Predicting Deep Water Intrusions to Puget Sound, WA (USA), and the Seasonal Modulation of Dissolved Oxygen

Abstract

Observations and predictions regarding oceanic intrusions at the entrance to Puget Sound, WA (USA), are presented. Four years of seabed observations at Admiralty Inlet show episodic periods high salinity, coincident with landward residual currents near the seabed. The observed residual currents are consistent with an estuarine exchange flow during minimal tidal mixing, coincident with neap currents and maximum diurnal inequalities. These subtidal intrusions can carry dense water into Puget Sound and, thereby, renew the bottom water that is trapped landward of the sill. The oxygen concentration levels of these intrusions are often low, though not hypoxic, and may influence Communicated by Carl T. Friedrichs R. Walter Deppe does not currently work at the University of Washington. The work for this project was conducted while he was a research assistant at the University of Washington. the oxygen levels in Puget Sound. The water mass properties of these intrusions are influenced strongly by regional dynamics, because the entrance to Puget Sound is connected to the open ocean via the Strait of Juan de Fuca. Coastal upwelling and discharge from the Fraser River discharge control the exchange flow in the Strait of Juan de Fuca and thus control the availability of dense water at the entrance to Puget Sound. The net effect of the tidal and regional dynamics is for intrusions with low oxygen levels to prevail in the late summer months. To predict intrusions in future years, an empirical method is developed and validated for daily application. The prediction method is based on publicly available operational data products and does not require in situ observations. In verification, 98% of intrusion events with dissolved oxygen less than 4.0 mg/L are successfully identified in a hindcast prediction for the 4-year observational record.

Seasonal variation in aragonite saturation in surface waters of Puget Sound – a pilot study

“…The deep waters entering Puget Sound originate in the Pacific Ocean and represent a mix of subtropical and subarctic water masses. Upwelling varies in strength and duration, with short-term intrusions over the sill at Admiralty Inlet (Deppe et al, 2013;Khangaonkar et al, 2017). The surface layer is influenced by vertical mixing with deep waters enhanced by circulation around the sills, and also by inputs of freshwater and primary productivity within the euphotic zone.…”

Estuarine Circulation, Mixing, and Residence Times in the Salish Sea

“…The exchange flow may also be influenced by the density of water upwelled on the coast during spring and summer (Babson et al, 2006), the seasonal variation of wind (D. A. Sutherland et al, 2011), and tides (Deppe et al, 2018).…”

“…Taking the Puget Sound volume (Figure 6) as an example, the biggest freshwater source in the Salish Sea is seaward of Puget Sound and peaks some months later than Puget Sound Rivers (Figure 1). The exchange flow may also be influenced by the density of water upwelled on the coast during spring and summer (Babson et al., 2006), the seasonal variation of wind (D. A. Sutherland et al., 2011), and tides (Deppe et al., 2018).…”

Methane Plume Emissions Associated With Puget Sound Faults in the Cascadia Forearc

“…Puget Sound bottom water is well-flushed tidally and the bottom waters of all three basins are very well-oxygenated (Deppe et al, 2018). Due to methanotrophic bacteria and a well-developed sulfate methane transition zone, the upper sediment layers of the estuary do not appear to be able to produce free-methane bubble streams without being previously accumulated within a sub-surface reservoir.…”