Modelling stratification and baroclinic flow in the estuarine transition zone of the St. Lawrence estuary

Abstract

This paper presents a hydrodynamic study of the St. Lawrence Estuary's estuarine transition zone, a 100 km region where fresh water from the river mixes with salt water from the estuary. The circulation of the estuarine transition zone is driven by strong tides, a large river flow, and well‐defined salinity gradients. For this study, a three‐dimensional hydrodynamic model was applied to the estuarine transition zone of the St. Lawrence Estuary and used to examine stratification and density‐driven baroclinic flow. The model was calibrated to field observations and subsequently predicted water level elevations, along‐channel currents, and salinity with mean errors of less than 9%, 11%, and 17%, respectively. The baroclinic density‐driven currents were distinguished from the tidal barotropic currents by using principal component analysis. Stratification and baroclinic flow were observed to vary throughout the estuarine transition zone on tidal and subtidal spring‐neap time scales. On a semidiurnal tidal time scale, stratification was periodic, and baroclinic flow was represented by pulses of sheared exchange flow, suggesting that neither buoyancy forcing nor turbulent mixing is dominant at this scale. On a subtidal spring‐neap time scale, stratification and baroclinic flow varied inversely with tidal energy, increasing on weak neap tides and decreasing on strong spring tides.     

The Rose Spit Eddy in Dixon Entrance: Evidence for its existence and underlying dynamics

Abstract

The existence and dynamics of the so‐called “Rose Spit Eddy” in Dixon Entrance, British Columbia, are investigated by (i) analysing published observations of low‐frequency Eulerian and Lagrangian currents in the region; (ii) interpreting tidal residuals produced by the Hecate Model (a non‐rotating hydraulic model of Hecate Strait and Dixon Entrance); and (iii) running a barotropic, non‐linear numerical tidal model over simplified topography to investigate residuals produced over the Rose Spit sill.

Observations have consistently revealed persistent basin‐wide, surface‐intensified cyclonic shears within central and eastern Dixon Entrance. The Hecate hydraulic tidal model also produced a tidal residual cyclonic gyre in central Dixon Entrance, but with velocities considerably larger than those observed. Barotropic numerical simulations of tidal streams flowing over a representation of the Rose Spit sill produced residual flows along the sill in reasonable agreement with observations and theory. A southward‐directed jet flow was produced off Cape Chacon. Elsewhere, tidal rectification was weak. Run without the Coriolis force, organized flow along the sill broke down, although the headland jet off Cape Chacon persisted.

We submit that the observed Rose Spit eddy results from interactions between buoyancy‐driven coastal currents and tidally rectified flows generated over the Rose Spit sill, and near Cape Chacon, and perhaps indirectly, over the western flank of Learmonth Bank (which although west of the Rose Spit eddy, contributes to the cross‐channel flow across the Entrance). These regions of localized tidal stress will each favour recirculation of a portion of the coastal current within the Entrance, helping to form the eddy.

We believe that the Hecate hydraulic model eddy was generated to a significant degree by phase errors introduced at the northern open boundary, where a rocking barrier was used to force currents. A second rocking barrier also produced a large cyclonic gyre, not supported by observations, near the model's southern boundary.     

Spatial and Temporal Variability of Ocean Temperature over the Labrador Shelf

Abstract

The mid‐to‐bottom waters of the Labrador Shelf are shown to exhibit an anomalous along‐shelf temperature gradient, with warmer waters found in the north. This feature is present in summer and autumn but appears to reverse in December. Inadequate data are available during winter and spring to draw firm conclusions regarding this feature. A time averaged heat loss of the shelf waters to the atmosphere would result on average, in colder waters in the south (because of north‐south advection); however, it is shown that there is a net annual‐mean input of heat to the shelf waters. An examination of the seasonal temperature cycle at standard depths reveals that its phase is almost uniform below 30‐m depth on the northern banks of the Labrador Shelf. The limited phase variation suggests the influence of a plume of well mixed water originating near Hudson Strait. It thus appears that mixing at the entrance to Hudson Strait imparts a phase anomaly to the seasonal cycle in the north that contributes to the observed inversion of the expected latitudinal temperature gradient.     

