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Shenn-Yu Chao

Abstract

Low-level winds over land-ocean boundaries are generally alongshore. Two types of forcing that generate alongshore jets in the lower atmosphere are considered here, using as simple a model as possible. The first type of forcing is the blocking of the low-level zonal winds by topographies. Winter alongshore wind off the west coast of North America is topography-forced. Our simple model produces wind fields that have horizontal structure similar to the observed. Based on the distribution of zonal wind stress with latitudes, the model suggests that the alongshore wind over the west coast of the United States reveres its direction near 40°N, in fair agreement with observations. However, the linear model underestimates the alongshore wind off the California coast by a factor of about 1.7–2. Consequently, linear prediction of nearshore wind stress is about 3–4 times smaller than the observed. The discrepancy is attributed to nonlinear effects. A nonlinear analysis is presented to show how nonlinearity increases the speed of the coastal jet. The second type of forcing is the temperature difference between the land and the ocean. The dynamic similarities and differences between a thermally driven coastal jet and a topography-forced alongshore jet are discussed.

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Shenn-Yu Chao

Abstract

A barotropic ocean model is used to study the bimodal behavior of the Kuroshio to the south of Japan. By considering the combined effects of the beta plane, the Kyushu coastal perturbation, the Izu Ridge and the SW-NE tilted coastline, the two frequently observed meander patterns have been numerically verified as the two quasi-steady solutions contained n the model, The small-meander state is identified as an upstream disturbance largely forced by the Izu Ridge; its width decreases as the strength of the current increases. The large meander state is forced by the presence of both the Kyushu wedge and the Izu Ridge topography. For small volume transports the large meander state behaves like the small meander state, in that the meander width decreases as the current speed increases; for large volume transports it behaves like a lee Rossby wave, in that the width of the meander increases as the current speed increases.

The birth of the large meander state occurs as a consequence of the ocean spin-down. Prior to the generation of the large meander state, the Kyushu wedge excites small meanders and eddies which propagate eastward at a speed of several miles a day. This model result agrees well with observations.

The bimodality of the Kuroshio is identified as an example of multiple equilibrium states (Charney and Fierl). Below a volume transport of 30 Sv, only the small meander state exists. The large meander state and the small meander state coexist in a range of volume transports from 30 to 60 Sv. Beyond 60 Sv, a strongly nonstationary solution develops and the structure of the meander can no longer be determined by the present model.

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Shenn-Yu Chao

Abstract

An analytical method is developed to compute the diffraction of a barotropic Kelvin wave by a localized topographic irregularity on an otherwise flat-bottom ocean with an arbitrary vertical stratification. The bump topography is assumed to be small in height compared to the water depth of the flat-bottom ocean. It is found that all baroclinic mode Kelvin waves will be generated downstream of the bump, with the first baroclinic mode having the largest amplitude. At subinertial frequencies (ω<f) localized disturbances are also generated with higher vertical modes trapped nearer to the bump. At superinertial frequencies (ω>f) cylindrical Poincard waves with certain anisotropy are generated at (x = x 0, y = 0) and (x = −x 0, y = 0), where (x 0,0) is the center of the bump topography, and the y axis is the coastline. The Poincaré waves favor the lowest few modes, with the baroclinic modes having stronger tendencies to be directionally anisotropic. The baroclinic Poincaré waves radiating offshore from the bump topography could contribute to the internal wave field in the open ocean and provide an alternative mechanism to dissipate the barotropic tides. Order-of-magnitude estimates show that an energy flux of ∼0.09 W per centimeter coastline could be converted from the M2 tide in the eastern Pacific.

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Shenn-Yu Chao

Abstract

The forced circulation over a continental shelf generated by the alongshore wind stress is studied within the frictional regime. The model alongshore wind stress has a finite extent in the alongshore direction and oscillates monochromatically, resembling a series of anticyclones traveling across the coast-line. Both the shelf-wave response and the localized non-wavelike response exist within the wind band. The numerical results show that the interaction of shelf waves with locally wind-forced response generates a large increase in the phase speed within the wind band. Given an alongshore variation in the alongshore wind stress, a right-bounded phase propagation is possible even in the absence of continental shelf waves. Given a reasonable friction currently accepted for the east coast of the United States, the resonance mechanism at the cutoff frequency may not be important, and lower frequency wind events generate larger amplitude continental shelf circulation. By reducing the friction, energy at cutoff frequencies leaks out of the wind band in both directions effectively. It also is shown that non-wavelike response depends only on the local wind stress and is not affected significantly by friction.

