Controls on Exchange through a Tidal Mixing Hotspot at an Estuary Constriction

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 depend 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 4-yr 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. Significance Statement The purpose of this study is to better understand how much water is exchanged between the open ocean and the coastal ocean. This exchange is important because the contents of the water from the open ocean impact the ecosystem of the coastal ocean. Our results show what controls this exchange, highlighting the importance of the density difference between the coastal and open ocean as well as the turbulence of the waters connecting them.