Circulation Near Submarine Canyons: A Modeling Study

Circulation near a submarine canyon is analyzed with a numerical model. Previous theoretical work indicated that stratification controlled the interaction of coastal flow with canyons, specifically, the ratio of canyon width to the internal radius of deformation. A wide canyon was thought to merely steer the flow, while a narrow canyon would create substantial cross-shelf exchange. Four cases are analyzed considering two directions of alongshore flow and two choices of initial stratification. The weakly stratified case has an internal radius about equal to the canyon width, while the strongly stratified case has one about 3 times the canyon width. The direction of the alongshore flow is shown in this study to be the more important of the two factors. In particular, right-bounded flow (flow with the coast on the right, looking downstream in the northern hemisphere) leads to shallow downwelling in the canyon and weak exchange across the shelf break, while left-bounded flow creates upwelling at the head of the canyon and strong exchange between the ocean and shelf. In left-bounded flow (upwelling), dense water is pumped onto the shelf, even for strong stratification. However, the stratification limits the vertical extent of the topographic influence so that the alongshore flow above the canyon is only weakly affected in the strongly stratified case. With any level of stratification the surface temperature (density) is not modified at all by the flow interaction with the submarine canyon. The important dynamics involve pressure gradients and Coriolis acceleration and how they interact with the bathymetric gradients but not advection of momentum. Advection of density is clearly important in the upwelling cases. Finally, continued upwelling onto the shelf acts as a drag mechanism and retards the alongshore coastal flow

Vorticity Dynamics of Seasonal Variations of the Antarctic Circumpolar Current from a Modeling Study

A one-layer numerical model was developed to analyze the vorticity dynamics of the seasonal variations of currents in the Southern Ocean. The model includes the continental geometry and bathymetry of the Southern Ocean and is forced by monthly climatological wind stress. Five cases are considered that compare (i) circulation over a flat bottom to that with bathymetry, (ii) effects of zonally averaged wind stress forcing versus the climatological forcing and (iii) anomaly wind stress (winds with the annual mean removed) versus the full stress. The individual terms in the vorticity conservation equation are calculated from the model solution along two latitude lines; 57.5-degrees-S, which passes through Drake Passage, and 43.5-degrees-S, which is in the subtropical gyres. In the zonal part of the flat bottom simulation, the curl of the surface stress balances bottom stress curl. However, in Drake Passage, beta (advection of planetary vorticity) balances bottom loss-the western boundary balance. Such vorticity interactions depend on the partial barrier of South America and, thus, do not occur in zonal channel models. The removal of vorticity occurs throughout the Southern Ocean for the seasonally varying winds but the mean circulation is balanced mainly by losses near Drake Passage. The location of the Antarctic Circumpolar Current (ACC) is controlled by the tip of South America rather than the structure of the wind. The seasonal changes in the model surface elevation in Drake Passage occur largely in the southern part of the passage, in agreement with pressure observations. The calculated ACC transport is similar for climatological and zonally averaged winds but the structure of the forced circulation is rather different for the two cases. Bottom topography changes the vorticity interactions so that the largest effects occur where the flow is forced over bathymetry creating relative vorticity by stretching, which is then removed by bottom friction. The major loss in the model occurs near Drake Passage, although there are smaller losses at other locations along 57.5-degrees-S. Bathymetry provides a strong counter-force to the wind stress and the transport is reduced by a factor of ten compared to the comparable uniform depth simulation. Friction plays a secondary role by determining the width of the currents and the spinup time but has only a weak effect on the total transport

