Abstract
Deep estuaries are often separated from the open ocean by sills and constrictions. These constrictions are areas of intense mixing often dominating the total estuarine mixing. Our example is the Salish Sea on the West Coast of North America with strong mixing through the Southern Gulf and San Juan Islands. The amount and depth of the estuarine exchange, and the tracers it carries, through these constrictions depends sensitively on the mixing and the densities of the waters on the two sides of the mixing region. Predicting the future of estuarine exchange in a given region requires a full understanding of the dynamics. Here we use a Lagrangian tracking method that allows us to directly separate the exchange flow from the recirculating flows (the reflux and efflux). Separating the components of the estuarine flow simplifies the dynamics; 95% of the variance in the exchange flow through the tidal mixing region can be explained by the density difference across the region combined with a Richardson Number based on the tidal velocities. The Lagrangian tracking is done on output from SalishSeaCast, a three-dimensional ocean model of the region. Using a four-year hindcast from the model we determine the amount, depth, and position of the outflow and inflow. The direct separation of the estuarine components shows their four-dimensionality.
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Nancy K. Soontiens’ current affiliation: Fisheries and Oceans Canada
Michael Dunphy’s current affiliation: Fisheries and Oceans Canada
Elise Olson’s current affiliation: Environment and Climate Change Canada
