Hypoxic intrusions to Puget Sound from the ocean

Oceanic intrusions of dense, hypoxic water regularly occur at the entrance to Puget Sound, WA (USA), and may be significant to regional dissolved oxygen levels. Seabed observations at Admiralty Inlet from 2009 to 2013 show a strong correlation of low dissolved oxygen concentrations with high salinity, coincident with residual currents directed inward to Puget Sound. These intrusions of dense water to Puget Sound likely are related to estuarine exchange flows, which are expected to occur during conditions for minimal tidal mixing. Observations are consistent with minimal mixing, which occurs during the neap tides and maximum diurnal inequalities (and especially during solar equinoxes, when these effects are combined). However, tidal conditions alone cannot predict intrusions of hypoxic ocean water to Puget Sound. Coastal upwelling and Fraser River discharge influence the availability of dense, hypoxic water outside of Puget Sound. This likely is related to the larger-scale exchange flow in the Strait of Juan de Fuca, which connects Puget Sound to the North Pacific Ocean. This large scale process adds a strong seasonal modulation to the intrusion of hypoxic water. This paper develops a method to diagnose hypoxic intrusion events at Admiralty Inlet. The method is based, empirically, on seabed observations, but application of the method relies on operational data products. Using only tidal elevation datum and indices for coastal upwelling and river discharge, 100% of events with dissolved oxygen less than 4.0 mg/L are identified in the 3 year record.