The cross‐channel flow at the entrance of Lancaster Sound

Abstract

A westward current flows along the northern side of Lancaster Sound and an eastward current flows along the southern side. A cross‐channel flow is commonly observed to link them near the eastern entrance of Lancaster Sound; this flow is modelled assuming inviscid flow and conservation of potential vorticity. It is shown that the westward decrease in depth is sufficient to cause a cross‐channel flow that couples the inflow to the outflow. The modelled cross‐channel flow takes place at a distance inside the entrance that is less than that observed for the surface current. Obviously stratification reduces the coupling of the surface current to the bathymetry. A more realistic result is obtained with the barotropic model if the bottom slope is halved.

An inviscid mean barotropic flow out of the channel is also modelled and found to be concentrated on the southern bank in order to conserve potential vorticity. It seems that barotropic instability and friction would limit the narrowing of the flow.     

Airborne measurements of the ocean's Ku‐band radar cross‐section at low incidence angles

Abstract

Ocean backscatter data obtained with a K_u‐band airborne radar are presented along with coincident altimeter and directional wave spectral estimates. These data were collected using one sensor, NASA's radar ocean wave spectrometer (ROWS). The measurements are compared with an electromagnetic scattering model for perfectly conducting Gaussian random surfaces. The normalized radar cross‐section (NRCS) data cover those incidence angles (0–20°) where both quasi‐specular and Bragg scattering mechanisms are expected. Under certain conditions, identification and separation of these two mechanisms is possible. The scanning radar allows observations of the azimuthal variations in NRCS that are at times indicative of short‐scale wave generation in the wind direction.     

The influence of Hudson Bay runoff and ice‐melt on the salinity of the inner Newfoundland Shelf

Abstract

The present study examines sources of the interannual variability in salinity on the Newfoundland continental shelf observed in a 40‐year time series from an oceanographic station known as Station 27. Specifically, we investigate, through lag‐correlation analysis, the a priori hypotheses that the salinity anomalies at Station 27 are determined by freshwater runoff anomalies from Hudson and Ungava bays and by ice‐melt anomalies in Hudson Bay and on the Labrador Shelf. Interannual variations of summer runoff into Hudson Bay were significantly negatively correlated with salinity anomalies on the Newfoundland Shelf with a lag (9 months) that is consistent with expected travel times based on known current velocities in Hudson Bay and along the Labrador Shelf. Sea‐ice extent over the Labrador and northern Newfoundland shelves was significantly negatively correlated with salinity at a lag of 3 to 4 months, corresponding to the time of minimum salinity at Station 27. It appears that ice‐melt over the Labrador‐northern Newfoundland Shelf is primarily responsible for the seasonal salinity minimum over the Newfoundland Shelf. Interannual variability in runoff into Ungava Bay and ice‐melt in Hudson Bay were not correlated with interannual salinity variations on the Newfoundland Shelf.     

On the mean and tidal currents in Hudson strait

Abstract

The vertical structures of the mean and tidal flows in Hudson Strait are described from moored current‐meter data collected during an 8‐week period in August to October of 1982. The residual flow in the strongly stratified waters off Quebec is directed along the Strait to the southeast, is highly baroclinic and is concentrated near shore (within an offshore length scale of approximately an internal Rossby radius). Maximum mean speeds of 0.3 m s^?1 were observed near‐surface (30 m). In the weakly stratified waters on the northern side of the Strait along Baffin Island the mean flow is northwestward. The maximum speeds are 0.1 m s^?1 near‐surface (30 m) and the current amplitudes decrease to 0.05 m s^?1 at 100 m. The mean southeastward transport is estimated to be 0.93 ±0.23 × 10⁶ m³ s^?1 with a northwestward transport of 0.82 ± 0.24 × 10⁶ m³ s^?1. Over most of the Strait the across‐channel residual currents are directed towards the Quebec shore with velocities ranging from 0.02 to 0.1 ms^?1. Current variability is dominated by the tides, the M₂ being the major tidal constituent. In the vicinity of the mooring the M₂ tide is primarily barotropic, progressive in nature, and has along‐channel current amplitudes varying across the Strait from 0.20 to 0.45 m s^?1. Observed differences in tidal sea‐level elevations across the Strait can be accounted for by the cross‐channel variations characteristic of Kelvin waves.     

