Interannual and seasonal water mass analysis in the Salish Sea using Lagrangian particle tracking

The Salish Sea is a semi-enclosed coastal sea between Vancouver Island and the coast of British Columbia and Washington State, invaluable from both an economic and ecologic perspective. Pacific inflow to the Sea is the main contributor of many biologically important constituents. The contribution of Pacific water masses to the flow through Juan de Fuca Strait (JdF), the Salish Sea’s primary connection to the Pacific Ocean, is explored. Quantitative Lagrangian particle tracking using Ariane was applied to two numerical ocean models (CIOPS-W in the shelf region, and SalishSeaCast in the Salish Sea) matched together within JdF. Water parcels seeded near the entrance of JdF were integrated forwards and backwards in time to assess water mass path (and properties while on this path) from the shelf region and once within the Salish Sea in more detail than previously possible. During summer upwelling, intermediate flow from the north shelf and offshore dominate inflow, while during winter downwelling, intermediate flow from the south shelf and surface flow from the Columbia River plume are the dominant sources. A weaker and less consistent estuarine flow regime in the winter led to less Pacific inflow overall and a smaller percentage of said inflow reaching the Salish Sea's inner basins than in the summer. Nevertheless, it was found that winter dynamics are the main driver of interannual variability, in part due to the strongly anti-correlated behaviour and distinct properties of the two dominant winter sources. This analysis extends the knowledge on the dynamics of Pacific inflow to the Salish Sea and highlights the importance of winter inflow to the interannual variability in biogeochemical conditions in the region.