Three-dimensional Circulation Research Articles

The role of quasi-stationary flaw polynya in formation of dense shelf waters in the wintertime and their subsequent slope cascading (the Laptev Sea case study)

Shelf areas in the region of the Severnaya Zemlya Archipelago in the Laptev Sea are characterized by existing quasi-stationary flaw polynya that periodically opens throughout the entire wintertime under the action of strong offshore winds, which occur during the passage of cyclones. In periods of the open water surface, a near-surface turbulent layer or forced convection layer is formed in the flaw polynya; the water in the layer formed undergoes intense salinization and its dense increases due to active volumetric frazil ice production. As a result of the gravity force action, intense three-dimensional convective circulation develops in the underlying layers. It leads to a fast convective adjustment of the entire water column, especially, in the late winter, when residual stratification in the area of polynya is weakened with the total action of salinization due to the background static ΣMsback and periodical local frazil ice formation ΣMsf. On the whole for the entire winter period ΣMsf is 3.4 times greater than ΣMsback, although, during one month, probable lifetime of polynya with open water surface is several days. However, in these periods, salt fluxes with frazil ice production exceed background salt fluxes in the congelation polynya and background salt fluxes under heavy ice (limiting the polynya) 10–80 times. Spreading outside the polynia, dense shelf waters form in the area of polynya mesoscale baroclinic circulation, first generating intense shelf cascading, then intense slope cascading, which is of a local and random character. Some estimates of elements of baroclinic circulation of a convective origin in the area of polynia were obtained from the laboratory modeling results and are confirmed by field observation data.

Process study of coastal circulation over the inner Scotian Shelf using a nested-grid ocean circulation model, with a special emphasis on the storm-induced circulation during tropical storm Alberto in 2006

This study examines main physical processes affecting the three-dimensional (3D) circulation and hydrographic distributions over the inner Scotian Shelf (ISS) in June and July 2006 using a nested-grid coastal ocean circulation modeling system known as the NCOPS-LB. The nested-grid system has five relocatable downscaling submodels, with the outermost submodel of a coarse horizontal resolution of (1/12)° for simulating storm surges and barotropic shelf waves over the Eastern Canadian shelf and the innermost submodel of a fine resolution of ∼180 m for simulating the 3D coastal circulation and hydrography over Lunenburg Bay of Nova Scotia in the default setup. The NCOPS-LB is driven by meteorological and astronomical forcing and used to study the storm-induced circulation over the ISS during tropical storm Alberto. Model results demonstrate that the coastal circulation and hydrographic distributions over the ISS are affected significantly by tides, local wind forcing, and remotely generated coastal waves during the study period.

How to Measure the Strength of the East Asian Summer Monsoon

Abstract Defining the intensity of the East Asian summer monsoon (EASM) has been extremely controversial. This paper elaborates on the meanings of 25 existing EASM indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979–2006 period. The existing indices can be classified into five categories: the east–west thermal contrast, north–south thermal contrast, shear vorticity of zonal winds, southwesterly monsoon, and South China Sea monsoon. The last four types of indices reflect various aspects of the leading mode of interannual variability of the EASM rainfall and circulations, which correspond to the decaying El Niño, while the first category reflects the second mode that corresponds to the developing El Niño. The authors recommend that the EASM strength can be represented by the principal component of the leading mode of the interannual variability, which provides a unified index for the majority of the existing indices. This new index is extremely robust, captures a large portion (50%) of the total variance of the precipitation and three-dimensional circulation, and has unique advantages over all the existing indices. The authors also recommend a simple index, the reversed Wang and Fan index, which is nearly identical to the leading principal component of the EASM and greatly facilitates real-time monitoring. The proposed index highlights the significance of the mei-yu/baiu/changma rainfall in gauging the strength of the EASM. The mei-yu, which is produced in the primary rain-bearing system, the East Asian (EA) subtropical front, better represents the variability of the EASM circulation system. This new index reverses the traditional Chinese meaning of a strong EASM, which corresponds to a deficient mei-yu that is associated with an abnormal northward extension of southerly over northern China. The new definition is consistent with the meaning used in other monsoon regions worldwide, where abundant rainfall within the major local rain-bearing monsoon system is considered to be a strong monsoon.