Oceanographic features of Hecate Strait and Queen Charlotte Sound in Summer

Abstract

We present evidence of previously unresolved oceanographic features in Queen Charlotte Sound and Hecate Strait using data collected in the summer of 1990 and interpreted using a three‐dimensional, finite‐element diagnostic numerical model for two separate simulations: baroclinic flow without wind‐forcing and barotropic flow with wind‐forcing. Features include a strong, prevailing southward flow along the east coast of Moresby and Kunghit Islands, clockwise circulation around the edge of Middle Bank and a cold‐water plume flowing from the shallows at the north end of Aristazabal Island toward the south and through the trough between Middle Bank and Goose Island Bank A persistent (near‐surface) outflow into the Pacific Ocean is found near the surface within 20 km of Cape St. James at the southern tip of the Queen Charlotte Islands and intermittent surface outflows are observed across the mouth of Queen Charlotte Sound. In central Hecate Strait, to the north of Middle Bank, prevailing along‐strait currents are weak and there is an east‐west interleaving of two water masses: warm water from the west side of the strait and cold water from the east side.     

Dynamical contribution to sea surface salinity variations in the eastern Gulf of Guinea based on numerical modelling

In this study, we analyse the seasonal variability of the sea surface salinity (SSS) for two coastal regions of the Gulf of Guinea from 1995 to 2006 using a high resolution model (1/12°) embedded in a Tropical Atlantic (1/4°) model. Compared with observations and climatologies, our model demonstrates a good capability to reproduce the seasonal and spatial variations of the SSS and mixed layer depth. Sensitivity experiments are carried out to assess the respective impacts of precipitations and river discharge on the spatial structure and seasonal variations of the SSS in the eastern part of the Gulf of Guinea. In the Bight of Biafra, both precipitations and river runoffs are necessary to observe permanent low SSS values but the river discharge has the strongest impact on the seasonal variations of the SSS. South of the equator, the Congo river discharge alone is sufficient to explain most of the SSS structure and its seasonal variability. However, mixed layer budgets for salinity reveal the necessity to take into account the horizontal and vertical dynamics to explain the seasonal evolution of the salinity in the mixed layer. Indeed evaporation, precipitations and runoffs represent a relatively small contribution to the budgets locally at intraseasonal to seasonal time scales. Horizontal advection always contribute to spread the low salinity coastal waters offshore and thus decrease the salinity in the eastern Gulf of Guinea. For the Bight of Biafra and the Congo plume region, the strong seasonal increase of the SSS observed from May/June to August/September, when the trade winds intensify, results from a decreasing offshore spread of freshwater associated with an intensification of the salt input from the subsurface. In the Congo plume region, the subsurface salt comes mainly from advection due to a strong upwelling but for the Bight of Biafra, entrainment and vertical mixing also play a role. The seasonal evolution of horizontal advection in the Bight of Biafra is mainly driven by eddy correlations between salinity and velocities, but it is not the case in the Congo plume.     

Variability of temperature,salinity and tidally‐averaged density in the middle estuary of the St Lawrence

Abstract

Surveys in the Middle Estuary of the St Lawrence have yielded a data base consisting of more than 15,000 T‐S pairs distributed over 62 13‐h profiling stations. Although the T‐S curves at each station are remarkably linear, the variability of the slopes and intercepts of the lines is considerable. The means and standard deviations of the temperature and salinity at each individual station are not explicable in terms of linear combinations of the parameters for location in the Estuary, the upstream water properties, the phase of the spring‐neap cycle and the tidal energies.

It is shown that the tidally‐averaged density structure is separable into horizontal and vertical components and that its vertical variation over the whole Estuary may be explained by any one of three different functional forms. However, its horizontal variation is not explicable in terms of linear combinations of the parameters mentioned in the paragraph above.

