The Role of Canyons, Promontories and Topography in DOES

1
The Role of Canyons, Promontories and Topography
in DOES

• Susan Allen,Department of Earth Ocean
  ScienceUniversity of British ColumbiaVancouver,
  Canada

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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

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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

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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
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Exchange

• Water onto the shelf
• ?
• Water off the shelf
• Bottom topography water onto the shelf
• Capes/promontories water off the shelf

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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.

• Geostrophy ?fv dp/dx ?fu -dp/dy
• Conserv. Volume ?u0 Implies dw/dz 0
• But w0 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.

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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)

• 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)

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Topography breaking the Constraints

• Decreased L
• Flows on the scale of the topography
• Induced instabilities
• Increased U
• Converging isobaths
• Mixing
• Internal wave breaking
• Enhanced shear

• 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.

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Capes and Promontories

• Capes cause
• Separation and instabilities
• Increased flow due to isobath convergence

COAS
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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
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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
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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
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Advection driven exchange over Canyons
Allen, 2004
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Observations from Astoria Canyon
Hickey 1997

• 6.5C water advected into the canyon and onto the
  shelf.

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Advection driven exchange over Canyons
Ro U/fR
Allen Hickey
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Flux Estimate (Astoria Canyon)
Flux through canyon
Surface Ekman flux
Mirshak Allen 2005

• Using laboratory experiments and theory we can
  formulate an estimate for upwelling flux through
  the canyon based on the incoming flow

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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
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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

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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

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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
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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
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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
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San Juan/Gulf Islands
Griffin LeBlond, 1990

• 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)

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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.