Baroclinic dynamics of wind-driven circulation in a stratified bay: A numerical study using models of varying complexity

The baroclinic response of a stratified coastal embayment (Lunenburg Bay of Nova Scotia) to the observed wind forcing is examined using two numerical models. A linear baroclinic model based on the normal mode approach shows skill at reproducing the observed isotherm movements and sub-surface currents during a time of strong stratification in the bay. The linear model also shows that the isotherm movement in Lunenburg Bay is influenced by the wind forcing and propagation of baroclinic Kelvin waves from neighbouring Mahone Bay. The effects of nonlinearity and topography are investigated using a three-dimensional nonlinear coastal circulation model. The nonlinear model results demonstrate that the nonlinear advection terms generate a gyre circulation at the entrance of Lunenburg Bay, and the slope bottom topography at the mouth of the bay strengthens the sub-surface time-mean inflow on the southern side of the bay. A comparison of model-calculated currents in different numerical experiments clearly shows that baroclinicity plays a dominant role in the dynamics of wind-driven circulation in Lunenburg Bay.

Structure of the subsurface counter current beneath the Tsushima warm current simulated by an ocean general circulation model

The subsurface counter current beneath the Tsushima Warm Current is simulated using a three-dimensional circulation model. The model well reproduces the counter current beneath the Tsushima Warm Current on the shelf break. The counter current appears as nearshore parts of the subsurface clockwise circulations from spring to early winter. The clockwise circulations are separated by developed shelves such as the Oki Spur and the Noto Peninsula, thus the counter current is not a continuous flow along the Japanese coast in this model. The vertical structure of the counter current can be explained by a density structure with the thermal wind relationship. The permanent and seasonal pycnoclines form mutually opposite horizontal density gradients near the Japanese coast in summer. Such a density structure results in a speed maximum of the counter current away from the bottom. It is remarkable that the second baroclinic mode is dominant in nearshore parts of the subsurface clockwise circulations in summer, which are attributed to the density structure. Similar density structures are also found in some coastal regions of the world oceans where subsurface counter currents are expected.

Localization of Multidecadal Variability. Part II: Spectral Origin of Multidecadal Modes

Abstract In a companion paper, the authors have shown that in an idealized Atlantic–Pacific Ocean configuration with a conveyor-type overturning circulation, localized multidecadal variability occurs in the Atlantic. Results suggest that the multidecadal variability originates from the instability of the three-dimensional thermohaline circulation and that the physics of the spatial patterns of the SST anomalies can be understood from a study of an Atlantic-only configuration. Specific internal (multidecadal) modes, which obtain a positive growth factor depending on the background thermohaline flow, are associated with the instability. In this paper, the spectral origin of these internal modes is studied using eigensolution continuation techniques. As in the single-hemispheric case, multidecadal modes arise through mergers of so-called SST modes. In the double-hemispheric case studied here, there actually are two types of multidecadal modes that lead to oscillatory behavior. Depending on the background conditions, one of these oscillatory flows is preferred.

Three-Dimensional Tidal Flow in an Elongated, Rotating Basin

Abstract The three-dimensional tidal circulation in an elongated basin of arbitrary depth is described with a linear, constant-density model on the f plane. Rotation fundamentally alters the lateral flow, introducing a lateral recirculation comparable in magnitude to the axial flow, as long as friction is not too large. This circulation is due to the imbalance between the cross-channel sea level gradient, which is in near-geostrophic balance with the Coriolis acceleration associated with the vertically averaged axial flow, and the Coriolis acceleration associated with the vertically sheared axial flow. During flood condition, for example, the lateral Coriolis acceleration near the surface exceeds the pressure gradient, tending to accelerate the lateral flow, while the converse is true near the bottom. As a result, with rotation, fluid parcels tend to corkscrew into and out of the basin in a tidal period. The axial flow is only weakly modified by rotation. When friction is small, the axial velocity is uniform in each section, except in a narrow bottom boundary layer where it decreases to zero. The boundary layer thickness increases with friction, so that with moderate or large friction, axial velocities are sheared from bottom to surface. When friction is large, the local and Coriolis accelerations are both small and the dynamics are governed by a balance between friction and the pressure gradient.

