Sensitivity of Flushing and Water Quality in Salish Sea Sub-basins to Exchange through Johnstone Strait Pathway – a modeling Study

The importance of tidally averaged inflow of upwelled Pacific Ocean water on flushing and water quality of the Salish Sea is well known. The general understanding is that this exchange flow enters Salish Sea along the bed, primarily through the Strait of Juan de Fuca, and is distributed north to the Strait of Georgia and south into Puget Sound. It has therefore been a common practice to neglect the connection of Salish Sea to the Pacific Ocean through the secondary pathway of Discovery Passage and the Johnstone Strait. The simplification of the Johnstone Strait boundary treatment in the leading predictive models of Salish Sea is a source of uncertainty and limitation that has not yet been characterized. A completely closed boundary approach assumes that there is zero exchange between the Pacific Ocean and the Salish Sea through Johnstone Strait and likely overestimates the strength of exchange flow. In this presentation we describe an improved Salish Sea Model developed using FVCOM-ICM where the limitation of the ocean boundary at Johnstone Strait has been eliminated. The model with expanded domain around Vancouver Island and covering the continental shelf extending from Coos Bay, Oregon in the south to Queen Charlotte Sound north of Vancouver Island was applied in a sensitivity analysis mode. A preliminary assessment previously provided an indication that significant fraction of tidally averaged flow exits the domain through the Johnstone Strait. The assessment has been refined with updated bathymetry provided by the Department of Fisheries and Oceans, Canada. Relative magnitudes of exchange of freshwater and nutrients between the Salish Sea and the Pacific Ocean through the Strait of Juan de Fuca and Johnstone Strait were re-characterized. The sensitivity of simplified/closed treatment of Johnstone Strait boundary on flushing times and water quality in Salish Sea sub-basins will be presented