Susan E. Allen

Topographically Generated, Subinertial Flows within A Finite Length Canyon

Abstract The presence of a canyon cutting the continental shelf has been observed to enhance wind-driven upwelling. In particular, in the vicinity of Juan de Fuca Canyon at the mouth of the Juan de Fuca Strait an eddy containing deep water (from a depth of approximately 450 m) has been documented. Strong upcanyon flows have been observed within numerous canyons including Astoria Canyon, which cuts the shelf offshore of the mouth of the Columbia River. The author develop a linear theory for wind-driven flow over an infinitesimally thin but finite length canyon to illustrate the basic mechanism. Two regimes are considered, the initial growing velocity field and a later steady velocity field. The flow toward the shore is enhanced by O (10) by the presence of the canyon in a homogeneous fluid. The presence of stratification introduces smaller horizontal length scales, the baroclinic Rossby radius, and allows further enhancement of the upcanyon flow. Numerical simulations show that the linear theory is a reaso...

The inorganic carbon system in the coastal upwelling region west of Vancouver Island, Canada

Abstract We present inorganic carbon data from the coastal upwelling region west of Vancouver Island, Canada (∼48.5° N , 126° W ) directly after an upwelling event and during summer downwelling in July 1998. The inner-shelf buoyancy current, the outer-shelf and the slope regions are contrasted for both wind regimes (up- and downwelling). Results show strong biological drawdown of the partial pressure of carbon dioxide ( p CO 2 ) in response to upwelling over the outer-shelf. In contrast, measured p CO 2 is exceptionally high (p CO 2 >1000 ppm ) in the inner-shelf current, where biological uptake of carbon is consistently large. The biological C:N uptake ratio appears to increase when nitrogen becomes limiting (during downwelling), while the POC:PON ratio is relatively constant (slightly lower than the Redfield ratio) suggesting that excess carbon uptake does not go into the POC pool. As expected, large cells dominate where measured primary productivity is greatest. Sub-surface inorganic carbon (and p CO 2 ) is high over the shelf. We suggest that carbon concentrations may be higher in coastal waters because of remineralization associated with high productivity that is confined to a smaller volume of water by bathymetry. At the coast these sub-surface concentrations are more efficiently mixed into the surface (especially during winter) relative to deeper offshore regions. Thus, despite high primary production, coastal waters may not aid in sequestration of atmospheric carbon.

Comment on ''Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?'' by M. Lyle et al

Accurately estimating the vertical flux of material reaching the seafloor from the overlying surface waters is essential for the paleoceanographic reconstruction of a wide variety of oceanic processes. Two approaches are currently being used. One consists of estimating mass accumulation rates (MAR) between dated horizons as the product of linear sedimentation rates, sediment dry bulk densities, and concentrations. One pitfall with this approach is that sediments can be redistributed on the seafloor by bottom currents, and their accumulation may not necessarily reflect the true vertical rain rate originating from the overlying water column. To address this problem, the method of 230Th normalization was developed [Bacon, 1984]. This method is based on the assumption that the rapid scavenging of 230Th produced in the water column by decay of dissolved uranium results in its flux to the seafloor always being close to its known rate of production. To the extent that this assumption is correct, scavenged 230Th can be used as a reference to estimate the settling flux of other sedimentary constituents and to correct for sediment redistribution on the seafloor [Henderson and Anderson, 2003; Francois et al., 2004].

The role of wind in determining the timing of the spring bloom in the Strait of Georgia

A coupled biophysical model of the Strait of Georgia (SoG), British Columbia, Canada, has been developed and successfully predicts the timing of the spring phytoplankton bloom. The physical model is a one-dimensional vertical mixing model, using a K-profile parametrization of the boundary layer, forced with high frequency meteorological data. The biological model includes one phytoplankton class (microphytoplankton) and one nutrient source (nitrate). The spring bloom in the SoG occurs when phytoplankton receive enough light that their growth rates exceed their loss rates. The amount of light that the phytoplankton receive is a function of solar radiation and the depth of mixing. The model was used to determine what physical factors are controlling the phytoplankton losses and the light received by the phytoplankton. Wind was found to control the spring bloom arrival time, with strong winds increasing the mixing-layer depth and delaying the bloom. The amount of incoming solar irradiance, through amount of ...

On subinertial flow in submarine canyons: Effect of geometry

Shelf break canyons on the west coast of Canada and the United States have been observed to be regions of enhanced upwelling during southward currents compared to the surrounding shelf break. Most shelf break canyons from Oregon north cross only part of the continental shelf cutting from the shelf break toward the coast but end on the continental shelf well below the mixed layer. Juan de Fuca canyon, on the other hand, cuts the continental shelf from the slope to, and actually continues into, the Strait of Juan de Fuca. This difference in geometry has a very strong effect on the subinertial flow around the canyon. Model canyon shapes, which include convergent bathymetric contours, are constructed and motivated for Juan de Fuca canyon and a typical shelf break canyon. Geostrophic analytic solutions show that the in-canyon flow in Juan de Fuca canyon is generated by first-order geostrophic dynamics, whereas in the majority of canyons, of which Astoria is an example, in-canyon flow is generated by higher-order effects. This difference is postulated to lead to the observed, very deep upwelling over Juan de Fuca canyon compared to more moderate, episodic upwelling over Astoria canyon.