Effects of Wind, Density, and Bathymetry on a One-Layer Southern Ocean Model

Steady solutions from a one-layer, wind-driven, primitive equation model are analyzed to determine the importance of wind forcing, pressure gradient force due to the climatological density distribution and bottom form drag on circulation in the Southern Ocean. Five simulations are discussed: three wind-forced simulations, with differing bathymetry (flat bottom, 15% bathymetry, and full bathymetry), one case with full bathymetry forced with the density-induced pressure force, and one case with full bathymetry forced by both wind and density-induced pressure gradients. The simulations presented here confirm the previous speculation (Munk and Palmen, 1951) that form drag is effective in balancing the driving force due to the surface wind stress. In fact, it has such a strong effect that bathymetry with only 15% of the true amplitude reduces the transport from over 480 x 106 m3 s-1 to about 190 x 106 m3 s-1. If the true bathymetry is used, the total transport is reduced to a value around 20 x 106 m3 s-1. Analysis of the zonally integrated momentum in the unblocked latitudes of the Southern Ocean shows that the bottom form drag balances the surface forcing, even for simulations that have viscosities that are in the upper range of acceptable values The vertically integrated pressure gradient due to the climatological density distribution produces a body force that accelerates the Antarctic Circumpolar Current, producing a transport of about 250 x 106 m3 s-1. Therefore the pressure gradient produced by the density structure of the Southern Ocean is an integral part of the dynamics of the Antarctic Circumpolar Current. It forces the flow across bathymetry that would, in the absence of the spatially varying density field, block the circulation. This result is in contrast to mid-latitude gyres in which the steady, wind-driven circulation is insulated from the influence of bathymetry by stratification (Anderson and Killworth, 1977)

Heat and Salt Changes on the Continental Shelf West of the Antarctic Peninsula Between January 1993 and January 1994

Hydrographic measurements from four cruises between January 1993 and January 1994 over the continental shelf west of the Antarctic Peninsula allow analysis of seasonal changes in heat and salt content of this region. Changes above the permanent pycnocline (about 150 m) follow a seasonal pattern of cooling and increasing in salt from summer to winter and warming and freshening from winter to summer. These near-surface changes expressed as net heating or salting rate, were above 80 W m(-2) and 4 mg salt m(-2) s(-1). The year to year difference was small compared to the seasonal changes. There was no seasonal pattern to the changes below the permanent pycnocline; heat and salt content increased or decreased together, with magnitudes about half (50 W m(-2) and 2.0 mg salt m(-2) s(-1)) that observed near the surface. Subpycnocline water warmed (10 W m(-2)) and increased salt (0.5 mg salt m(-2) s(-1)) from one January to the next. Exchange of Upper Circumpolar Deep Water (UCDW), an oceanic water mass, and West Antarctic Peninsula modified Circumpolar Deep Water, a, cooled version of UCDW on the shelf, is responsible for these changes. During the exchange process, UCDW cools by loosing heat to the cold, near-surface Winter Water left by the deep mixing during the previous winter. Subpycnocline heat and salt changes occur as a difference between onshore and vertical diffusion with vertical diffusivities of (1.0, 0.36) x 10(-4) m(2) s(-1), for heat and salt, respectively, and a horizontal diffusivity of 37 m(2) s(-1). The annual change is due to a net flux of UCDW onto the shelf during 1993, with most of the exchange occurring fall and winter. Meandering of the Antarctic Circumpolar Current along the shelf break in this area seems to cause these exchanges. Deep across-shelf cuts in the bottom topography may also play a role

Heart failure and “payment in full”: What does the law say?

One of the major practical manifestations of health law and ethics for practitioners in private practice occurs in their interactions with medical schemes. This article sets out the complete legal framework applicable to medical schemes cover as a manifestation of the human right of access tohealthcare and the right of access to social security. It is proposed that the legal framework can and should be used as neutral ground so as to ensure that patients obtain funding that provides meaningful cover of the costs ofappropriate care

Thermohaline Structure of an Eddy-Resolving North Atlantic Model: The Influence of Boundary Conditions

