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Peter Baker and Stephen Pond

Abstract

Many aspects Of the low-frequency response of a stratified inlet have not been previously observed because of the lack of simultaneous observations of runoff, wind, currents, and density over the entire body of water. Month-long observations throughout the water column of Knight Inlet, British Columbia, both outside and inside the sill, during the spring (1988) and summer (1989) runoff regimes are presented. These data are detided with harmonic analysis and used to investigate the subdiurnal residual response with respect to the wind, runoff, and deep water renewal. Near the surface, response to alongchannel winds was found to dominate with a coherence squared of greater than 0.8. The coherence was therefore used to directly estimate the wind influence, and dewinded residuals were formed by subtracting these estimates from the detided records. Structures were found in the dewinded residuals that correspond to a near-surface estuarine circulation vertically nested with deep water renewal.

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Michael W. Stacey and Stephen Pond

Abstract

A numerical model that uses a level-2½ turbulence closure scheme is used to compare two boundary conditions for the turbulent energy at the air–sea interface. One boundary condition, the most commonly used, sets the turbulent kinetic energy proportional to the friction velocity squared, while the other sets the vertical diffusive flux of turbulent kinetic energy proportional to the friction velocity cubed. The first boundary condition arises from consideration (simplification) of the turbulence closure scheme near boundaries, and the second arises from consideration of the influence of surface gravity waves on the transfer of turbulent kinetic energy from the wind to the water. Simulations using these two boundary conditions are compared to month-long observations of velocity, temperature, and salinity (as shallow as 2 m from the surface) from Knight Inlet, British Columbia, Canada. The circulation in the inlet is strongly influenced by the wind, tides, and freshwater runoff. The two boundary conditions produce simulations that are different down to a depth of at least 5 m. Somewhat more accurate simulations are produced by the second boundary condition. Also, simulations using the second boundary condition are more sensitive to variations in the roughness length. Based on the simulations, roughness lengths as large as 1 m (or greater) are possible.

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Scott W. Tinis and Stephen Pond

Abstract

The energy budget of a tidally active, shallow silled fjord is discussed. Constriction of the flow over the shallow sill causes a reduction in tidal amplitude and a phase lag across the sill. A generalized expression for the total power extracted from the barotropic tide by dissipation at the sill is derived as a function of the tidal amplitude difference and phase lag of the tidal elevation. Using tide gauge data from both sides of the sill at the entrance to Sechelt Inlet, British Columbia, this generalized expression yields estimates for the energy flux of the barotropic tide, which approach 100 MW during spring tides. From direct measurements of the currents, the estimated frictional dissipation is equal to the flux out of the barotropic tide (within experimental error). A small amount of the energy flux (∼5%) is estimated to go into the generation of a tidal jet, which dissipates within a few kilometers of the sill and contributes to the formation of a mid-water layer.

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DANIEL E. FRYE, STEPHEN POND, and WILLIAM P. ELLIOTT

Abstract

An examination of the spectrum of winds along the Oregon coast shows a major diurnal peak at a period of 24 hr and a small microscale peak at about 50 s. The 24-hr peak is thought to be associated with the diurnal variation of land–sea temperature difference.

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Michael W. Stacey, Stephen Pond, and Zenon P. Nowak

Abstract

During spring 1988 (a period of low freshwater runoff) and summer 1989 (a period of high freshwater runoff), month-long observations of velocity, temperature, and salinity were made throughout the water column in Knight Inlet, both up-inlet and down-inlet of the sill. Measurements were made at depths of 2, 4, 6, 9, and 12 m using S4 current meters, at depths down to about 200 m using profiling current meters, and at depths deeper than 200 m using Aanderaa current meters. Anemometers were deployed at two locations along the inlet.

A laterally integrated, two-dimensional numerical model of the inlet that uses the Mellor and Yamada level 2.5 turbulence closure scheme and that accounts for the combined influence of the winds, tides, and freshwater runoff has been used to produce 30-day simulations of the velocity and density field in the inlet. The vertical coordinate is transformed in the model so that very fine vertical resolution can be attained near the surface even though the tidal range is large. Therefore, the thin but distinct, surface layer that exists in the inlet during times of high, freshwater runoff can be resolved by the model.

With a single set of empirical constants, that is, horizontal diffusion and drag coefficients, the model successfully reproduces much of the tidal, estuarine, and wind-forced components of the circulation in the inlet during the two observation periods, as long as the vertical diffusion coefficients of Mellor and Yamada are augmented by an extra stratification-dependent term. This term can be interpreted as representing the mixing caused by breaking internal waves.

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Michael W. Stacey, Stephen Pond, Paul H. LeBlond, Howard J. Freeland, and David M. Farmer

Abstract

A description of the low-frequency (ap;10 to 30 days period) current fluctuations in the Strait of Georgia is presented. Velocity time series from four cyclesonde moorings and five current meter mooring, spanning the time interval from June 1984 until January 1985, are analyzed. Emphasis is placed on identifying the forcing mechanisms and determining the spatial structure of low-frequency flow.

The nonlinear interaction of semidiurnal tidal constituents with bottom topography caused a near-bottom, low-frequency oscillation that was coherent over the span of the experimental array (ap;11 km). The tides were important elsewhere in the water column too, and altogether directly accounted for 37% of the low-frequency energy in the Strait.

There is evidence of significant wind forcing. An empirical orthogonal function analysis of the vertical structure of the current fluctuations yields strong evidence for the existence of wind-forced Ekman spirals. Typically, the orthogonal modes that dominate the variance near the surface rotate clockwise with depth and are coherent with the wind.

Longitudinal and transverse velocity correlations imply that at some depths the low-frequency current fluctuations are consistent with horizontally nondivergent, isotropic flow. They also suggest horizontal scales of less than 8 km.

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