FY 2025

The limits of oceanic forcing on the exchange flow of the Salish Sea

Sanchez, R., S.N. Giddings, and E. Lemagie

J. Geophys. Res., 130(5), e2025JC022384, doi: 10.1029/2025JC022384, View Open Access article at AGU/Wiley (external link) (2025)

The dynamics of ocean-estuary exchange depend on a variety of local and remote ocean forcing mechanisms where local mechanisms include those directly forcing the estuary such as tides, river discharge, and local wind stress; remote forcing includes forcing from the ocean such as coastal wind stress and coastal stratification variability. We use a numerical model to investigate the limits of oceanic influence, such as wind-driven upwelling, on the Salish Sea exchange flow and salt transport. We find that along-shelf winds substantially modulate flow throughout the Strait of Juan de Fuca until flow reaches sill-influenced constrictions. At these constrictions the exchange flow variability becomes sensitive to local tidal and river forcing. The salt exchange variability is tidally dominated at Admiralty Inlet and upwelling has little impact on seasonal salt exchange variability. While within Haro Strait, the salt exchange variability is driven by a mix of coastal upwelling and local forcing including river discharge. There, the transition from oceanic to local control of salt exchange occurs over a longer distance and is primarily identifiable in the increasing variability of bulk outflowing salinity values. The differences between the two locations highlight how ocean variability interacts with both tidal pumping and gravitational circulation. We also distinguish between transient ocean forcing which can modify fjord properties near the mouth of the strait and seasonal ocean forcing which primarily affects along-strait pressure gradients. The results have implications for understanding the transport variability of biogeochemical variables that are influenced by both along-shelf winds and local sources.

Plain Language Summary. Over periods of time longer than a day, the circulation of estuaries is generally composed of inflowing salty water at depth and relatively fresh outflowing water near the surface. This pattern of circulation, called the exchange flow, is responsible for replenishing water in an estuary and transporting waterborne material including nutrients, larvae, and pollutants. This circulation is maintained by freshwater from rivers and mixing of fresh and salty water by tides. However, the circulation can also be influenced by changes in ocean conditions. For example, summer coastal wind-driven upwelling can bring dense water into an estuary which increases density gradients and increases the circulation. In this study we use a simulation of the Salish Sea in the Pacific Northwest to quantify how important ocean forcing is in modifying an estuary's circulation. We find that the coastal wind-driven circulation is the most important force modifying circulation until the flow reaches channel constrictions. These results imply that the exchange of nutrients is also dominated by ocean conditions deep into the estuary. At the constrictions, the circulation becomes more sensitive to local factors such as tides and river flow.