A T-S volumetric census, with a resolution of 0.2 degrees C and 0.1 psu, for years 20-25 of the World Ocean Circulation Experiment Community Modeling Effort eddy-resolving simulation of the equatorial and North Atlantic Ocean, reveals how the thermohaline character of the model has changed from the initial conditions, which were taken from the Levitus climatology. Any changes in the thermohaline structure, other than stirring, mixing, or geostrophic adjustment of smoothed climatology, must be due to the boundary conditions, which are imposed at the surface and at four sponge layers (northern boundary, southern boundary, Labrador Sea and Mediterranean Sea), where water temperature and salinity are nudged toward climatological conditions. Several unrealistic thermohaline features appear in the solution, which can be traced to these surface and lateral sponge boundary conditions. 1) Water masses from the Arctic Ocean are overrepresented in the model. The volume transport across the northern sponge is twice the value estimated from observations. The heat flux is approximately correct, while the salt flux is large by a factor of 4. 2) Water masses from the South Atlantic are underrepresented. The transport of water across the southern sponge is about two-thirds of the observed value, but the salt flux is comparable with estimates. However, the heat flux is only 10% of measured values due to a missing equatorward motion of warm surface waters. 3) Water masses from the Labrador Sea and Baffin Bay are overrepresented. The volume nux is twice that observed, while the heat flux from the sponge is realistic. The salt flux is about 20% of the observed value. 4) Finally, Mediterranean Water is underrepresented. Even though the volume transport across the sponge is eight times the observed value, the net salt flux is small by a factor of 400, leading to an insufficient production of salt. All of these difficulties with the model T-S structure are traced to three general problems. First, the flow at the outer edge of the sponges is strongly barotropic in spite of the fact that the temperature and salinity fields are from climatology. Part of the problem with the sponges may be the smoothed nature of the climatology, which has the effect of reducing density gradients, thereby reducing geostrophic shears. In all cases, except the southern sponge, the volume transport across the sponge is two to eight times larger than the value expected from other analyses or observations. Since the vertical structure of the now is set by the climatology, the only way to create this additional transport is through barotropic now. The reason for the additional transport is not entirely clear, but it may be due to the excessive vertical velocities that are demanded by the conversion process in the sponges. These vertical motions create bound vortices in the sponge layers that drive recirculation in the vicinity of the sponges;increasing the transport without changing the heat or salt flux. The second problem is due to geometric effects within the sponges. One such problem is that Iceland blocks the exchange along the northern sponge. Another problem is that the ocean bathymetry is specified in the sponge layer. For example, the inner Mediterranean sponge is so shallow (around 100 m) that there is very little area in which to modify the water. Similar conditions occur in the Labrador sponge where the water is also 100 m deep. The third general problem is the use of relaxation to climatology to represent surface freshwater fluxes, which leads to unrealistic surface forcing if the currents are displaced from climatological locations. The combination of a displaced Gulf Stream and the relaxation of surface salinity to climatology produces mode waters that are unrealistically cool and fresh

Exchange Across the Shelf Break at High Southern Latitudes

Exchange of water across the Antarctic shelf break has considerable scientific and societal importance due to its effects on circulation and biology of the region, conversion of water masses as part of the global overturning circulation and basal melt of glacial ice and the consequent effect on sea level rise. The focus in this paper is the onshore transport of warm, oceanic Circumpolar Deep Water (CDW); export of dense water from these shelves is equally important, but has been the focus of other recent papers and will not be considered here. A variety of physical mechanisms are described which could play a role in this onshore flux. The relative importance of some processes are evaluated by simple calculations. A numerical model for the Ross Sea continental shelf is used as an example of a more comprehensive evaluation of the details of cross-shelf break exchange. In order for an ocean circulation model to simulate these processes at high southern latitudes, it needs to have high spatial resolution, realistic geometry and bathymetry. Grid spacing smaller than the first baroclinic radius of deformation (a few km) is required to adequately represent the circulation. Because of flow-topography interactions, bathymetry needs to be represented at these same small scales. Atmospheric conditions used to force these circulation models also need to be known at a similar small spatial resolution (a few km) in order to represent orographically controlled winds (coastal jets) and katabatic winds. Significantly, time variability of surface winds strongly influences the structure of the mixed layer. Daily, if not more frequent, surface fluxes must be imposed for a realistic surface mixed layer. Sea ice and ice shelves are important components of the coastal circulation. Ice isolates the ocean from exchange with the atmosphere, especially in the winter. Melting and freezing of both sea ice and glacial ice influence salinity and thereby the character of shelf water. These water mass conversions are known to have an important effect on export of dense water from many Antarctic coastal areas. An artificial dye, as well as temperature, is used to diagnose the flux of CDW onto the shelf. Model results for the Ross Sea show a vigorous onshore flux of oceanic water across the shelf break both at depth and at the surface as well as creation of dense water (High Salinity Shelf Water) created by coastal polynyas in the western Ross Sea

