models solve the linear equation ∂p/∂z = −gρ(S, T, z). This modification essentially replaces the total pressure, which includes a time-dependent signal, with an approximate time-independent pressure associated with the depth of a model grid point. In this paper, the authors argue that the inclusion of the total pressure when solving the hydrostatic equation can generate a depth-dependent baroclinic pressure gradient equivalent to a geostrophic velocity of several centimeters per second. Further, this effective velocity can increase with depth and is largest in dynamically important areas like western boundary currents. These points suggest that the full feedback of pressure on density should be included in numerical models. Examples of the effect using oceanic data and output from a typical primitive equation model run are discussed. Finally, algorithms for both rigid-lid and free surface models that explicitly include full pressure are derived, and some related numerical issues are discussed.
Abstract
Many state-of-the-art numerical ocean models calculate pressure using the hydrostatic balance, or an equation derived from it. The proper form of this deceptively simple-looking equation, ∂p/∂z = −gρ(S, T, p) (where notation is standard), is nonlinear in the pressure p. In contrast, most numerical models solve the linear equation ∂p/∂z = −gρ(S, T, z). This modification essentially replaces the total pressure, which includes a time-dependent signal, with an approximate time-independent pressure associated with the depth of a model grid point. In this paper, the authors argue that the inclusion of the total pressure when solving the hydrostatic equation can generate a depth-dependent baroclinic pressure gradient equivalent to a geostrophic velocity of several centimeters per second. Further, this effective velocity can increase with depth and is largest in dynamically important areas like western boundary currents. These points suggest that the full feedback of pressure on density should be included in numerical models. Examples of the effect using oceanic data and output from a typical primitive equation model run are discussed. Finally, algorithms for both rigid-lid and free surface models that explicitly include full pressure are derived, and some related numerical issues are discussed.
Abstract
The California Undercurrent (CUC) flows poleward mostly along the continental slope. It develops a narrow strip of large negative vertical vorticity through the turbulent boundary layer and bottom stress. In several downstream locations, the current separates, aided by topographic curvature and flow inertia, in particular near Point Sur Ridge, south of Monterey Bay. When this happens the high-vorticity strip undergoes rapid instability that appears to be mesoscale in “eddy-resolving” simulations but is substantially submesoscale with a finer computational grid. The negative relative vorticity in the CUC is larger than the background rotation f, and Ertel potential vorticity is negative. This instigates ageostrophic centrifugal instability. The submesoscale turbulence is partly unbalanced, has elevated local dissipation and mixing, and leads to dilution of the extreme vorticity values. Farther downstream, the submesoscale activity abates, and the remaining eddy motions exhibit an upscale organization into the mesoscale, resulting in long-lived coherent anticyclones in the depth range of 100–500 m (previously called Cuddies) that move into the gyre interior in a generally southwestward direction. In addition to the energy and mixing effects of the postseparation instability, there is are significant local topographic form stress and bottom torque that retard the CUC and steer the mean current pathway.
Abstract
The California Undercurrent (CUC) flows poleward mostly along the continental slope. It develops a narrow strip of large negative vertical vorticity through the turbulent boundary layer and bottom stress. In several downstream locations, the current separates, aided by topographic curvature and flow inertia, in particular near Point Sur Ridge, south of Monterey Bay. When this happens the high-vorticity strip undergoes rapid instability that appears to be mesoscale in “eddy-resolving” simulations but is substantially submesoscale with a finer computational grid. The negative relative vorticity in the CUC is larger than the background rotation f, and Ertel potential vorticity is negative. This instigates ageostrophic centrifugal instability. The submesoscale turbulence is partly unbalanced, has elevated local dissipation and mixing, and leads to dilution of the extreme vorticity values. Farther downstream, the submesoscale activity abates, and the remaining eddy motions exhibit an upscale organization into the mesoscale, resulting in long-lived coherent anticyclones in the depth range of 100–500 m (previously called Cuddies) that move into the gyre interior in a generally southwestward direction. In addition to the energy and mixing effects of the postseparation instability, there is are significant local topographic form stress and bottom torque that retard the CUC and steer the mean current pathway.
