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The circulation and energetics of the Sechelt Inlet system, British Columbia Tinis, Scott Wayne


The sill at the entrance to Sechelt Inlet, 50 km northwest of Vancouver, is tuned almost perfectly to extract the maximum power from the barotropic tide. However, despite the enormous up-inlet energy flux from the barotropic tide (~42 MW), the diffusive processes in the deep-water of the inlet basin are weak, and the energy flux of the internal tide generated at the sill is relatively small (< 200 kW). The large tidal energy flux is almost completely dissipated at the sill, and most of the energy that manages to escape the sill region does so in the form of a turbulent jet that dissipates at mid-depth near the sill; very little energy is left over for mixing the deep-water. Using estimates of the change in potential energy of the water column by vertical diffusion, the mixing efficiency (flux Richardson Number) of the breaking internal tide is determined to be between 3 and 8%. Because the transfer of energy to diffusive processes is so inefficient, the vertical diffusion of salt decreases the deep-water density by only ~0.0l kg m⁻³ per month. The decrease of the deep-water density conditions the inlet for eventual deep-water replace ment; however, hydrographic surveys in Sechelt Inlet suggest that while the mid-depth basin water is replaced once a year, the deep-water (below ~150 m) is replaced only about every five years. The wind has relatively little influence on the circulation. Much of the wind energy is contained in the diurnal seabreeze, and its effect on the overall baroclinic energy flux is estimated to be no more than 5%. The coherence of the wind to the currents in the lower frequency bands (f < 0.929 cpd) is small over most of the water column, with the frequency band between 0.2 ≤ f ≤ 0.66 cpd having the highest coherence. Empirical orthogonal function (EOF) analysis also indicates that the wind is not primarily responsible for the low frequency variability: the EOF analysis could not consistently identify the wind-driven mode as one that accounted for a significant percentage of the low frequency variance. The four-layer flow that was identified as the dominant mode of low frequency variability is also a persistent feature of the monthly mean currents, and is probably formed by density gradients created by the tidal mixing near the sill. The mean circulation is similar to that proposed in an earlier study of Sechelt Inlet by Lazier (1963).

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