Stephen Pond

A Numerical Model of the Circulation in Knight Inlet, British Columbia, Canada

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 larg...

Deep and intermediate water replacement in the Strait of Georgia

Abstract We have studied deep‐water replacement processes in the Strait of Georgia using data from two different observational programs. From the monthly hydrographic data of Crean and Ages (1971) we have recognized the propagation of cold, brackish and well oxygenated waters northwards from Boundary Passage at depths between 75 and 200 m. We found a significant correlation over the years 1967–78 between surface cooling and temperature drops at those depths some months later. Measurements at an array of moorings in the central Strait of Georgia (Stacey et al., 1987) revealed the presence during summer months of currents concentrated near the bottom and varying with fortnightly and monthly periods. We have interpreted this phenomenon in terms of gravity currents originating from Boundary Passage during periods of neap tides and introducing at depth salty waters from the Strait of Juan de Fuca. Our analysis confirms in part the validity of the deep‐water replacement mechanisms advanced by Waldichuk; however...

On the Mellor–Yamada Turbulence Closure Scheme: The Surface Boundary Condition for q2

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 con...

The low-frequency residual circulation in Knight Inlet, British Columbia

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.

An Analysis of the Low-Frequency Current Fluctuations in the Strait of Georgia, from June 1984 until January 1985

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 var...

A Wind-Forced Ekman Spiral as a Good Statistical Fit to Low-Frequency Currents in a Coastal Strait

Ekmans classical analysis of wind-driven currents is a fundamental component of the modern circulation theory of the oceans, but there have been few good observations of the predicted Ekman spiral, where the velocity vector rotates clockwise in direction (in the Northern Hemisphere) and decays exponentially in magnitude with increasing depth. An analysis of recent cyclesonde velocity measurements based on the use of empirical orthogonal functions, however, suggests that a classical Ekman spiral was a good statistical fit to a significant portion of the low-frequency current fluctuations in the Strait of Georgia, British Columbia, for fluctuation periods of about 5 to 10 days.

The Deepwater Exchange Cycle in Indian Arm, British Columbia

Abstract A silled fjord on the British Columbia coast, Indian Arm, was used to study the exchange processes controlling deepwater exchange in fjords. Bottom-water renewal took place during the winter of 1984/85. Hydraulic control of the exchange is exerted over the long sill which restricts access to the fjord. The maximum density at the sill occurs during neap tides. No distinct peaks in the velocity at the sill are observed during periods of inflow. Reduced vertical mixing over the long sill during periods of neap tides sets the timing of the bottom water exchange. During each inflow over the 1984/85 winter about 20% of the water in the fjord was replaced. These exchanges occurred over periods of 5–10 days. Between the exchange years, diffusion of the water in the basin is important in conditioning the water for the next exchange event. The vertical diffusion coefficient ( K t ) was determined from an analysis of the salinity data and was found to be related to the buoyancy frequency N by the relationship, K v αN −1·6 .

Estimates of entrainment in the Fraser river plume, British Columbia

Abstract Estimates of the entrainment rate of salt water into the Fraser River plume have been made using two independent methods. A value of k = 2 × 10 ‐4 is obtained for he entrainment coefficient relating the vertical to the horizontal velocity from salt conservation arguments at a series of profiles along the plume and from calculations of surface divergence, as measured by drifting drogues. It is also found that entrainment contributes significantly to the deceleration of the river plume after it issues into the Strait of Georgia.

Tidal Energy Dissipation at the Sill of Sechelt Inlet, British Columbia

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.

Low‐frequency subsurface current and density fluctuations in the strait of Georgia

Abstract The depth‐time structure of low‐frequency subsurface current and density fluctuations in central Strait of Georgia is described. The measurements were made between February and May, 1981 with profiling Cyclesondes. The depth range of approximately 50 to 290 m was sampled in water depths of about 325 m. The mean current was cross‐strait and decreased with depth. The low‐frequency current fluctuations occurred predominantly at periods exceeding a week and also decreased with depth. Below about 200 m, there was an increasing tendency for the fluctuations to be aligned with the local topography. The maximum vertical separation for components of current to be significantly coherent was 100 m and was in the lowest frequency band. Currents separated by 4.1 km in the horizontal were correlated in the more energetic north‐south component.