Distributed Entrainment Sink Approach for Modeling Mixing and Transport in the Intermediate Field

In densely populated coastal cities in Asia, wastewater outfalls are often located not far from sensitive areas such as beaches or shellfisheries. The impact and risk assessment of effluent discharges poses particular technical challenges, as pollutant concentration needs to be accurately predicted both in the near field and intermediate field. The active mixing close to the discharge can be modeled by proven plume models, while the fate and transport far beyond the mixing zone can be well-predicted by three-dimensional (3D) circulation models based on the hydrostatic pressure approximation. These models are usually applied separately with essentially one-way coupling; the action of the plume mixing on the external flow is neglected. Important phenomena such as surface buoyant spread or source-induced changes in ambient stratification cannot be satisfactorily addressed by such an approach. A Distributed Entrainment Sink Approach is proposed to model effluent mixing and transport in the intermediate field by dynamic coupling of a 3D far field shallow water circulation model with a Lagrangian near-field plume model. The action of the plume on the surrounding flow is modeled by a distribution of sinks along the plume trajectory and an equivalent diluted source flow at the predicted terminal height of rise. In this way, a two-way dynamic link can be established at grid cell level between the near and far-field models. The method is demonstrated for a number of complex flows including the interaction of a confined rising plume with ambient stratification, and the mixing of a line plume in cross flow. Numerical predictions are in excellent agreement with basic laboratory data. The general method can be readily incorporated in existing circulation models to yield accurate predictions of mixing and transport in the intermediate/far field.

Stomatal conductance changes due to increasing carbon dioxide levels: Projected impact on surface ozone levels

The impact of increasing levels of carbon dioxide on stomatal conductance and surface ozone levels was investigated using a global three-dimensional general circulation model (GCM) coupled to an interactive land surface scheme and a chemistry model. Pre-industrial, present day and doubled present day levels of carbon dioxide were used. This approach was used to examine the sensitivity of modelled surface ozone levels to changes in stomatal conductance via dry deposition. A doubled level of carbon dioxide was found to increase surface ozone levels by between 2 and 8 ppb in all four seasons owing to reduced dry deposition fluxes, although the location and extent of the changes were very different between each season. No change in levels of nitrogen oxides (NO and NO2) was modelled. A similar experiment to examine the same effect on modelled pre-industrial ozone levels showed that the ozone levels over Europe were only slightly smaller (by 1–1.5 ppb) when the CO2 level was decreased from 369 ppm to 280 ppm.

Progress in the 3-D circulation of the eastern equatorial Pacific in a climate ocean model

The Pacific Equatorial Undercurrent (EUC) displays important seasonal, interannual and decadal variability. The correct representation of the EUC in ocean general circulation models (OGCM) is of primary importance for an adequate simulation of the Tropical Pacific Ocean, and for the ENSO (El Nino Southern Oscillation) cycle. This study aims at better understanding the EUC termination and its interactions with the three-dimensional circulation in the eastern Pacific in a climate-type OGCM. It also aims at improving the realism of the EUC in this region where some deficiencies were previously noticed. Several sensitivity experiments have been performed from 1993 to 2000, and the model results are compared to TAO observations. The sensitivity to vertical and lateral eddy mixing coefficients is explored, and we find that the 3-D equatorial circulation is highly sensitive to a reduction of the background vertical eddy coefficients in the surface layers. This reduction is only possible because of the change in the advection scheme and leads to a great improvement in the annual mean and the seasonal cycle of the modelled EUC, increasing both the EUC and the tropical cells transports. However, it appears that in the eastern Pacific, the increase in EUC transport is largely independent of the increase in the 3-D circulation. This is also true for the seasonal cycle transport. Reducing the isopycnal mixing coefficients further improves the mean EUC speed, but does not modify the associated 3-D circulation. Adding the Galapagos Archipelago to the model bathymetry only induces very local changes in the equatorial 3-D circulation. However, it leads to important changes in the pathways of EUC waters that reach the South American coast.