Plots of the horizontal variations in temperature, salinity or density may only be meaningful if the data are collected synoptically, and even then cannot be considered to be accurate over time‐scales longer than one tidal cycle.     

On the relationship between arctic sea‐ice anomalies and fluctuations in Northern Canadian air temperature and river discharge

Abstract

A lagged cross‐correlation analysis of climatic data from the period 1953–1984 was carried out for three regions of Northern Canada (Beaufort Sea, Hudson Bay, Baffin Bay/Labrador Sea) to determine the relationships between sea‐ice anomalies and surface air temperature and river discharge anomalies. Significant negative correlations at the 95% level were found between sea‐ice and temperature anomalies. A significant correlation at the 95% level was found between sea‐ice and river discharge anomalies in only one of two subregions studied.     

The response of an open stratified bay to wind forcing

Abstract

Current and temperature measurements collected during the summers of 1974, 1975 and 1979 are used to investigate the wind‐induced response of St Georges Bay, Nova Scotia. A multivariate‐frequency response analysis shows that temperature and the along‐bay component of current in the lower layer are coherent with the local wind stress at periods of 2–6 d, with the wind stress accounting for 35–65% of the observed variances. Winds are also coherent with the surface currents but account for only 20–25% of the variance. Two dynamically different regions are identified. Near the entrance to the bay, cross‐bay wind stresses (τ_x) produce Ekman drift with compensating flow in the lower layer. In the interior of the bay, the near‐bottom currents are coherent with along‐bay (τ_y) wind stresses and are directed upwind. These currents are topographically steered and a response to surface pressure gradients set up by the wind. The surface gradients are believed to be part of the wind‐induced set‐up within the southern Gulf of St Lawrence.     

Skew eddy fluxes as signatures of non‐linear tidal current interactions,with application to Georges Bank

Abstract

The eddy flux of a conservative scalar in a time‐dependent rotary velocity field may have a component that is normal to the scalar gradient. This component is the “skew flux”, which consists of the scalar transport by the Stokes velocity and a part that is always non‐divergent (and hence does not affect scalar evolution). Since tidal velocity fields usually have rotary features, tidal‐band eddy scalar fluxes may include a skew component that can be useful in indicating the occurrence of non‐linear current interactions.

The skew temperature flux associated with the semidiurnal tide in a continental shelf region is demonstrated using simple models, and moored current and temperature observations from Georges Bank. The observed fluxes on the Bank are largely directed along isobaths, with apparent contributions from the topographic rectification of the barotropic tidal current over the Bank's side and from the rotary tidal ellipses in a frontal region. Simple models indicate that the weaker cross‐isobath fluxes can arise through the influence of frictionally induced vertical structure on topographic tidal rectification, a baroclinic tidal current interaction, or the interaction of baroclinic and barotropic tidal currents. In some cases, the simple models show qualitative agreement with the observed fluxes and currents but, in general, more realistic models and better estimates of the background mean temperature field are required to obtain quantitative estimates of the relative importance of these interactions and other processes. Nevertheless, the observations and models suggest that non‐linear interactions involving both barotropic and baroclinic tidal currents are occurring on Georges Bank.     

Advection,Surface Area,and Sediment Load of the Fraser River Plume Under Variable Wind and River Forcing

The Fraser River is the source of most particulate matter in the Strait of Georgia, and its dispersal is modulated by the Fraser’s plume. Here we examine the plume’s shape, location, and area, and the variation of these parameters with changes in wind and river forcing by examining a 13-year time series of Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery for the concentration of suspended particulate matter (SPM). Plume shape and its variations are quantified by dividing the 904 images in this time series into subsets showing conditions under specified wind and river flow conditions and forming a composite image for each subset. Quantitative analysis of scalar quantities like plume area and mean plume SPM are based on calculated values for all individual images. The plume area increases linearly with river flow, changing by a factor of 10 between lowest and highest flows. Mean plume SPM also changes with flow but only by a factor of two. The surface area of the plume is almost completely unaffected by wind conditions. Plume location, however, is very sensitive to both wind speed and direction. It can reach across the Strait at highest river flows and is advected either northwest or southeast along the Strait in the same direction as winds on daily time scales. We also estimate the residence time of sediment in the plume to be only a few days, allowing the plume itself to reshape rapidly over short time scales in response to weather conditions.     

