Hypoxic Intrusions to Puget Sound from the Pacific Ocean

Abstract

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 landward bottom residual currents. Intrusions of dense water to Puget Sound are likely related to estuarine exchange flows expected to occur during conditions for minimal tidal mixing. Observations agree with minimal mixing occurring during neap tides and maximum diurnal inequalities. 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, likely due to the larger-scale exchange flow in the Strait of Juan de Fuca. This large-scale process adds a strong seasonal and interannual modulation to the intrusions of hypoxic water. This thesis develops a method to diagnose hypoxic intrusion events at Admiralty Inlet. The method is based, empirically, on seabed observations, but its application relies on operational data products. Using only tidal elevation signals and indices for coastal upwelling and river discharge, 98% of events with dissolved oxygen less than 4.0 mg/L are identified in the 3 year record. Two-layer exchange flow conditions during intrusions are confirmed with surface observations and CTDO casts. Effects of landward propagation of hypoxic intrusions are shown to be very diffuse and more prominent in the probable cumulative response. Hypoxic water in Hood Canal cannot be directly tied to these oceanic intrusions.