Ariane Results for the Manuscript: Controls on Exchange through a Tidal Mixing Hotspot at an Estuary Constriction

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. The amount and depth of the estuarine exchange depends sensitively on the mixing and the densities of the waters on the two sides of the mixing region. Thus, the density, nutrient concentration, oxygen saturation, and dissolved inorganic carbon content of the incoming flow depend on local tidal mixing processes and large scale buoyancy dynamics. We have investigated this process using a numerical model (SalishSeaCast) of the Salish Sea on the West Coast of North America. The region receives considerable freshwater dominated by the Fraser River. The Fraser River first flows into the deep Strait of Georgia but the freshwater must traverse passages through the Southern Gulf and San Juan Islands with strong tidal currents before it reaches the Pacific Ocean. The model correctly reproduces the deep water flow into the Strait of Georgia as evaluated against observations. Using a four-year hindcast from the model we determine the amount, depth and position of the outflow and inflow. We show that 95\% of the variance of the 4-day average baroclinic water flux through the tidal mixing region can be explained by the density difference across the region and a Richardson Number based on the tidal velocities. The outgoing flux includes both surface and intermediate waters and the incoming flux includes both intermediate and deep waters. Laterally, fluxes into and out of the Strait of Georgia and across Victoria Sill show the impact of the Coriolis force and local bathymetry.