The rose report — A review

Abstract

Estimates of the entrainment rate of salt water into the Fraser River plume have been made using two independent methods. A value of k = 2 × 10 ^‐4 is obtained for he entrainment coefficient relating the vertical to the horizontal velocity from salt conservation arguments at a series of profiles along the plume and from calculations of surface divergence, as measured by drifting drogues. It is also found that entrainment contributes significantly to the deceleration of the river plume after it issues into the Strait of Georgia.     

Assimilation of ERS‐2 scatterometer winds using the Canadian 3D‐var

Abstract

The goal of this study is to evaluate the impact of incorporating the marine surface winds retrieved from the ERS‐2 scatterometer in the Canadian three‐dimensional variational analysis system, (3D‐var). The aspects of the 3D‐var most relevant to the assimilation of surface ‐wind observations and a general method for resolving the directional ambiguity of the retrieved scatterometer ‐winds are first described. A comparison ‐with 6‐h forecasted winds is then made to demonstrate that these data are of high quality, but exhibit a speed bias that can be removed by increasing their amplitudes by about 5%. The analysis increment from a single scatterometer wind observation is calculated to illustrate the response of the 3D‐var to surface wind observations. As a consequence of the forecast error covariance model, the assimilation of surface wind observations produces meteorologically consistent increments for both the rotational and divergent wind components and the mass field. The results from a series of cross‐validation experiments using ship‐based wind data demonstrate a positive impact of assimilating scatterometer winds and the effectiveness of a simple method for estimating and removing the speed bias. The impact of assimilating scatterometer data within a short assimilation cycle is also evaluated. Overall, the results show that including scatterometer data in the analysis decreases the 6‐h forecast error of surface wind by 13%. Over the northern extra‐tropics the improvement is only 4% and for the southern extra‐tropics it is 16%. Results from a series of two‐day forecasts produced using the analyses from the assimilation cycles with and without retrieved scatterometer winds included are also presented. Using radiosonde observations at 850 hPa, 500 hPa, 250 hPa and 100 hPafor verification, the impact on the forecasts is nearly neutral in the northern hemisphere and the tropics. Conversely, a significant positive impact is found on both wind and mass fields in the southern hemisphere over the entire two‐day forecast.     

Current meter observations from the Labrador and Newfoundland shelves and comparisons with barotropic model predictions and IIP surface currents

Abstract

Recent current measurements from the southern Labrador and northeastern Newfoundland shelves confirm the presence of inshore and offshore branches of the Labrador Current with high mean currents and low standard deviations. At mid‐shelf weaker and more variable currents occur over the banks, and cross‐shelf flows are found to be associated with the shelf topography. An annual cycle of the inshore branch, in phase with wind forcing, is significant on the NE Newfoundland Shelf but not detectable on Hamilton Bank. The phase of the annual cycle in the offshore branch is consistent with buoyancy, not wind forcing. The observations compare reasonably well with results from a barotropic model for the region and the International Ice Patrol (IIP) surface current map. Differences occur particularly in regions of high bathymetrie curvature or an ill‐defined shelf break. The model location of the Labrador Current lies inshore of that indicated by the data, suggesting the need for better definition of the northern inflow boundary condition and the inclusion of baroclinicity. The HP surface current map agrees well with observations offshore, but shows an unrealistic, broad inshore branch, especially on the Grand Bank These differences have important implications for the drift models.     

Northern hudson bay and foxe basin: Water masses,circulation and productivity

Abstract

Total alkalinity and total carbonate determinations together with salinity and temperature are used to characterize water masses in Foxe Basin, Hudson Bay and Hudson Strait. From these measurements, we are able to infer the amounts of fresh water from river runoff and from sea‐ice meltwater. The average ice cover is estimated to be 1.9 m, and the residence time of river runoff in Hudson Bay is 3—4 years. Estimates of biological productivity were made by “correcting” total carbonate measurements for effects of biological processes, giving a value of 24 gC m a for new production.