Numerical model simulations of continental shelf flows off northern California

Abstract The three-dimensional circulation on the continental shelf off northern California in the wind events and shelf transport (WEST) experiment region during summer 2001 is studied using the primitive equation regional ocean modeling system (ROMS). The simulations are performed with realistic topography and initial stratification in a limited-area domain with a high-resolution grid. Forcing consists of measured wind-stress and heat flux values obtained from a WEST surface buoy. The general response shows a southward coastal upwelling jet of up to 1 m s - 1 and a weakening or reversal of currents inshore of the jet when upwelling winds relax. Model results are compared to WEST moored velocity and temperature measurements at five locations, to CODAR surface current observations between Pt. Reyes and Bodega Bay, and to hydrographic measurements along shipboard survey lines. The model performs reasonably well, with the highest depth-averaged velocity correlation (0.81) at the inshore mooring (40 m water depth) and lowest correlation (0.68) at the mid-depth mooring (90 m depth). The model shows generally stronger velocities than those observed, especially at the inshore moorings, and a lack in complete reversal of southward velocities observed when upwelling winds relax. The comparison of surface velocities with CODAR measurements shows good agreement of the mean and the dominant mode of variability. The hydrography compares closely at the southern and northern edges of the survey region (correlation coefficients between 0.90 and 0.97), with weaker correlations at the three interior survey lines (correlation coefficients between 0.44 and 0.76). Mean model fields over the summer upwelling period show slight coastal jet separation off Pt. Arena and significant separation off Pt. Reyes. The cape regions also experience relatively strong bottom velocities and nonlinearity in the surface flow. Across-shelf velocity sections examined along the shelf reveal a double jet structure that appears just north of Bodega Bay and shows the offshore jet strengthening to the south. We examine the dynamics during an upwelling and subsequent relaxation event in May 2001 in which the WEST measurements show evidence of a strong flow response. The alongshelf variability in the upwelling and relaxation response introduced by Pt. Reyes is evident. Analysis of term balances from the depth-averaged momentum equations helps to clarify the event dynamics in different regions over the shelf. A clear pattern in the nonlinear advection term is due to the spatial acceleration of the southward jet around the capes of Pt. Arena and Pt. Reyes during upwelling. Results from a three-dimensional Lagrangian analysis of water parcel displacement show significant southward displacement in the coastal jet region, including a strong signal from the double jet. Alongshelf variability in parcel displacements and upwelling source waters due to the presence of Pt. Arena and Pt. Reyes is also apparent from the Lagrangian fields. A cyclonic eddy-like recirculation feature offshore of Pt. Arena prior to the upwelling event causes large patches of onshore-displaced parcels. Additionally, across-shelf variability in the response of water parcels along the D line includes decreased vertical displacement and increased alongshelf displacement in the offshore direction.

