Flood–ebb and spring–neap variations of mixing,stratification and circulation in Chesapeake Bay

A three-dimensional hydrodynamic model is used to investigate intra-tidal and spring–neap variations of turbulent mixing, stratification and residual circulation in the Chesapeake Bay estuary. Vertical profiles of salinity, velocity and eddy diffusivity show a marked asymmetry between the flood and ebb tides. Tidal mixing in the bottom boundary layer is stronger and penetrates higher on flood than on ebb. This flood–ebb asymmetry results in a north–south asymmetry in turbulent mixing because tidal currents vary out of phase between the lower and upper regions of Chesapeake Bay. The asymmetric tidal mixing causes significant variation of salinity distribution over the flood–ebb tidal cycle but insignificant changes in the residual circulation. Due to the modulation of tidal currents over the spring–neap cycle, turbulent mixing and vertical stratification show large fortnightly and monthly fluctuations. The stratification is not a linear function of the tidal-current amplitude. Strong stratification is only established during those neap tides when low turbulence intensity persists for several days. Residual circulation also shows large variations over the spring–neap cycle. The tidally averaged residual currents are about 50% stronger during the neap tides than during the spring tides.     

Observations of the spring–neap modulation of the gravitational circulation in a partially mixed estuary

The long-term variability of the non-tidal circulation in Southampton Water, a partially mixed estuary, was investigated using 71-day acoustic Doppler current profiler (ADCP) time series. The data show evidence that the spring–neap tidal variability of the turbulent mixing modulates the strength of the non-tidal residual circulation, with subtidal neap tide surface flows reaching 0.12 m s^–1 compared to <0.05 m s^–1 at spring tides. The amplitude of the neap-tide events in this non-tidal circulation is shown to be related to a critical value of the tidal currents, illustrating the strong dependence on tidal mixing. The results suggest that the dominant mechanism for generating these neap-tide circulation events is the baroclinic forcing of the horizontal density gradient, rather than barotropic forcing associated with ebb-induced periodic stratification. While tidal turbulence is thought to be the dominant control on this gravitational circulation, there is evidence of the additional effect of wind-driven mixing, including the effects of wind fetch and possibly wave development with along-estuary winds being more efficient at mixing the estuary than across-estuary winds. Rapid changes in atmospheric pressure also coincided with fluctuations in the gravitational circulation. The observed subtidal flows are shown to be capable of rapidly flushing buoyant material out of the estuary and into the coastal sea at neap tides.Responsible Editor: Iris Grabemann     

西沙群岛西北深水海域的内潮特征分析

基于海南岛至西沙群岛之间深水海域一长达5年的锚定潜标测流资料,采用谱分析、调和分析和动力模态分解等方法主要分析了局部海域内潮的基本特征.结果表明:研究海域的正压和斜压潮均以全日振荡为主,500~900m的海洋中层全日等密度线垂向振幅可达40m;全日内潮主要沿垂直于陆坡方向传播,与天文潮锁相的全日内潮可占全日内潮总能量的41%;海洋上层,O1内潮垂向平均振幅与局地海面高度呈显著正相关关系,K1内潮则表现为夏、冬季增强的半年循环特征;超过70%的O1分潮能量集中于第一、二斜压模态上,K1分潮在第三斜压模态上亦有相当能量.     

Tidally induced upwelling off Yangtze River estuary and in Zhejiang coastal waters in summer

MASNUM wave-tide-circulation coupled numerical model (MASNUM coupled model, hereinafter) is de-veloped based on the Princeton Ocean Model (POM). Both POM and MASNUM coupled model are ap-plied in the numerical simulation of the upwelling off Yangtze River estuary and in Zhejiang coastal waters in summer. The upwelling mechanisms are analyzed from the viewpoint of tide, and a new mechanism is proposed. The study suggests that the tidally inducing mechanism of the upwelling in-cludes two dynamic aspects: the barotropic and the baroclinic process. On the one hand, the residual currents induced by barotropic tides converge near the seabed, and upwelling is generated to maintain mass conservation. The climbing of the residual currents along the sea bottom slope also contributes to the upwelling. On the other hand, tidal mixing plays a very important role in inducing the upwelling in the baroclinic sea circumstances. Strong tidal mixing leads to conspicuous front in the coastal waters. The considerable horizontal density gradient across the front elicits a secondary circulation clinging to the tidal front, and the upwelling branch appears near the frontal zone. Numerical experiments are de-signed to determine the importance of tide in inducing the upwelling. The results indicate that tide is a key and dominant inducement of the upwelling. Experiments also show that coupling calculation of the four main tidal constituents (M2, S2, K1, and O1), rather than dealing with the single M2 constituent, im-proves the modeling precision of the barotropic tide-induced upwelling.     

