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Hsien Wang Ou

Abstract

Two conceptual models of a two-layered frontal system are presented to study the wintertime shelf-slope front. The first model examines the geostrophic adjustment over a step topography after the fall overturning and applies only over short time scale before the nonconservative processes become important. The second model, on the other hand, examines the thermodynamic balance over longer time scales when some dissipative and mixing effects are included.

From the geostrophic-adjustment model, it is found that the flat-bottom solution of a less-dense shelf water with respect to the slope water is little modified by the presence of a step. But in the case of denser shelf water, the solution shows the detachment of the spillage when the depth ratio across the step is greater than two, resembling some regional observations.

In the frictional model, the wind generated entrainment is demonstrated to provide a virtual momentum source to maintain the along-front current against friction and thus can account for the persistence of the front through the winter season. The entrainment also decreases the buoyancy of the exported shelf water, the distribution of which however, varies greatly with the external parameters. For parameter values applicable to the Middle Atlantic Bight, an inflection point, corresponding to a weakened lateral buoyancy gradient, is predicted above the front, consistent with observation.

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Hsien-Wang Ou

Abstract

A reduced-gravity model is used here to investigate the dynamics of a buoyant flow through a strait driven by pressure difference of the adjoining basins. Assuming the flow to be hydraulically controlled so that the transport is maximized, flow structures in the upstream basin, during its transit through the strait and along the downstream coast, are determined.

It is found in particular that the combined effect of friction exerted by the sill and stretching of the buoyant layer as it exits the strait may cause the downstream flow to exhibit two velocity maxima—along the layer outcrop and the coastal boundary. When applied to the Tsushima Current of the Japan/East Sea, the required conditions for branching are amply satisfied, the model thus provides a plausible explanation of this observed feature. In addition, a favorable comparison between predicted and observed transports supports the hydraulic control of the flow.

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Hsien Wang Ou

Abstract

A simple two-dimensional model is used to determine the geostrophically adjusted state of an initially motionless fluid with lateral density gradient. It is an extension of the much studied two-layer models to the case when the initial stratification is continuous, and allows a more detailed examination of the density and current fields. In addition, the model has demonstrated that geostrophic adjustment with its accompanying transverse circulation can produce regions of convergence and thus steepen the initial density gradient. In cases when the initial density transition is sharp enough, fronts, or density discontinuities, are formed at these convergence regions, along which the surface fluid particles drawn in from both sides of the fronts move into the interior. The density jump across the fronts is the greatest at the top and bottom surface and diminishes toward the interior where differential vertical motion during the adjustment can significantly level the isopycnals. One observed frontal structure south of Nantucket Shoals exhibits some qualitative feature predicted by the model.

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Hsien-Wang Ou

Abstract

A model of tidal rectification in a homogeneous ocean (Part I) is extended here to include a front that separates shelf and slope waters. The front is approximated as a density discontinuity, the stratification and anchoring position of which are given, but which otherwise is coupled to the flow field. The dynamical closure is formulated through vorticity balance of the two layers and a parameterization of potential vorticity (PV) flux in terms of local tidal amplitude and the mean field.

As an example of the frontal effect on the tidally rectified flow, a solution is calculated for the case of negligible interfacial stress and PV flux in the bottom boundary layer and compared with that of a homogeneous ocean. It is found that the mean along-isobath flow outside the frontal zone remains largely unchanged, but is qualitatively altered in the frontal zone. Specifically, the mean flow above the sloping front—being insulated from bottom friction—is greatly intensified by PV mixing, consistent with observed seasonal change over Georges Bank. The mean flow below the frontal interface on the other hand is weakened by enhanced frictional effect—to nearly zero at the foot of the front. The vanishing Ekman transport would strengthen the front as a barrier to offshore transport of a passive tracer.

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Hsien Wang Ou

Abstract

Motivated by the observed branching of the Equatorial Undercurrent in the Gulf of Guinea, an idealized model is developed here to examine the termination of an equatorial jet in a gulf. Similarity solutions are, found that can satisfy the boundary conditions along an idealized gulf coast and hence represent realistic flow fields. It is found that friction tends to retard the northern branch, resulting in a southward flow at the equator, and that as the incident jet intensifies, the cross-equatorial flow is increasingly prohibited, resulting in a greater return flow within the gulf.