Modeling growth of Atlantic cod larvae on the southern flank of Georges Bank in the tidal-front circulation during May 1999

Abstract Cruises were conducted in spring 1999 to describe the interaction between tidal-front processes and the transport, retention, and growth of cod larvae and their prey during the seasonal transition to a stratified water-column along the southern flank of Georges Bank. All the physical and biological observations were integrated in coupled circulation-trophodynamic simulations. The three-dimensional circulation fields were modeled using data assimilation methods described in Aretxabaleta et al. [2005. Data assimilative hindcast on the Southern Flank of Georges Bank during May 1999: frontal circulation and implications. Continental Shelf Research 25, 849–874]. The individual-based model (IBM) of Lough et al. [2005. A general biophysical model of larval cod growth applied to populations on Georges Bank. Fisheries Oceanography 14, 241–262] was used to consider trophodynamic effects on the growth and survival of larval cod. Prey fields were specified for mixed and stratified water columns from field surveys and allowed to adjust in the circulation model. Encounter and ingestion rates of larvae were functions of prey concentration, larval search patterns, light, swimming speeds of predator and prey, and turbulence. Model outputs provide hourly depth-dependent estimates of growth rates, prey biomass ingested, and larval length and weight. Simulations were conducted along a 2-D transect across the tidal front, from mixed to stratified water columns, before and after a wind event. Pre-storm, observed larval cod growth rates, based on RNA–DNA analysis, were highest in the surface 20 m at the stratified and front stations. Post-storm, larval growth rates decreased 1–2% d −1 at the stratified and front stations, corresponding with a

Intercomparison of the Southern Ocean Circulations in IPCC Coupled Model Control Simulations

Abstract The analyses presented here focus on the Southern Ocean as simulated in a set of global coupled climate model control experiments conducted by several international climate modeling groups. Dominated by the Antarctic Circumpolar Current (ACC), the vast Southern Ocean can influence large-scale surface climate features on various time scales. Its climatic relevance stems in part from it being the region where most of the transformation of the World Ocean’s water masses occurs. In climate change experiments that simulate greenhouse gas–induced warming, Southern Ocean air–sea heat fluxes and three-dimensional circulation patterns make it a region where much of the future oceanic heat storage takes place, though the magnitude of that heat storage is one of the larger sources of uncertainty associated with the transient climate response in such model projections. Factors such as the Southern Ocean’s wind forcing, heat, and salt budgets are linked to the structure and transport of the ACC in ways that have not been expressed clearly in the literature. These links are explored here in a coupled model context by analyzing a sizable suite of preindustrial control experiments associated with the forthcoming Intergovernmental Panel on Climate Change’s Fourth Assessment Report. A framework is developed that uses measures of coupled model simulation characteristics, primarily those related to the Southern Ocean wind forcing and water mass properties, to allow one to categorize, and to some extent predict, which models do better or worse at simulating the Southern Ocean and why. Hopefully, this framework will also lead to increased understanding of the ocean’s response to climate changes.

Modelling the hydrodynamics and the fate of passive and active organisms in the Guadiana estuary

Hydrodynamics, salinity intrusion and stratification in the Guadiana estuary were characterized using two- and three-dimensional models for fresh water flows covering the range of monthly averaged river flows for dry to wet years. Two- and three-dimensional circulation model results were then used to investigate the flushing properties of the lower estuary for passive organisms and for organisms with vertical migration capabilities, respectively. Stratification can occur in the Guadiana estuary for river flows as small as 10 m 3 s −1, depending on the tidal amplitude. For homogeneous or weakly stratified conditions, salinity can be found upstream of Alcoutim (km 40) during neap tides, and extends even further for spring tides. For a river flow of 10 m 3 s −1, euhaline conditions reach 25 km upstream of the mouth during spring tides. For larger river flows, the region under stratified conditions is located in the downstream 10 km, and its extent depends mainly on the tidal phase. For river flows between 50 and 300 m 3 s −1, salinity intrusion does not extend beyond km 20. As the river flow increases, the upper limits of the several haline regions approach one another, creating a lower estuary where salinity conditions have a wide range of horizontal variability. Residence times of passive organisms in the lower Guadiana estuary depend on the magnitude of the river flow, tides and other mechanisms, varying from a few hours to 60 days in the range of river flows considered. The variability of residence times with different environmental conditions is similar for all river flows, the upper limit of residence times being about one order of magnitude larger than the lower limit. For river flows above 200 m 3 s −1, both limits of residence times are small (from a few hours to a few days), showing the dominant role of river flow in these conditions. However, active organisms, with vertical migration capabilities, can remain inside the Guadiana estuary under strong river flows, by taking advantage of the phase lag along the vertical profiles of velocity under stratified conditions. Permanence conditions improve for organisms located in the main channel, due to the larger stratification in deeper areas. The tidal amplitude also plays an important role, spring tides being more favourable to permanence than neap tides.