On the vertical structure of the Rhine region of freshwater influence

An idealised three-dimensional numerical model of the Rhine region of fresh water influence (ROFI) was set up to explore the effect of stratification on the vertical structure of the tidal currents. Prandle’s dynamic Ekman layer model, in the case of zero-depth-averaged, cross-shore velocities, was first used to validate the response of the numerical model in the case of barotropic tidal flow. Prandle’s model predicted rectilinear tidal currents with an ellipse veering of up to 2%. The behaviour of the Rhine ROFI in response to both a neap and a spring tide was then investigated. For the given numerical specifications, the Rhine plume region was well mixed over the vertical on spring tide and stratified on neap tide. During spring conditions, rectilinear tidal surface currents were found along the Dutch coast. In contrast, during neap conditions, significant cross-shore currents and tidal straining were observed. Prandle’s model predicted ellipse veering of 50%, and was found to be a good indicator of ellipticity magnitude as a function of bulk vertical eddy viscosity. The modelled tidal ellipses showed that surface currents rotated anti-cyclonically whereas bottom currents rotated cyclonically. This caused a semi-diurnal cross-shore velocity shearing which was 90° out of phase with the alongshore currents. This cross-shore shear subsequently acted on the horizontal density gradient in the plume, thereby causing a semi-diurnal stratification pattern, with maximum stratification around high water. The same behaviour was exhibited in simulations of a complete spring–neap tidal cycle. This showed a pattern of recurring stratification on neaps and de-stratification on springs, in accordance with observations collected from field campaigns in the 1990’s. To understand the increase in ellipticities to 30% during neaps and the precise shape of the vertical ellipse structure, stratification has to be taken into account. Here, a full three-dimensional numerical model was employed, and was found to represent the effect of de-coupling of the upper and lower layers due to a reduction of mixing at the pycnocline.     

Variability of baroclinic tidal currents on the Mackenzie Shelf,the Southeastern Beaufort Sea

Semidiurnal tidal currents on the outer shelf of the Mackenzie Shelf in the Beaufort Sea were found to be strongly influenced by the locally generated baroclinic tide. Two primary factors are involved in this process: (1) the sharp shelf break along the northeastern Mackenzie Shelf, promoting the generation of vigorous internal tidal waves; and (2) the proximity to critical latitudes for M₂ and N₂ motions locking these waves and preventing them from leaving the source region. As a result, internal tides are resonantly trapped between the shelf and critical latitudes. The physical properties and temporal variations of tidal motions were examined using current meter measurements obtained from 1987–1988 at four sites (SS1, SS2, SS3, and SS4) offshore of the shelf break at depths of ∼200 m. Each mooring had Aanderaa RCM4s positioned at ∼35 m below the surface and ∼50 m above the bottom. Complex demodulation was used to compute the envelopes (amplitude modulation) of these components. A striking difference in the variability of clockwise (CW) and counterclockwise (CCW) tidal currents was found. The CW tides are highly variable, have greater amplitude, exhibit a burst-like character associated with wind events and contain about 80% of the total energy of the semidiurnal tidal currents. In contrast, the CCW components have a more regular temporal regime with distinct monthly, fortnightly and 10-day modulation at astronomical periodicities associated with frequency differences M₂–N₂ (0.03629 cpd), S₂–M₂ (0.06773 cpd), and S₂–N₂ (0.10402 cpd). Significant horizontal correlation of the CW current envelopes was found only between stations near the northeast Mackenzie Shelf, indicating this to be the main area of baroclinic internal wave generation.     

