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The Role of Canyons, Promontories and Topography in DOES

Susan Allen, Department of Earth & Ocean Science University of British Columbia Vancouver, Canada. The Role of Canyons, Promontories and Topography in DOES. Outline. Some definitions Limitation of shelf-break exchange Eddy shedding and instability: capes and promontories

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The Role of Canyons, Promontories and Topography in DOES

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  1. Susan Allen,Department of Earth & Ocean ScienceUniversity of British ColumbiaVancouver, Canada The Role of Canyons, Promontories and Topography in DOES

  2. Outline • Some definitions • Limitation of shelf-break exchange • Eddy shedding and instability: capes and promontories • Advection effects : canyons and ridges • Mixing effects: canyons, banks and deep channels

  3. Ocean & the Shelf • Exchange: it is not enough to simply bring ocean water inside the shelf-break line but it needs also to “upwell” to shelf depths

  4. Deep Channels • However, deep channels can be extremely important in bringing ocean water into shelf regions where mixing or other processes can bring water to shelf-depth. DFO

  5. “Exchange” • Water onto the shelf • ≠ • Water off the shelf • Bottom topography: water onto the shelf • Capes/promontories: water off the shelf

  6. Geostrophy: ρfv = dp/dx ρfu = -dp/dy Conserv. Volume: ∇∙ū=0 Implies dw/dz = 0 But w=0 at the surface and as the bottom condition is: w = -u dh/dx - v dh/dy flow at the bottom must follow the isobaths Furthermore, using the density equation, Brink (1998) shows that the flow must follow the isobaths upto a depth where the flow is zero. Limitations of Flow over the Shelf-break • Purely geostrophic flow is constrained to follow the isobaths near the bottom • “Near the bottom” is given by the scale depth NL/f where N is the Brunt-Vaisala frequency, f is the Coriolis parameter and L is an appropriate horizontal length scale for the flow.

  7. Bottom boundary layers probably play a smaller role than I originally thought in canyon upwelling. Slopes are steep and the water is stratified. On a slope of 0.02 and a with a stratification of 0.003 s-1, the bottom boundary layer will arrest on a timescale of 0.9 days. (MacCready and Rhines, 1992) Breaking the Constraints • In order to move deep water over the shelf-break, one needs to break the constraints of geostrophy: • Time-dependence • Advection • Friction • (turbulence)

  8. Decreased L: Flows on the scale of the topography Induced instabilities Increased U: Converging isobaths Mixing: Internal wave breaking Enhanced shear Topography breaking the Constraints • Advection: governed by the Rossby Number Ro = U/Lf. Topography usually works by decreasing L but can also increase U. • Turbulent Mixing: Topography can induce increased mixing.

  9. Capes and Promontories • Capes cause: • Separation and instabilities • Increased flow due to isobath convergence COAS

  10. Capes : Separation in Eastern Boundary Currents • For eastern boundary currents, -effect is destabilizing. • For anti-cyclonic currents, stretching is destabilizing. (Marshall & Tansley, 2001) • Shelf-current off Oregon/California is “inherently unstable”. Probably kept stable by winds increasing to the south. • Flow can re-attach or eddies become trapped by the topographic slope and not actually lead to exchange. J. Gower

  11. Isobath Convergence • If near-bottom geostrophic flow follows the isobaths, if the isobaths converge the flow accelerates. • Flow that was initially low Ro number can have elevated Ro numbers and cross-isobaths. Allen, 2000

  12. Reduced Ro number • Any topography that has a length scale small compared to the along-shelf current or the shelf width will increase the Rossby number. • If the Rossby number is sufficiently large, cross-isobath flow will occur F. Shepard

  13. Advection driven exchange over Canyons Allen, 2004

  14. Observations from Astoria Canyon • 6.5°C water advected into the canyon and onto the shelf. Hickey 1997

  15. Advection driven exchange over Canyons Ro =U/fR Allen & Hickey

  16. Flux Estimate (Astoria Canyon) Flux through canyon Surface Ekman flux • Using laboratory experiments and theory we can formulate an estimate for upwelling flux through the canyon based on the incoming flow Mirshak & Allen 2005

  17. Draining through Canyons • Canyons can guide deep shelf water out to the open ocean • Chapman (2000) shows limitations on water created near the shelf actually getting into the canyon Wahlin, 2002

  18. Exchange due to Rough Slope • We looked at diffusion of a tracer from the coast to the open ocean in a homogeneous fluid. • Topography included a shelf, slope and deep ocean with significant small scale topography on the slope

  19. Exchange due to Rough Slope • Tracer contours are packed near shelf-break but are obviously less packed than they would be without the roughness • Exchange is advective with flow shoreward in the canyons and oceanward over the ridges

  20. Enhanced Mixing due to Topography • Canyons,ridges and banks have been shown to be regions of enhanced mixing due to breaking internal waves, boundary layer separation and hydraulic processes. • However, mixing in many of these cases do not lead directly to exchange. Klymak & Gregg, 2004

  21. Canyons Carter & Gregg, 2002 • Extremely large values of diapycnal mixing have been seen over canyons, in particular Monterey Canyon. • Deep ocean water can be advected into the canyon and then mixed up into the water column and advected onto the shelf

  22. Head of Laurentian Channel • The deep Laurentian channel carries oceanic water toward the Saguenay region. • Here intense tidal mixing pumps deep water and the associated nutrients toward the surface Saucier, 2000

  23. San Juan/Gulf Islands • The Juan de Fuca canyon and Strait of Juan de Fuca similar give a deep channel from the Pacfic toward the Strait of Georgia • In the Gulf/San Juan islands intense tidal mixing between the deep inflowing water and the surface buoyant water of the Strait of Georgia form a new high nutrient water mass Griffin & LeBlond, 1990

  24. San Juan/Gulf Islands • This mixed water both fills the Strait of Georgia with nutrient rich water but also flows seaward and provides up to 2/3 of the nutrients to the productive West Vancouver Island shelf. (Crawford & Dewey, 1989) Griffin & LeBlond, 1990

  25. Summary • Topography can induce cross-shelf exchange by increasing the Rossby number leading to flow separation/instabilities or advective crossing of isobaths. • Topography can induce cross-shelf exchange by a combination of delivering deep water into the shelf area through canyons or deep channels and then by enhancing mixing. • Topography can also act in tandem with other exchange process to enhance them: for example time dependent flows.

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