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Y. Hsueh

Abstract

The role of topography in the mean structure of low-level winds is investigated. Because the gravitational force vector has a component locally parallel to the sloping terrain, the low-level temperature anomaly, created by the necessary thermal adjustment of the air stream to a fixed ground temperature, results in an Archimedean effect in the Ekman layer. Thus, the viscous boundary layer and the thermal boundary layer are coupled.

A set of three second-order differential equations which approximately governs the dynamics of the thermal-viscous boundary layer is solved. It is shown that topographical features can account for the overall spatial distribution of the wind structure in the southerly flow over the Great Plains and over Kenya. It is also shown that such a boundary layer can provide a low-level convergent environment which is conducive to rainfall activities.

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Y. Hsueh

Abstract

Doppler wind data obtained over the Indian Ocean during the 1964 Indian Ocean Expedition are analyzed in terms of power spectrum estimates. Peculiarities in the computed distribution of the mean horizontal kinetic energy associated with mesoscale (∼1–100 km) motions are noted in light of energy input from mean wind shear and energy export to the density field in a stably stratified atmosphere.

The data indicate that quasi-horizontal eddies 10 km in size derive their energy mainly from mean horizontal wind shear at a wavenumber around 1−2 cycle km−1, and that eddies of a few kilometers in diameter are approximately two-dimensionally isotropic.

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Dongliang Yuan
and
Y. Hsueh

Abstract

An assimilation of routine sea surface temperature observations is conducted to estimate the sea surface heat flux in the Yellow Sea during the winter of 1986. Ten-day mean SST compilations, published by the Japan Meteorological Agency, are used. The time window is from 0000 Japan Local Time (JLT) 21 January to 0000 JLT 21 February 1986. Because there are only three frames of the observed temperature available for the time window, only a steady-state distribution of the heat flux is determined. A tonguelike feature of the optimized heat flux distribution is consistent with the warm SST anomaly at the center of the Yellow Sea trough. The optimized heat flux generates improved simulation of the cooling trend of the temperature time series.

The variational method is the assimilation procedure employed. The developed scheme is able to optimize simultaneously the initial temperature condition and the sea surface heat flux without a priori knowledge of either. A coarse-resolution Hessian is used to evaluate errors of the assimilation.

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Y. Hsueh
and
Dongliang Yuan

Abstract

A vertically integrated model that incorporates horizontal temperature variations is used to study the circulation of the Yellow Sea in a wintertime period for which velocity and temperature measurements are available at several moorings locations along a central trough. The model features realistic bottom topography and is forced with wind stress and heat flux fields from 13 January to 22 February 1986. The model also incorporates, as a boundary condition, sea-level fluctuations derived from coastal and insular tide gauge stations along model boundaries in open waters.

The model reproduces well sea level fluctuations along the coasts of both China and Korea. The hindcast velocity time series, particularly for the north–south component, track those obtained from direct measurements at the moorings. The model momentum balance indicates that the northward flow in the trough is driven by a sea level setup to the south in response to northerly wind bursts in the winter monsoon. The sea-level fluctuations propagate around the Yellow Sea embayment in a counterclockwise sense and exhibit a northward increase in amplitude along both the China and Korea coasts, apparently due to the general shallowness of the northern reaches of the embayment. The lack of a suitable initial condition in temperature and the presence of large biases in the sea surface heat flux distribution preclude the hindcast of the temperature field. Yet trajectories of model fluid displacement confirm an overall northward transport of mass, and hence heat and salt, even though the northerly wind-pulse-dominated current fluctuates with a small mean.

While wintertime currents in the Yellow Sea appear dominated by the wind forcing, empirical orthogonal function analysis of model sea-level fluctuations attributes 48% of the variance to a mode whose time variation follows those of sea-level heights imposed along the open model boundaries. The mode with a time variation similar to that found in the wind stress magnitude time series accounts for only 28% of the variance. This suggests the domination of sea-level fluctuations by low-frequency fluctuations in the Kuroshio.

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Y. Hsueh
and
Richard D. Romea

Abstract

Analyses of low-pass filtered coastal sea-level data and geostrophic winds derived from surface pressure charts for the period of 1 December 1980 through 31 March 1981 indicate that the wintertime dynamics in the Northeast China Sea is strongly influenced by the passage of rapidly moving cold fronts from northwest to southeast across the region. Along the west coast of Korea, sea-level fluctuations are highly coherent with the north–south wind in two bands centered, respectively, at about 0.17 and 0.36 cpd, and propagate to the south for the low-frequency band. Removal of the wind-forced component of the sea-level signal yields freely propagating fluctuations in bands centered at about 0.20, 0.34 and 0.50 cpd, that travel northward at approximately the phase speed expected for barotropic Kelvin waves, indicative of a relaxation in the sea-level field following the successive passages of cold fronts. Coherence studies of winds and sea-level differences in the Tsushima Strait indicate a correlation, at low frequencies (ω < 0.25 cpd), between the along-strait sea-level slope and winds oriented north–south, indicative perhaps of the difference in sign in offshore Ekman transports, under a broad-scale north–south wind, between the east and west coasts of Korea. For 0.5 > ω > 0.08 cpd, fluctuations in sea-level records from stations in the Ryukyu island chain are wind-forced and propagate eastward with similar speeds as the wind forcing, while for ω < 0.08 cpd the island sea levels are not well correlated with the winds. These latter fluctuations propagate eastward with much smaller phase speeds and may be associated with fluctuations in the Kuroshio.