Cross-shelf transport into nearshore waters due to shoaling internal tides in San Pedro Bay, CA

In the summer of 2001, a coastal ocean measurement program in the southeastern portion of San Pedro Bay, CA, was designed and carried out. One aim of the program was to determine the strength and effectiveness of local cross-shelf transport processes. A particular objective was to assess the ability of semidiurnal internal tidal currents to move suspended material a net distance across the shelf. Hence, a dense array of moorings was deployed across the shelf to monitor the transport patterns associated with fluctuations in currents, temperature and salinity. An associated hydrographic program periodically monitored synoptic changes in the spatial patterns of temperature, salinity, nutrients and bacteria. This set of measurements show that a series of energetic internal tides can, but do not always, transport subthermocline water, dissolved and suspended material from the middle of the shelf into the surfzone. Effective cross-shelf transport occurs only when (1) internal tides at the shelf break are strong and (2) subtidal currents flow strongly downcoast. The subtidal downcoast flow causes isotherms to tilt upward toward the coast, which allows energetic, nonlinear internal tidal currents to carry subthermocline waters into the surfzone. During these events, which may last for several days, the transported water remains in the surfzone until the internal tidal current pulses and/or the downcoast subtidal currents disappear. This nonlinear internal tide cross-shelf transport process was capable of carrying water and the associated suspended or dissolved material from the mid-shelf into the surfzone, but there were no observation of transport from the shelf break into the surfzone. Dissolved nutrients and suspended particulates (such as phytoplankton) transported from the mid-shelf into the nearshore region by nonlinear internal tides may contribute to nearshore algal blooms, including harmful algal blooms that occur off local beaches.     

Scales and structure of frontal adjustment and freshwater export in a region of freshwater influence

Sea surface temperature satellite imagery and a regional hydrodynamic model are used to investigate the variability and structure of the Liverpool Bay thermohaline front. A statistically based water mass classification technique is used to locate the front in both data sets. The front moves between 5 and 35 km in response to spring–neap changes in tidal mixing, an adjustment that is much greater than at other shelf-sea fronts. Superimposed on top of this fortnightly cycle are semi-diurnal movements of 5–10 km driven by flood and ebb tidal currents. Seasonal variability in the freshwater discharge and the density difference between buoyant inflow and more saline Irish Sea water give rise to two different dynamical regimes. During winter, when cold inflow reduces the buoyancy of the plume, a bottom-advected front develops. Over the summer, when warm river water provides additional buoyancy, a surface-advected plume detaches from the bottom and propagates much larger distances across the bay. Decoupled from near-bed processes, the position of the surface front is more variable. Fortnightly stratification and re-mixing over large areas of Liverpool Bay is a potentially important mechanism by which freshwater, and its nutrient and pollutant loads, are exported from the coastal plume system. Based on length scales estimated from model and satellite data, the erosion of post-neap stratification is estimated to be responsible for exporting approximately 19% of the fresh estuarine discharge annually entering the system. Although the baroclinic residual circulation makes a more significant contribution to freshwater fluxes, the episodic nature of the spring–neap cycle may have important implications for biogeochemical cycles within the bay.     

Circulation and connectivity on the Australian North West Shelf

Australia's North West Shelf supports a diverse range of tropical habitats and marine communities, as well as being Australia's most economically significant marine region. This study is the first attempt to describe the ocean circulation across the North West Shelf on time-scales from hours to years, and space scales from 10 km over the entire shelf to 1 km in a selected focus area around the Dampier Archipelago. A series of nested circulation models have been developed with forcing by realistic winds, tides, and larger scale oceanographic conditions (taken from a global circulation model). Dispersion and connectivity patterns have also been estimated over the shelf using particle-tracking techniques. The simulations covered a period of more than 6 years, allowing the tidal, seasonal, and interannual characteristics to be investigated. Model results confirm that the instantaneous current patterns are strongly dominated by the barotropic tide and its spring–neap cycle. However, longer term transports over the inner- and mid-shelf were mainly controlled by wind-driven flow, which followed the seasonal switch from summer monsoon winds to southeasterly trades in winter. Results were shown to be relatively insensitive to adjustable model parameters and sub-model structures. However, model performance was strongly dependent on the quality of the forcing fields. Connectivity results have been represented in terms of a comprehensive set of statistical probabilities that have been made available online (http://www.per.marine.csiro.au/connie). The potential regional-scale connectivity between coral reefs on the North West Shelf has been used to illustrate the approach.     