Flow Near Submarine Canyons Driven by Constant Winds

Circulation over coastal submarine canyons driven by constant upwelling or downwelling wind stress is simulated and analyzed with a primitive equation ocean model. Astoria Canyon, on the west coast of North America, is the focus of this study, and model results are consistent with most major features of mean canyon circulation observed in Astoria Canyon. Near-surface flow crosses over the canyon, while a closed cyclone occurs within the canyon. Upwelling prevails within the canyon and is larger than wind-driven upwelling along the adjacent shelf break. Water rises from depths reaching 300 m to the canyon rim and, subsequently, onto the adjacent shelf. Onshore flow within the canyon is driven by the onshore pressure gradient force, due to the free surface slope created by the upwelling wind, and is enhanced by the limitation to alongshore flow by the canyon topography. Density gradients largely compensate the surface slope with realistic stratification, but continual upwelling persists near the edges of the canyon. Within the upper canyon (50-150 m below the canyon rim) a cyclone is created by flow turning into the canyon mouth, separating from the upstream edge, and advecting toward the downstream rim. Below this layer the cyclone is created by vortex stretching due to the upwelling. Downwelling winds create nearly the opposite flow, in which compression and momentum advection create a strong anticyclone within the canyon. Momentum advection is found to be important both in creating strong circulation within the canyon and in allowing the surface flow to cross the canyon undisturbed. Model results indicate that Astoria-like submarine canyons produce across shore transport of sufficient volume to flush a continental shelf in a few (2-5) years

Is Oyster Shell a Sustainable Estuarine Resource?

The decline of the eastern oyster (Crassostrea virginica) as an estuarine resource is well documented for many estuaries on the United States east coast. This decline is often associated with a decline in the shell resource and ultimately the disappearance of the shell bed. We develop a model that expressly and conjointly evaluates oyster abundance and surficial shell quantity and examine whether stability in the stock and the habitat can be simultaneously achieved. Simulations suggest that a steady-state shell content exists for any set of recruitment and natural mortality rates and that the amount of shell present at steady state varies over a wide range as recruitment and natural mortality vary. Shell mass is maximized at a natural mortality rate near the rate observed in unfished populations unimpacted by disease. A species dependent on the maintenance of hard substrate for survival, as is the oyster, might have a life span adapted to maximize the accretion of carbonate; thereby sustaining the substrate on which it depends. Relatively small changes in the recruitment rate produce large changes in abundance and consequently shell mass and the scale of variation dwarfs that of natural mortality or fishing. Only variations in the rate of shell loss or the average size of animals at death produce equivalent excursions in shell mass. In comparison, the ambit of natural mortality imposed by the disease process fortuitously occurs in a range that restrains the change in carbonate mass, probably because increased mortality reduces abundance but also increases the death rate, thus adding more shell. Simulations covering a range of fishing rates indicate that no fishing rate exists that is likely to be sustainable of the shell resource over the long term. Fishing will always abet the taphonomic and depositional processes conspiring to debilitate the oyster bed. Successful management of the oyster shell resource is obstructed by the simple fact that no additional mortality, whether imposed by disease or through fishing, can occur that will not result in habitat loss at some rate. The shell resource is maximized when the population is at predisease natural mortality rates and unfished. Thus, if fishing is to be permitted or if disease has increased persistently the natural mortality rate, the only recourse of the manager is the perpetual addition of shell in compensation to the loss or the acceptance of the degradation of the shell bed