Development and application of a three-dimensional orthogonal coordinate semi-implicit hydrodynamic model

A three-dimensional, orthogonal coordinate semi-implicit hydrodynamic model in spherical coordinates that can be applied to estuarine, coastal sea, and continental shelf waters is presented. A generalized orthogonal coordinate transformation on the horizontal and a sigma coordinate transformation on the vertical, are applied to the governing equations. The governing equations are decomposed into exterior and interior modes and solved using a semi-implicit solution technique. Second-order accurate spatial and temporal discretization schemes are used on a space staggered grid. A simple flooding and drying technique is used to model the tidal flats. The model results are tested against analytical solutions for tidal circulation in an annular channel and steady residual flow generated by wind, and density differences in a rectangular channel. The predictions from the model showed very good comparison with analytical solutions for all the test cases. Three-dimensional circulation in Narragansett Bay was then studied using the developed model. The model predicted surface elevations, three-dimensional instantaneous and mean currents, salinities, and temperatures in Narragansett Bay are compared with the observations. Mean errors in the model predicted surface elevations and velocities are less than 3% and 15%, respectively. The spring and neap cycles, the shorter duration but stronger ebb dominant currents and the double flood phenomena seen in the observations are reproduced by the model. The mean estuarine currents, and the sub-tidal currents seen in the observations are also well reproduced by the model. Correlation coefficients for salinity and temperatures exceed 0.95 and 0.87, respectively.

Circulation and thermal structure in Lake Huron and Georgian Bay: Application of a nested-grid hydrodynamic model

A nested-grid hydrodynamic modelling system is used to study circulation and temperature distributions in Lake Huron (LH) and adjacent areas. This nested system is based on the three-dimensional, primitive-equation z-level ocean model. The nested system consists of two sub-components: a coarse-resolution outer model covering LH and Georgian Bay (GB) with a horizontal resolution of roughly 2.5 km, and the fine-resolution inner model covering eastern LH and northwestern GB with a horizontal resolution of roughly 900 m. Both the outer and inner models have 30 z-levels in the vertical. To assess the model performance, we simulate the three-dimensional circulation and temperature distributions of LH and GB in 1974–1975 and compare the model results with observations made in the lake. We demonstrate that outer model of the nested system simulates reasonably well the large-scale circulation and seasonal evolution of thermal stratifications in LH and GB, and the inner model produces reasonably well the three-dimensional flow and thermal structure over the coastal boundary layer close to the eastern shore of the lake.

Mechanisms for the formation of headland-associated linear sandbanks

Headland-associated linear sandbanks are a specific type of sandbanks which are located in the lee of coastal topographic features such as headlands and islands. Interaction between tidal currents and topographic features generate complex three-dimensional circulation patterns that significantly influence the distribution of sediments in the vicinity of the feature. A morphological model, using a three-dimensional hydrodynamic model, has been developed in order to determine the mechanisms responsible for the formation of the sandbanks. The model has been applied to an idealised gaussian shaped headland and long-term simulation have been undertaken. Sandbanks are created on each side of the headland but are not located in the centre of the tidal residual eddy as suggested previously. The sandbanks modify the tidal current residual as they grow. Their growth is controlled by the tidal regime, the availability of sand, the sediment grain size and the presence of secondary flows. Sandbanks are formed due to the asymmetry in the sediment transport on each side of the bank, due to the reversing tidal flow. They initially develop as a circular shape and then evolve into a more elongated shape.

