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William K. Dewar

Abstract

Convection in the world's oceans often occurs in small, semienclosed basins where bottom slopes and nearby continental shelf breaks are commonplace. The evolution of convectively generated heat anomalies in such settings is studied using quasigeostrophic finite-difference and point vortex models. The displayed behaviors divide essentially into two categories: whole fluid column convection, in which bottom-slope effects are felt immediately, and partial fluid column convection, in which the topographic effects can be delayed. In both cases, topography significantly modifies the evolution of convective patches from that occurring over flat bottoms. Vertical walls induce strong self-propagation mechanisms that accelerate alongslope heat transport, while the continental shelf slope is repulsive and rejects lower-layer anticyclones. These anomalies are then “stranded,” being too far offshore to interact with the shelf break and having lost their heton partner in the interaction. Weaker deep ocean topographic slopes disrupt heton formation and disperse convective patches by topographic mechanisms. Partial fluid column convection, with stratification under the mixed layer, proceeds through a cascade from small to large length scales. In oceanically relevant regimes, smaller scales are shielded from bottom slopes and can disperse as small hetons. Larger-scale structures are prevented by the topography from forming into hetons and instead evolve as if in a sloping-bottom two-layer system. The small hetons, when encountering shelf breaks, can experience topographic repulsion and stranding. Comparisons with the Mediterranean Sea suggest alternative interpretations for some observations, and several observed Labrador Sea mesoscale convective characteristics can be ascribed to topographic effects.

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William K. Dewar

Abstract

Point vortex and finite-difference methods are used to study baroclinic eddies advected into weak and strong encounters with topography. It is argued that weak interactions often scatter radially symmetric eddies into generalized hetons. The dipole moments so generated within the eddy result in eddy propagations at various angles to the current. Strong interactions can result in the complete separation of the upper- and lower-layer circulations. Subsequent evolution in this case depends on many factors, although strong topographic obstacles (i.e., seamounts) permit a reorganization of the centers into a coherent structure. Weaker topography, confined to the deep ocean, can disrupt the lower center, although the upper center typically survives. Disassociation of the centers with both retaining their integrity is also possible. Heton generation can occur for eddies with weak lower-layer expressions, demonstrating a potentially strong control of shallow eddy propagation by deep sea bathymetry. Analytical and numerical estimates of the induced propagation speeds are sizable, arguing topographic scattering is a potentially powerful mechanism influencing eddy propagation.

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William K. Dewar

Abstract

Ocean adjustment on annual to interdecadal scales to variable forcing is considered for a more nonlinear general circulation than has previously been studied. The nature of the response is a strong function of forcing frequency and importantly involves the inertial recirculations rather than linear baroclinic waves. The spatial expression of this variability is concentrated near the separation latitudes of the Gulf Stream extension, a model region corresponding to an area in the real ocean of well-known strong ocean–atmosphere buoyancy exchange. “Turn-on” cases, periodically forced cases, and stochastically forced cases are considered. The first set of experiments clarifies the adjustment timescales and dynamics of a nonlinear circulation. The second set examines modifications to that adjustment rendered by time-dependent forcing. The last set is perhaps the most realistic in terms of the atmospheric forcing of the ocean, because wind spectra are not strongly peaked beyond a few weeks. Multidecadal forcing is argued both to excite a novel, rapid mode of adjustment and to resonate with a considerably slower, nonlinear mode. Stochastic forcing seems clearly to excite the fast mode and to contribute to the slower mode, although the latter also derives considerable variance from intrinsic sources. These conclusions are based on a suite of distinct spatial and temporal characteristics of the dominant ocean variability patterns under various forcing scenarios and comment on the ocean dynamics likely to be important to decadal timescale midlatitude climate variability.

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William K. Dewar
and
John M. Bane

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.

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William K. Dewar
and
John M. Bane

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.

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William K. Dewar
and
Glenn R. Flierl

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.

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William K. Dewar
and
Trevor J. McDougall

Abstract

The numerical solution of the vertical advection–diffusion equation in layered coordinates is revisited. The objectives of this work are to propose a generalization of the discontinuous layered representation of the ocean tracer field to higher-order, smoother representations (while retaining the quasi-Lagrangian character of the coordinate) and compare the solutions generated by several approaches in order to illustrate their respective advantages and disadvantages. The one-dimensional advection–diffusion equation is chosen as a test bed for layered coordinates because ocean simulation for climatic purposes requires the inclusion of dianeutral diffusive processes.

The layered approach is generalized by replacing the traditional stack of well-mixed layers by stacks of piecewise smooth profiles. All the well-known properties of quasi-Lagrangian coordinates are retained. Comparisons of the quasi-Lagrangian solutions with coarse- and fine-resolution fixed grid solutions illustrates the efficiency of the adaptive, quasi-Lagrangian coordinate.

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Liang Gui Chen
and
William K. Dewar

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.

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William K. Dewar
and
Christine Gailliard

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).

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William K. Dewar
and
John C. Marshall

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.

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