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Shenn-Yu Chao

Abstract

The effect of wind forcing on existing estuarine plumes in a coupled estuary-shelf environment is studied here using a three-dimensional primitive-equation model. The emphasis is on wide estuaries so that the Coriolis force cannot be ignored. Over the shelf, the plume responds mostly to the wind-induced surface Ekman drift. Under downwelling-favorable wind, an additional downwind coastal jet occurs that elongates the plume along the shore. For cross-shelf winds, the nearshore Ekman drift is considerably retarded by the sea level setup or setdown. The retardation is particularly effective when the stratification increases. These properties of wind-driven coastal circulations determine the first-order plume responses.

Inside the estuary, the down estuary wind reinforces the gravitational circulation, but the up-estuary wind opposes it. Both winds are effective local forcings that make the remotely forced signals from the shelf unlikely to be detected. Compared to longitudinal winds, cross-estuary winds are less effective in diving the local circulation, allowing the remotely forced signals from the shelf to stand out better. The upwelling-favorable wind enhances the gravitationally induced longitudinal currents. In consequence, remote signals from the shelf are barely detectable. The downwelling-favorable wind first neutralizes longitudinal currents. Thereafter, the remotely forced signal becomes the dominant response.

Two types of wind-induced vertical mixing are produced by the model. The first is typically observed during seaward wind, which enhances the vertical current shear and destratifies the water column from the surface downward. The second is typically during the landward or downwelling-favorable wind, which weakens and then reverses the surface current and its associated vertical shear, transports heavier water atop the light water pool, and destratifies the water column.

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Shenn-Yu Chao

Abstract

The development, maintenance, and dissipation of river-forced estuarine plumes with and without seaward sloping bottom are studied by use of a three-dimensional, primitive-equation model. Inside the estuary, discussion is focused on how the Coriolis force induces lateral asymmetries in the circulation. Four physical processes dictate the transient as well as the quasi-steady circulation in moderately stratified estuaries: 1) upper-layer convergence during the transient spinup phase, 2) upward entrainment leading to the two-layer steady circulation pattern, 3) upward stretching of the lower-layer vortex during the spindown period, and 4) in the presence of a seaward bottom slope, the left-bounded tendency of the landward undercurrent for all phases of development. Deviations from the laterally averaged circulation pattern caused by these processes are discussed. Over the shelf, various types of plumes are defined according to the visual appearance of the surface salinity field. The four types of plumes, namely, the supercritical, subcritical, diffusive-supercritical, and diffusive-subcritical are classified by an empirical Froude number and another dimensionless parameter indicating the amount of dissipation acting on density currents.

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Shenn-Yu Chao

Abstract

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Shenn-Yu Chao

Abstract

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Shenn-Yu Chao

Abstract

Off an estuary mouth, the nonlinear transfer of vorticity from the oscillating onshore-offshore tidal currents to the mean field generates a pair of counter-rotating eddies that flows seaward near the center axis of the inlet and landward on both sides. The Coriolis deflection strengthens the anticyclonic eddy but weakens the cyclonic one. A seaward sloping shelf enhances these eddies but a seaward sloping estuary weakens them. These tidal residual eddies strengthen and hasten the expansion of a river-forced plume off an estuary mouth, but weaken and retard the development of the coastal jet farther downcoast. To lowest order, this is the primary effect of tides on an estuarine plume in a coupled estuary-shelf system. Tidal effects on the subtidal river-forced circulation inside the estuary are secondary.

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Shenn-Yu Chao

Abstract

A two-dimensional dry model for the atmosphere is coupled with a two-dimensional primitive equation model for the ocean to investigate how cold fronts interact with the Gulf Stream and its adjacent waters during cold-air outbreaks. The development of a cross-stream frontal circulation under the influence of the ocean surface heating and its impact on the ocean circulation are determined for various frontal and synoptic conditions. The diabatic heating under an offshore wind is shown to generate a convective boundary layer over the ocean, which deepens seaward. The interaction of the convective boundary layer and the seaward-moving cold front decreases the speed of the frontal propagation and induces a downdraft behind the front and an updraft near the nose of the front. This process is largely independent of the vertical shear of the offshore wind. The postfrontal downdraft over the coastal ocean intensifies the low-level southward wind, which in turn weakens the Gulf Stream transport. To lowest order, this sequence of events is the primary effect of coastal air–sea interaction on a weekly time scale identified by the model. Conventional oceanography models that treat the ocean cooling as a forcing independent of the wind stress may grossly underestimate its effects during cold-air outbreaks. The present model also suggests that the conventional meteorological practice of fixing the sea surface temperature is a reasonable approximation on time scales less than one week, as far as the dry two-dimensional model is concerned. It is speculated that the accuracy of this meteorological approximation will deteriorate if moisture effects and alongshore variations are incorporated; this requires future model verification.

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