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W. R. Young

Abstract

The rectification of oscillatory tidal currents on the sloping sides of a low submarine bank is discussed using the moment method. This method has been previously used in shear dispersion studies where it is used to analyze the advection-diffusion equation. In the present problem it is applied to the barotropic potential vorticity equation linearized about an oscillatory, spatially uniform tidal velocity. To apply the method it is necessary to assume that the topography produces only a small change in depth. The method economically provides the most important qualitative properties (e.g., transport, location and width) of the time averaged current.

These results are obtained without making an harmonic truncation. They can then be used to assess the accuracy of the harmonic truncation approximation used by other authors. It is shown that harmonic truncation correctly predicts the transport and location of the rectified current when the bank is low. However if the width of the bank is much less than a tidal excursion distance, harmonic truncation may give a very mistaken impression of the width of the rectified current.

Finally, lateral vorticity diffusion is included in the moment calculation. It is shown that this dissipative process does not change the transport or location of the rectified current. It does however increase its width.

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W. R. Young

Abstract

The density of the mixed layer is approximately uniform in the vertical but has dynamically important horizontal gradients. These nonuniformities in density result in a vertically sheared horizontal pressure gradient. Subinertial motions balance this pressure gradient with a vertically sheared velocity. Systematic incorporation of shear into a three-dimensional mixed layer model is both the goal of the present study and its majority novelty.

The sheared flow is partitioned between a geostrophic response and a frictional, ageostrophic response. The relative weighting of them two components is determined by a nondimensional parameters μ≡1/fτ U , where τ U is the timescale for vertical mixing of momentum and f −1 is the inertial timescale.

If μ is of order unity, then the velocity has vertical shear at leading order. Differential advection by this shear flow will tilt over vertical isosurfaces of heat and salt so as to “unmix” or “restratify” the mixed layer. The unmixing process is balanced by intermittent mixing events, which drive the mixed layer back to a state of vertical homogeneity.

All of these processes are captured by a new set of reduced or filtered dynamics called the subinertial mixed layer (SML) approximation. The SML approximation is obtained by expanding the equations of motion in both Rossby number and a second small parameter that is the ratio of the vertical mixing timescale to the dynamic time scale. The subinertial dynamics of slab mixed layer models is captured as a special case of the SML approximation by taking the limit μ → ∞.

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W. R. Young

Abstract

A variety of two-dimensional advection-diffusion models are investigated analytically with the goal of understanding the role of boundary layer in gyre-scale ocean mixing.

It is assumed throughout that the Péclet number of the flow in the Sverdrup interior, Pe = UL/k is large. (Here, L) is the length of the gyre, U is the velocity scale in the interior and k is the explicit diffusivity.) There are then two limits depending on the size of (I/L) where l is the width of the western boundary layer.

First, if (I/L)P is large, the diffusion is weak everywhere in the gyre and the tracer makes repeated passages through the boundary layer before mixing. In this case, the time taken to mix is (Ll/k). Second, if (l/L)P is small, the diffusion is strong in the boundary layer and the time taken to mix is the circulation time L/U. In any case, the mixing time is substantially less than the diffusion time based on the length scale of the gyre.

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J. Pedlosky
and
W. R. Young

Abstract

A model for the vertical structure of the oceanic circulation is presented that combines elements of the theory of the ventilated thermocline, given by Luyten, Pedlosky and Stommel, with the theory of Rhines and Young for the wind driven circulation of an unventilated ocean.

Our model consists of a ventilated thermocline region above an unventilated zone in which motion is limited to pools of constant potential vorticity. The model is nonlinear and hence the presence of ventilation affects the dynamics of the unventilated motion and vice-versa.

The planetary geostrophic equations are used and so the quasi-geostrophic assumption of Rhines and Young is relaxed, allowing large isopycnal excursions.

It is shown that the presence of ventilation generally shrinks and weakens the size and vigor of the subsurface pools of homogenized potential vorticity. At the same time, within those domains, the strength of circulation in the ventilated zone is somewhat diminished as the subsurface layers carry a portion of the Sverdrup transport.

We argue that the (mathematically) consistent circulation in the absence of sub-thermocline constant potential-vorticity pools is unstable.

The non-uniqueness of the nondissipative Sverdrup dynamics is demonstrated by the ambiguity in the specification of potential vorticity in the deeper, unventilated layers. The study emphasizes the subtle importance of dissipation in selecting a unique solution.

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W. R. Young
and
Lianggui Chen

Abstract

The density of the mixed layer (ML) is approximately uniform in the vertical, but there are dynamically important horizontal gradients. The subinertial mixed layer (SML) approximation is a small Rossby number filtering of the primitive equation that isolates the low frequency (ω ≪ f) dynamics.

A linear stability analysis based on the SML approximation shows that the horizontal density gradients within the mixed layer (ML) support baroclinically unstable waves with inverse wavenumbers in the range 1 to 10 km. This conclusion follows from both a slab ML model, in which the horizontal velocity has no vertical shear, and a geostrophic ML model, in which the horizontal velocity is sheared according to the thermal wind relation. In the geostrophic case the instability is identical to the long wavelength limit of baroclinically unstable Eady waves.

