Deep-water Intrusions in the Puget Sound

Deep-water Intrusions in the Puget Sound

Sally WarnerCoastal & Estuarine Fluid Dynamics class

Friday Harbor LabsSummer 2006

Project goals

- Was an intrusion of dense ocean-water occurring during our 36-hour cruise?

- What processes were working to allow or prevent this intrusion?

Estuary dynamics

Fjord dynamics

Why are intrusions essential to fjords?

What happens in the Puget Sound?

What controls the onset of intrusions?

Intrusions

Puget Sound and Admiralty Inlet

depth

30.8 psu

The Intrusion

30.8 psu water intruding down the estuary side of the sill

Barotropic vs. Baroclinic Forcing

31.2

Advection distance of 31.2 psu water:3.89 and 4.03 km in 5.3 and 5.0 h.

Tidal current

time

ubaroclinic ≈Ladvection − uTdt

tn

tn+1

∫tn+1 −tn

=15 cm s-1

t1t0 t2

Barotropic excursion distance

Barotropic excursion lengths:0.95 and 1.60 km

ADCP velocity

ubaroclinic = 25 cm s-1

Baroclinic circulation is essential for the propagation of the intrusion

t3

ubarotropic = 27 cm s-1

Historical Data: sx vs. mixing

38 historical transects and our cruise

• “Bottom water renewal is controlled by changes in the intensity of mixing due to fortnightly variations in the tidal amplitude.” Geyer & Cannon (1982)

• “The onset of intrusions is a result of the fluctuations in the horizontal density gradient caused by salinity variations across the sill.” Cannon et. al. (1990)

Our cruise: slack = 14 h s = 1.5 psu, sx=0.03 psu km-

1

Conclusions

- Yes, an intrusion was occurring when we visited Admiralty Inlet

- Reduced tidal velocities allowed baroclinic circulation to develop which carried 30.8 psu water over the sill.

- Both the spring/neap change in tidal energy and variation in the horizontal salinity gradient are important mechanisms controlling intrusions of deep water into the Puget Sound, as seen from historical data.