The Role of Canyons, Promontories and Topography in DOES Susan Allen, Department of Earth & Ocean Science University of British Columbia Vancouver, 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) S =( N sin(theta)/fcos(theta))^2 tau = 1/S^2 f cos(theta) if s=0.01 theta = 0.01, sin(theta) = 0.01, cos(theta)=1 S = N^2 s^2/f^2 = (10^-3)^2 (10^-2)^2/(10^-4)^2 = (10^-1)^2 = 10^-2 tau = 1/S^2 f = 1/(10^-2)^2 10^-4 =10^8 s ==> 3 years if s=0.02 N = 5e-3 then S = 1 tau = 10000 ==>1/10 of a day or N=3e-3 gives 0.9 day

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.

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 Mirshak & Allen 2005 Using laboratory experiments and theory we can formulate an estimate for upwelling flux through the canyon based on the incoming flow

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 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

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 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

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)

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.