The Role of Canyons, Promontories and Topography in DOES

Susan Allen,Department of Earth & Ocean Science

University of British ColumbiaVancouver, Canada

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

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

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

“Exchange”

Water onto the shelf

≠

Water off the shelf

Bottom topography: water onto the shelf

Capes/promontories: water off the shelf

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: 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.

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)

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.

Decreased L:

Flows on the scale of the topography

Induced instabilities

Increased U:

Converging isobaths

Mixing:

Internal wave breaking

Enhanced shear

Capes and Promontories

Capes cause:

Separation and instabilities

Increased flow due to isobath convergence

COAS

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

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

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

Advection driven exchange over Canyons

Allen, 2004

Observations from Astoria Canyon

Hickey 1997

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

Advection driven exchange over Canyons

Ro =U/fR

Allen & Hickey

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

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

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

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

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

Canyons

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

Carter & Gregg, 2002

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

San Juan/Gulf IslandsThe 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

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

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.