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A mixing budget for the Strait of Georgia, British Columbia Samuels, Geoffrey

Abstract

A mixing budget for the Strait of Georgia was prepared summarizing mixing processes and effects upon the physical oceanography of the strait. A mixing budget was defined as an itemized estimate of mixing mechanisms and the total state of mixing in the strait. Mixing effects in the Strait of Georgia were estimated from hydrographic data and from monthly summaries of meteorological data for four months: February, May, August and November of 1968. Mixing was estimated qualitatively by depth-averaging temperature, salinity, density and oxygen concentrations for four representative layers, each 25 metres thick and by producing contour plots for each variable for the layers (two upper layers, an intermediate layer and a deep layer). Quantitative estimates of mixing were made by computing the potential energy density and the energy density needed for total mixing for the representative layers plus the potential energy density of total stratification (a fresh water layer over a layer with oceanic salinity). Changes in potential energy were due to mixing plus advection. Estimates of the energy available for mixing were made for the different mixing mechanisms acting upon the strait: wind mixing, buoyancy flux (convection) mixing, tidal mixing, mixing by internal waves and entrainment mixing. The Strait of Georgia has three different domains which are influenced by different mixing mechanisms: the southern passages, the upper layers of the northern strait and the deep waters. The southern passages (including the San Juan Archipelago and Haro and Rosario Straits) are the site of intensive tidal mixing which keeps the entire water column well mixed} convection and wind mixing are also important during the winter. The upper layers of the northern strait (north of Boundary Pas sage) are well mixed by the wind in the winter and by the wind and convection in the autumn. The uppermost brackish layer is highly stratified in the spring and summer from the effects of surface heating and fresh water runoff; intermediate layers are mixed by entrainment upwelling. The deep waters of the strait are influenced chiefly by advection processes: estuarine flow and seasonal intrusions of new water masses. In the winter, cold low-salinity water intrudes and displaces the warm water left from the previous summer while in the summer, warm saline water replaces the cold winter water.

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