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Hsien Wang Ou

Abstract

Through a simple model, it is demonstrated that earth's sphericity (the beta effect) imposes a severe constraint on the discharge pattern near the equator. Using either bottom or lateral friction to counter the beta effect in the vorticity balance, the flow in the far field is confined to boundary layers either along the solid boundaries that are open on the anticyclonic side or along the equator to the west of the point source. Thus, for all possible orientation of the radial boundaries flanking the point source, there are either one or two branches receptive of the discharge in the far field depending on whether the open angle spanned by the two boundaries excludes the east direction. Even when the branching is permissible, it is further argued, based on symmetry of the governing equation, that the outflow is strongly favored toward the branch that deviates more from the east direction. Numerical solutions show that the bulk of the diversion of the discharge to this branch occurs within one frictional scale of the point source. Since this distance, as crudely estimated, can be of order 100 km, the model can explain the sharp deflection of the Amazon outflow to the northern coast over the shelf proper without requiring asymmetric external forcings.

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Hsien-Wang Ou

Abstract

In previous studies of tidal generation of mean flow over varying topography, the rectification mechanism has generally invoked bottom friction as a source of tidal flux of momentum and vorticity (hence referred as“friction” mechanism). The author proposes a different mechanism based on horizontal mixing of potential vorticity. Drawing analogy from tidal dispersion of passive tracers, this mixing is parameterized through a diffusivity (hence called “diffusivity” mechanism) that is quadratic in the tidal amplitude. In this, Part 1, the mean along-isobath flow near a shelf break is determined for a homogeneous ocean and contrasted with that induced by friction mechanism. In Part 2, the effect of a front will be considered.

It is found that although the mean flow is pointing in the same direction as that induced by friction mechanism (i.e., to the right when facing deep water in the Northern Hemisphere), it varies more slowly with the tidal amplitude. In the typical situation when the mean shear is small compared with the Coriolis parameter, this dependence is linear rather than quadratic, as is the case for the friction mechanism. This linear dependence compares more favorably with observations over Georges Bank.

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Hsien-Wang Ou

Abstract

A global-mean model is used here to elucidate possible bounds on the surface temperature of a simplified ocean–atmosphere system. Extending previous one-dimensional models, it has included as internal variables the low-level and high-level cloud covers and the turbulent wind at the surface. The main hypothesis for the model closure is that the conversion rate from the solar to the kinetic energy—or, equivalently, the rate of internal entropy production—is maximized, which has been applied with considerable success in past latitudinal models. From the model derivation, it is found that the surface temperature is narrowly bounded below by the onset of the greenhouse effect and above by the rapid increase of the saturation vapor pressure. Because both are largely intrinsic properties of water, the resulting surface temperature is mostly insensitive to detailed balances or changing external conditions. Even with a 50% change of the solar constant from its present-day value, the model temperature has varied by only about 10 K. The reason that the heat balances can be maintained is an internal adjustment of the low cloud cover, which offsets the solar effect. The model offers a plausible explanation of an equable climate in the geological past so long as there is a substantial ocean.

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Hsien-Wang Ou

Abstract

In the Antarctic, dense shelf water is formed in coastal polynyas and is differentiated from the fresher surface water by the wind-induced ice motion that displaces offshore the ice melt from production zones. Where the shelf water discharges into the deep ocean, the Antarctic Slope Front (ASF) is V shaped, separating the shelf and surface waters (referred to as “frontal” waters) from the Circumpolar Deep Water (CDW). To elucidate basic constraints on frontal properties, a minimal model of homogeneous water masses forced by offshore wind and freshwater input in a perpetual winter is considered. With the surface water stirred by—and hence aligned with—the ice cover, there is little leakage of ice or meltwater from the frontal system, so ice production and melt merely redistribute heat and salt between frontal waters. As such, the heat loss to the atmosphere needs to be supplied by entraining CDW, which then necessitates the shelf water discharge on account of the mass balance. Because of the freshwater input, the discharged shelf water may not be saltier than the CDW, which thus may descend the slope to form bottom water only because of its coldness. With both frontal waters cooled to the freezing point, dominant balances are formulated to determine their salinity and exchange rate with the ambient CDW. Although extremely crude, the model derivations are favorably compared with observations, which thus may provide physically based parameterizations for the bottom-water formation that can be incorporated into global models.

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Hsien Wang Ou

Abstract

It is found that for a continuously stratified fluid which remains so during the geostrophic adjustment, the energy conversion ratio γ (≡ΔKEPE) is ½, in contrast to the value of ⅓ for a two-layer fluid. Since the two-layer fluid is an asymptotic limit of a continuously stratified fluid, it is deduced that γ decreases smoothly from ½ to ⅓ when density discontinuities are formed during the adjustment. Furthermore, the total energy released during the adjustment is found to be inversely proportional to the dimensionless horizontal scale (scaled by the baroclinic radius of deformation) characterizing the density variation in the initial state.

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