Abstract
The study of barotropic structure and its effects on oceanic ring stability has yielded seemingly conflicting results. Some studies suggest that the stability of a given ring profile is as sensitive to the sense of the barotropic mode as it is to the vertical shear, while others suggest the vertical shear is the sole dominant effect. Here numerical evidence that supports both views is presented. Warm rings with a favorable barotropic structure can retain their monopole nature while cold rings do not. These results are of interest given the observed long lifetimes of oceanic rings.
As evidence a series of initial value integrations is presented. The initial ring profile consists of an exponential profile decaying as the cube of the radial distance, rather than as the squared decay law of the commonly used Gaussian. The reasons for this choice are that previous studies have examined the Gaussian initial condition extensively and recent analysis suggests the Gaussian profile has special stability properties.
The authors find that the barotropic mode affects the coherence of warm rings, yielding essentially stable, monopolar structures for the case that the initial deep flow is in the same sense as the surface flow (i.e., in the“co-rotating” case), even if the initial underlying ring is linearly unstable. Thus, warm rings remain dominantly monopolar, although an underlying, weak tripole is often seen in the final state. Cold rings in the oceanic parameter regime, on the other hand, experience no such stabilizing effects from deep structure. Quasigeostrophic dynamics fails to capture the stabilization tendencies of warm rings with corotating deep flow, suggesting the effect is related to the finite-amplitude thickness changes of a warm ring. The transition from an unstable, warm monopolar initial state to an effectively stable, warm initial monopolar state is a sensitive function of the barotropic mode. Finally, beta-plane experiments demonstrate the robustness of the primitive equation result.
Thus, it is suggested that the barotropic component of a warm ring can enhance ring stability as a monopole by providing for the existence of a nearby tripolar state to which the ring evolves and thereafter remains. The observed stability of cold rings, however, remains a mystery.
Abstract
The study of barotropic structure and its effects on oceanic ring stability has yielded seemingly conflicting results. Some studies suggest that the stability of a given ring profile is as sensitive to the sense of the barotropic mode as it is to the vertical shear, while others suggest the vertical shear is the sole dominant effect. Here numerical evidence that supports both views is presented. Warm rings with a favorable barotropic structure can retain their monopole nature while cold rings do not. These results are of interest given the observed long lifetimes of oceanic rings.
As evidence a series of initial value integrations is presented. The initial ring profile consists of an exponential profile decaying as the cube of the radial distance, rather than as the squared decay law of the commonly used Gaussian. The reasons for this choice are that previous studies have examined the Gaussian initial condition extensively and recent analysis suggests the Gaussian profile has special stability properties.
The authors find that the barotropic mode affects the coherence of warm rings, yielding essentially stable, monopolar structures for the case that the initial deep flow is in the same sense as the surface flow (i.e., in the“co-rotating” case), even if the initial underlying ring is linearly unstable. Thus, warm rings remain dominantly monopolar, although an underlying, weak tripole is often seen in the final state. Cold rings in the oceanic parameter regime, on the other hand, experience no such stabilizing effects from deep structure. Quasigeostrophic dynamics fails to capture the stabilization tendencies of warm rings with corotating deep flow, suggesting the effect is related to the finite-amplitude thickness changes of a warm ring. The transition from an unstable, warm monopolar initial state to an effectively stable, warm initial monopolar state is a sensitive function of the barotropic mode. Finally, beta-plane experiments demonstrate the robustness of the primitive equation result.
Thus, it is suggested that the barotropic component of a warm ring can enhance ring stability as a monopole by providing for the existence of a nearby tripolar state to which the ring evolves and thereafter remains. The observed stability of cold rings, however, remains a mystery.