An interesting difference between the slab and geostrophic ML is the dynamics of thermal and saline anomalies. In the slab case, thermohaline anomalies are advected without shear dispersion, and the initial TS relation is preserved. In the geostrophic case, the shear dispersion associated with the thermal wind produces a flux of heat and salt orthogonal to the buoyancy gradient. This flux varies as the cube of the thermohaline gradients, and it acts so as to mix heat and salt while leaving buoyancy unchanged on fluid particles. The mechanism tighten an initially diffuse TS relation so that a cloud of points in the TS plane condenses onto a curve.

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Luanne Thompson
and
W. R. Young

Abstract

Laboratory experiments show that ageostrophic instability can “break up” a parallel flow into a sequence of axisymmetric eddies. This is a plausible scenario for the generation of sub-mesoscale coherent Vortices (SCVs). Here we show that conservation of mass, energy and potential vorticity enables one to very simply calculate the radius of the axisymmetric eddy and the wavelength of the nonlinear instability. The latter agrees more closely with laboratory experiments than does the wavelength predicted by linearized stability theory. It energy is not conserved, say because of wave radiation into the lower layer, then the preceding calculation establishes an upper bound on the radius of the eddy. We offer this as an explanation of the observed small size of SCVs.

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Lianggui Chin
and
W. R. Young

Abstract

Density-compensated temperature and salinity gradients are often observed in mixed layer fronts. A possible explanation Of this Observation is that there is a systematic relation between the “strength” of a front, defined as the buoyancy jump across the front, and the thickness of a front. If stronger fronts tend to be thicker, then in an ensemble of random fronts, in which the temperature and salinity jumps am independent random variables, the temperature and salinity gradients will he correlated. This correlation between the thermohaline gradients is such that heat and salt make antagonistic contributions to the buoyancy gradient–-that is, them is buoyancy compensation. The statistics of heat and salt fluxes across nearly compensated fronts are counterintuitive: strong heat fluxes can occur across a front with weak thermal gradients and strong salinity gradients, and vice versa.

As a specific model that relates the width of a front to the strength of a front, a pair of coupled nonlinear diffusion equations for heat and salt are used. The nonlinear diffusion coefficient, proportional to the square of the buoyancy gradient, arises from quasi-steady shear dispersion driven by thermohaline gradients. This nonlinear mixing prevents stirring by mesoscale advection from indefinitely filamenting mixed layer tracer distributions. The model predicts that the thickness of a front varies as the square root of the strength and inversely as the one-quarter power of the mesoscale strain.

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G. R. Ierley
and
W. R. Young

Abstract

The stability of the western boundary layer is studied by idealizing it as a parallel flow and solving the Orr–Sommerfeld equation, generalized to include the gradient of planetary vorticity. The critical Reynolds number, at which the idealized flow first becomes unstable, is found to be between 20 and 100 depending on the details of the profile. The modes themselves are trapped within the boundary jet because their phase speeds exceed that of the fastest free Rossby wave with the same meridional wavenumber. However, in the important case of a jet with a broad exponential decay, corresponding to a highly inertial flow, we find that the phase speed of the critical mode exceeds that of a free Rossby wave by a very small amount. Consequently, the trapped mode has a very slowly decaying oscillatory tail and so is much wider than the basic state that supports it. The Reynolds stresses in the tail region induce a mean Eulerian flow opposite in direction to the basic state jet. However, Stokes drift is substantial so that the mean Lagrangian flow is an order of magnitude smaller than the mean Eulerian.

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G. R. Ierley
and
W. R. Young

Abstract

A sub-basin scale recirculation can be driven by imposing low values of potential vorticity in the northwest corner of a β-Plane box. Mesoscale eddies parametrized by lateral potential vorticity diffusion, carry this anomaly into the interior and establish the mean flow. While the structure of the flow is not sensitive to details of the boundary forcing or to the size of the diffusion coefficient, κ, the amplitude and length scale are. For instance, as, κ is reduced, the maximum transport scales as κ½ and the Reynolds number as κ−½.

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G. R. Ierley
and
W. R. Young

Abstract

The question posed in the title of this paper is answered in the affirmative by investigating a two-layer, quasi-geostrophic model of the wind-driven circulation. The two layers model the thermocline rather than the whole depth of the ocean. The wind stress is balanced by interfacial and bottom drag. This is perhaps the simplest baroclinic extension of Stommel's (1948) barotropic circulation model. It differs from an earlier model of Welander (1966) in that the vortex stretching nonlinearity is of primary importance.

In this model the dynamics of the frictional western boundary layer determine the vertical structure of the wind-driven flow in the Sverdrup interior. Thus, in a sense, the boundary layer is “active” and cannot be appended to an arbitrary interior flow; rather it partially determines the interior circulation by setting the functional relationship between the streamfunction and the potential vorticity in the lower layer.

In previous studies (Rhines and Young 1982b) this functional relationship has been calculated using a generalized Prandtl-Batchelor theorem. This result does not apply to the present calculation because every lower layer streamline passes through a frictional boundary layer.

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