Modeling the effects of environmental variability on Balanus glandula larval development

Planktonic larvae encounter variability in the marine environment as they develop, and rates of larval development are affected by changes in certain environmental variables. We examine the effects of variation in environmental parameters on the duration of the naupliar period of Balanus glandula larvae. Naupliar development rate is modeled as a function of temperature and food concentration. Simulations of the time course of development were driven by time series of temperature and food concentration collected at two locations on the US west coast. Predicted naupliar durations are reasonable in comparison to previous observations. Simulations of larval development and dispersal were driven by output of a three-dimensional physical circulation model and remotely sensed chlorophyll. These simulations demonstrate the impact of spatial variation in current velocity, temperature and chlorophyll on the duration of the larval period, the dispersal trajectory of these organisms and ultimately on recruitment success. Our naupliar development model coupled with a physical circulation model is a useful tool for addressing questions of intertidal population ecology.

A numerical model of seasonal primary production within the Chukchi/Beaufort Seas

A coupled three-dimensional circulation and ecological model provided numerical analysis of daily carbon/nitrogen cycling by the planktonic and benthic components of western Arctic shelf/basin ecosystems during 2002, when extensive field data were obtained by American and Canadian ice-breakers. Seasonal model budgets of April–May, July–August, and September–October 2002 allowed both interpolation and extrapolation of these validation data, suggesting that the most productive shelf regime of the Chukchi/Beaufort Seas was that of summer. Yet, during this period of July–August, a combination of light-limitation and nutrient-limitation limited shelf-wide mean simulated net photosynthesis to only � 709 mg C m � 2 day � 1 for shelf waters of o220 m depth. This modeled seasonal carbon fixation then accounted for � 45% of the annual shelf primary production of 97.4 g C m � 2 yr � 1 Identification of the relative importance of natural control factors of light and nutrients by the coupled model provided insight into possible consequences of future global climatic changes at these high latitudes. The model’s seasonal penetration of relatively saline, nutrient-rich Anadyr Water of Pacific origin into the eastern Chukchi Sea replicated the time series of observed salinity fields. A similar fidelity of the simulated nitrate, silicate and dissolved inorganic carbon fields with the observed ones yielded an assessment of nutrient uptake and photosynthesis during a natural fertilization experiment. The simulated chlorophyll, dissolved organic carbon (DOC), and NH4 stocks also mimicked these shipboard observations. We found that the spring 2002 stocks of new nutrients were stripped by the end of summer, with little fall nutrient resupply by physical and biotic factors, when incident light waned. However, because of extensive ice cover and nutrient-poor upper waters within the Canadian Basin, the slope regions remained

Maintenance of headland-associated linear sandbanks: modelling the secondary flows and sediment transport

Linear sandbanks are located globally in areas where there are strong currents and an abundance of sand. In the recent years, these sandbanks have become of strategic interest as a potential source of marine aggregates (sand and gravel) and mineral deposits. They form the seaward boundary of the nearshore zone and therefore are important for the stability of the coastal system. They also commonly reach the sea surface and thus pose a threat to navigation. Headland-associated linear sandbanks are a specific type of sandbanks which are located in the lee of coastal topographic features such as headlands and islands. Interaction between tidal currents and topographic features generate complex three-dimensional circulation patterns that significantly influence the distribution of sediments in the vicinity of the feature. Field and numerical model investigations of the three-dimensional flow structure have been undertaken on the Levillain Shoal, a headland-associated linear sandbank present in the lee of Cape Levillain (Shark Bay, Western Australia). The field data indicated the presence of secondary flows near the tip of the cape and around the bank which were re-produced in the numerical simulations. Numerical results have shown that residual eddies are not representative of the sediment transport and that secondary currents enhance the convergence of sediment towards the sandbank. Maintenance processes have been investigated. Sediment transport paths near the cape and the bank indicate that the sandbank is part of a sand circulation cell where the sand is circulating around the bank with exchanges between the sandbank and the headland.