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John M. Bane Jr.
and
Y. Hsueh

Abstract

Free, stable, coastal-trapped waves propagating in a continental shelf regime typical of coastal upwelling areas are studied using a two-layer analytical model. Two cases of interfacial geometry and mean current simulate the hydrographic and current fields before and after upwelling. The completely stratified shelf representing conditions before upwelling allows baroclinic waves to be trapped near the coastline with a cross-shelf scale of l −1, where 1 is the alongshore wavenumber. A geostrophic, baroclinic mean flow is found to give rise to stationary waves for which the phase speed is zero. After-upwelling conditions are modeled by allowing the density interface to warp upward and intersect the sea surface some distance away from the coast (∼10 km). The band of homogeneous water between the surface density front and the coast is found to support a barotropic wave motion that co-oscillates with the baroclinic waves trapped offshore of the front. At the surface front, surface wave elevation and cross-shelf transport matching conditions lead to the dispersion relation for this “complementary mode.” The complementary mode has the appearance of a nearshore oscillatory barotropic jet that is coherent with vertical pycnocline motions farther offshore. The phase speeds for complementary mode waves are found to be between those of a before-upwelling baroclinic wave and a purely barotropic wave. Before and after upwelling, the purely barotropic mode is essentially the same as a quasi-geostrophic edge wave.

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Y. Hsueh
and
Ig-Chan Pang

Abstract

The coastally trapped long-wave theory is adapted to a double shelf bottom topography in the development of a barotropic long-wave model for calculating wintertime wind-driven currents and sea level fluctuations in the Yellow Sea. The low frequency, free wave solutions consist of two infinite sets of modes, one set trapped to each side of the trough that separates the two plane shelves. The phase propagation of either set of waves depends upon the slopes of both shelves. Forced wave motions are considered for geostrophic winds calculated at 35°N, 125°E for the period of January–April 1986 for which point measurements of currents and sea level fluctuations are available. Comparison to these measurements shows that the model reproduces reasonably well upwind flow events in the trough that dominate the wind-driven wintertime regime. The anatomy of a typical event of this type shows that the Korea-side Kelvin wave is mainly responsible for the establishment of the downwind setup in sea level and in the ultimate generation of an adjustment drift to the right of the wind necessary for the initiation of the upwind flow. The qualitative similarity found between the model output in alongshore velocity and the observation in coastal waters of Korea, in which no clearly dominant features exist, suggest the importance of the influence of wave modes trapped on one shelf in the determination of motions on the other.

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Y. Hsueh
and
Hsien-wang Ou

Abstract

The steady-state circulation on a continental shelf under the combined influence of a wind stress, a surfacedensity distribution, and a longshore current over the shelf break is investigated in a linear, β-plane modelthat allows a longshore pressure gradient. The problem is quasi-two-dimensional and lends itself readily toa standard boundary-layer analysis. For the range of parameters considered, the hydrostatic Lineykin layerallows a vertical mass transport into the surface Ekman layer to compensate for the one-sided divergencecreated by the wind stress at the coast and is, therefore, primarily responsible for coastal upwelling. Anequatorward longshore current over the shelf break, on the other hand, contributes to a shelf break upwelling due to the Sverdrup relation. There is, in this case, also a possibility for a poleward undercurrentover the continental shelf. When the equatorward longshore velocity at the shelf break bottom is sufficientlylarge, however, the poleward undercurrent may not exist at all, and the whole shelf water may move equatorward. The resulting onsho're transport in the bottom Ekman layer then causes upward motion in the Stewartson E½ layer, and allows for an appearance of coastal upwelling in the presence of upwelling at the continental shelf break. The interior density anomaly in the model is always diffusive and admits an upwellingcirculation beneath sharp surface contrasts with a shoreward gradient. While such contrasts in surfacedensity anomaly can, and do, occur at mid-shelf points, the intensity of upwelling generally remains thegreatest in the coastal boundary layer.

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Y. Hsueh
and
Chi-Yuan Lee

Abstract

Hindeasts of bottom currents on the southern Washington continental shelf are made on the basis of a long-wave approximation to the vorticity equation for a barotropic flow. Resistance of the shelf bottom to the flow is incorporated with the specification of a bottom stress that is proportional to the bottom geostrophic velocity. Observed longshore bottom currents on the Oregon continental shelf off Newport and winds at a Newport jetty from 21 July to 26 August 1972 enter as forcing functions. Comparison of the hindcast results with the observation of two current meter mooring sites off the southern Washington coast shows that more than half of the observed along-isobath variance in bottom velocity components parallel to local depth contours becomes accountable when a bottom resistance coefficient on the order of 0. 10 g cm−2 s−1 is used. The accountability of similar variance in cross-isobath components is nearly as high but suffers from relatively large absolute errors in the computed flow field. Coherence analyses also show that the hindcast generates along-isobath velocity time series whose contents at low frequencies (∼0.14 cpd) are nearly statistically identical to those observed. An examination of hindcast results with and without the Newport jetty wind indicates that the wind-forced low-frequency response contained in alongshore currents is in phase with the wind. This result has been attributed to the fact that a significant amount of the onshore transport that compensates for the surface wind drift at the coast is carried by the bottom boundary layer.

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Y. Hsueh
and
Georges L. Weatherly

Abstract

Because continental margins are inclined, the boundary layer over them is tied to bottom density as well as to the barotropic pressure gradient. Through the no-flux condition at the sea floor, the bottom boundary layer, in turn, constrains the interior density field. The barotropic pressure gradient and the baroclinic velocity are thus coupled. Theoretical analysis in the framework of a model of linear dynamics demonstrates this coupling and resolves it.

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