Event Title

Optimal-detail circulation models for fjords and sea lochs: an application to the Puget Sound

Presentation Abstract

The wild salmon population has been declining over the last 30 years in the Puget Sound, a fjord near Seattle in the USA. A hypothesis for this decline is the impact of circulation and salinity variations on plankton production. Here we create a new fjords circulation model that runs over several decades in order to investigate the long-term changes in estuarine circulation due to regional climate variabilities. The approach includes a vertical and longitudinal fixed Cartesian grid with momentum and salinity equations averaged over the channel width and the tidal cycle to improve speed-efficiency. The model uses an analytical solution for the quasi-steady momentum equation and a numerically solved tracer equation which takes into account the salinity adjustment time. However, a new mixing parameterization for the eddy viscosity is required due to the fjords distinctive sills and deep bathymetry. Using MoSSea, a detailed 3D simulation of the Puget Sound, the mixing pattern is analyzed to build an efficient equivalent tuning for subtidal estuarine dynamics. We show that where the estuary is deep, different physical processes drive the mixing in the bottom and surface layers. For the bottom layer, the eddy viscosity relies only on the tidal velocity. However for the surface layer, wind stress also needs to be considered. Near the sills, the mixing is enhanced and the eddy viscosity is explained by the tidal velocity alone. To validate the resulting model we use the 2001 drought, an event responsible for producing a unique salinity profile. Using a few decades hindcast with historic data of river discharge, wind and tides as forcing, the reproducibility of this event is tested. Finally, this model can provide a physical base which can be coupled with a bio-chemical model to discern long-term drivers for the degradation of salmon food-webs in the Puget Sound.

Session Title

Posters: Data & Information Management

Conference Track

SSE18: Posters

Conference Name

Salish Sea Ecosystem Conference (Seattle, WA : 2018)

Document Type

Event

SSEC Identifier

SSE18-29

Start Date

5-4-2018 11:30 AM

End Date

5-4-2018 1:30 PM

Type of Presentation

Poster

Contributing Repository

Digital content made available by University Archives, Heritage Resources, Western Libraries, Western Washington University.

Geographic Coverage

Salish Sea (B.C. and Wash.)

Rights

This resource is displayed for educational purposes only and may be subject to U.S. and international copyright laws. For more information about rights or obtaining copies of this resource, please contact University Archives, Heritage Resources, Western Libraries, Western Washington University, Bellingham, WA 98225-9103, USA (360-650-7534; heritage.resources@wwu.edu) and refer to the collection name and identifier. Any materials cited must be attributed to the Salish Sea Ecosystem Conference Records, University Archives, Heritage Resources, Western Libraries, Western Washington University.

Type

text

Language

English

Format

application/pdf

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Apr 5th, 11:30 AM Apr 5th, 1:30 PM

Optimal-detail circulation models for fjords and sea lochs: an application to the Puget Sound

The wild salmon population has been declining over the last 30 years in the Puget Sound, a fjord near Seattle in the USA. A hypothesis for this decline is the impact of circulation and salinity variations on plankton production. Here we create a new fjords circulation model that runs over several decades in order to investigate the long-term changes in estuarine circulation due to regional climate variabilities. The approach includes a vertical and longitudinal fixed Cartesian grid with momentum and salinity equations averaged over the channel width and the tidal cycle to improve speed-efficiency. The model uses an analytical solution for the quasi-steady momentum equation and a numerically solved tracer equation which takes into account the salinity adjustment time. However, a new mixing parameterization for the eddy viscosity is required due to the fjords distinctive sills and deep bathymetry. Using MoSSea, a detailed 3D simulation of the Puget Sound, the mixing pattern is analyzed to build an efficient equivalent tuning for subtidal estuarine dynamics. We show that where the estuary is deep, different physical processes drive the mixing in the bottom and surface layers. For the bottom layer, the eddy viscosity relies only on the tidal velocity. However for the surface layer, wind stress also needs to be considered. Near the sills, the mixing is enhanced and the eddy viscosity is explained by the tidal velocity alone. To validate the resulting model we use the 2001 drought, an event responsible for producing a unique salinity profile. Using a few decades hindcast with historic data of river discharge, wind and tides as forcing, the reproducibility of this event is tested. Finally, this model can provide a physical base which can be coupled with a bio-chemical model to discern long-term drivers for the degradation of salmon food-webs in the Puget Sound.