On vertical advection truncation errors in terrain-following numerical models: Comparison to a laboratory model for upwelling over submarine canyons

[1] Submarine canyons which indent the continental shelf are frequently regions of steep (up to 45°), three-dimensional topography. Recent observations have delineated the flow over several submarine canyons during 2-4 day long upwelling episodes. Thus upwelling episodes over submarine canyons provide an excellent flow regime for evaluating numerical and physical models. Here we compare a physical and numerical model simulation of an upwelling event over a simplified submarine canyon. The numerical model being evaluated is a version of the S-Coordinate Rutgers University Model (SCRUM). Careful matching between the models is necessary for a stringent comparison. Results show a poor comparison for the homogeneous case due to nonhydrostatic effects in the laboratory model. Results for the stratified case are better but show a systematic difference between the numerical results and laboratory results. This difference is shown not to be due to nonhydrostatic effects. Rather, the difference is due to truncation errors in the calculation of the vertical advection of density in the numerical model. The calculation is inaccurate due to the terrain-following coordinates combined with a strong vertical gradient in density, vertical shear in the horizontal velocity and topography with strong curvature.

Bottom-trapped subinertial motions over midocean ridges in a stratified rotating fluid.

Abstract Linear analytical solutions for bottom-trapped subinertial oscillatory flow over simple ridge topographies in a stratified (two-layer) rotating fluid are presented. Results are compared to moored current meter observations of bottom-intensified motions over the Endeavour Segment of Juan de Fuca Ridge in the northeast Pacific. The solutions reproduce many of the observed features including preferential amplification of the clockwise rotary component of velocity over the ridge and increased velocity amplification with proximity to the ridge crest. For a given internal deformation radius, the degree of current amplification increases with increased bottom slope, ridge height, and oscillation frequency. Amplification decreases with increased width of the ridge relative to the deformation radius.

The influence of canyons on shelf currents: A theoretical study

The influence of submarine canyons on shelf currents is studied using the Rossby adjustment method for a homogeneous, inviscid fluid on an f plane. The canyon in the model is assumed to have vertical edges and constant width. The geostrophic flow around a canyon is found to be dependent upon two geometric parameters: the ratio of the depth of the canyon to the depth of the shelf and the ratio of the width of the canyon to the Rossby radius over the canyon. Moreover, a single parameter determines most of the properties of the geostrophic state. This parameter is called the canyon number and is a combination of the two basic geometric parameters. In the geostrophic state an infinitely long flat-bottom canyon will act as a complete barrier to an approaching shelf flow. The approaching flow is asymmetrically diverted along the canyon, and a net flux is generated to the left of the flow in the northern hemisphere. If the canyon cuts a shelf between the shelf break and the coast and connects to a strait (the geometry of Juan de Fuca Canyon) an in-canyon (out-canyon) current will be generated when the shelf break current flows keeping the shelf at its left (right) in the northern hemisphere. If the canyon has a stepped or sloped bottom, the geostrophic flow has a singularity where the step or slope meets the left canyon edge (looking upcanyon) in the northern hemisphere. Flow can cross the canyon edge through the singularity, so the canyon is no longer a complete barrier to the approaching shelf flow. In this case, as above, a net flux is generated to the left of the approaching shelf flow.

The generation of internal waves on the continental shelf by Hurricane Andrew

Observed currents, temperature, and salinity from moored instruments on the Louisiana continental slope and shelf reveal multiple baroclinic oscillations during Hurricane Andrew in August 1992. These measurements are supplemented by numerical models in order to identify possible internal wave generation mechanisms. The Princeton Ocean Model is run with realistic topography, stratification, and wind forcing to extend the observations to Mississippi Canyon and other areas on the shelf. A two-layer isopycnal model is used with idealized topography and spatially uniform winds to isolate internal waves generated in and around the canyon. The combination of the observations and the results from the numerical models indicates several possible mechanisms for generating long internal waves: (1) near-inertial internal waves were generated across the slope and shelf by dislocation of the thermocline by the wind stress; (2) interaction of inertial flow with topography generated internal waves along the shelf break, which bifurcated into landward and seaward propagating phases; (3) downwelling along the coast depressed the thermocline; after downwelling relaxes, an internal wave front propagates as a Kelvin wave; and (4) Poincare waves generated within Mississippi Canyon propagate seaward while being advected westward over the continental slope. These processes interact to produce a three-dimensional internal wave field, which was only partly captured by the observations.

Hydraulic Physical Modeling and Observations of a Severe Gap Wind

Abstract Strong gap winds in Howe Sound, British Columbia, are simulated using a small-scale physical model. Model results are presented and compared with observations recorded in Howe Sound during a severe gap wind event in December 1992. Hydraulic theory is utilized to explain along-channel variation in wind. Field observations affirm the findings of the physical modeling with both, indicating the presence and location of controls and hydraulic jumps in the wind layer. Hydraulic behavior is found to change as the synoptic pressure gradient and the flow rate increase. In particular, field results indicate two distinct hydraulic situations: one during relatively weak wind, the other, which is more strongly controlled, during the period of peak wind. An additional comparison is made with output from the computer model hydmod of Jackson and Steyn. Numerical simulations, configured for the conditions present in Howe Sound during the December 1992 event, indicate channel hydraulics (and thus spatial wind spee...