Tides off‐shore: Transition from California coastal to deep‐sea waters‡

Abstract

Tidal pressures and currents were measured with self‐contained capsules dropped to the sea floor for one month at distances of 175, 190, and 500 nautical miles from San Diego. These observations, together with a one‐week bottom pressure record by Filloux at 750 n miles, and three half‐week bottom current records by Isaacs et al, at intermediary distances, were analyzed for tidal components by cross‐correlation with a noise‐free reference time series. (For short records this method has some merit over classical tide analysis.) It was found that the tide decays seaward to e^‐1 times the coastal amplitude over a distance of order 1000 km for the semidiurnal species, slower for the diurnal species. Tidal currents turn counterclockwise, and are polarized with maximum flow parrallel to shore in the direction of tidal propagation (320°T) at local high tide. The current amplitude is roughly 2 cm/sec for the semidiurnal component, 1 cm/sec for the diurnal component. Superimposed baroclinic tidal currents lead to poor signal: noise ratios (between 1:1 and 10:1) for the barotropic currents. In contrast, the ratio is typically 1000:1 for the bottom pressures and generally exceeds that for coastal tide stations of comparable duration. Published I.H.B. tidal constants for exposed California coastal stations indicate “upshore” (towards 320°T) propagation at 140 m/sec for semidiurnal tides. 214 m/sec for diurnal tides.

To interpret these diverse observations, we have computed the dispersion laws for all possible rotationally‐gravitationally trapped waves against a straight coast with shelf. Trapped solutions are conveniently portrayed in terms of a parameter μ such that ? = sin μ = iu/v and f = ‐ cos μ = η/v define the ellipticity and impedance of the wave motion, η, u and v being off‐shelf dimensionless elevation, normal‐to‐shore and longshore components of velocity, respectively. We then attempt to fit the observations by a superposition of the possible wave classes, all of the same tidal frequency: (a) a free Kelvin‐like edge wave with small μ (mostly trapped by rotation, but somewhat slowed by the shelf); (6) a free Poincare‐like leaky wave; and (c) a forced wave (the distortion of the sea bottom by the tide producing forces plays a significant role). The mod el can account for the main features of the observed tidal heights, and gives relative amplitudes at the coast of 54:16:4 cm for components a:b:c in the case of the semidiurnal tides, 21:24:9 cm for the diurnal tides. The results place a semidiurnal amphidrome about midway between San Diego and Hawaii. Tidal currents are not well fitted by the model, and there are problems associated with the separation of barotropic and baroclinic modes, and with the benthic boundary layer. Coastal energy dissipation is small in the sea under investigation, but a “ capacitive “ phase delay appears to be associated with Northern California harbors and inland waters.     

Sill dynamics and energy transformation in a jet fjord

A detailed set of observations are presented of the tidal forcing and basin response of Loch Etive, a jet-type fjordic system on the west coast of Scotland. The characteristics of the tidal jet observed during a spring tide are discussed in detail, and with reference to laboratory studies of Baines and Hoinka (1985). Although the system is categorized as a jet basin during spring tides (when the mode-1 densimetric Froude number exceeds 1) and a wave basin during neap tides (when the Froude number remains below 1), a mode-1 baroclinic wave response is observed throughout the spring/neap cycle. Of the total incident tidal energy, 16% is lost from the barotropic tide. The ratio between loss to bottom friction, barotropic form drag and baroclinic wave drag is estimated to be 1:4:1 (1:4:3.3) at springs (neaps). Despite this, during a spring tide, a 20-m amplitude baroclinic mode-1 wave is observed to propagate along the full length of the basin at a speed of 0.2 m s^–1, somewhat slower than the predicted linear mode-1 phase speed. A hydrographic section supports the implication of the dissipation of the baroclinic wave towards the loch head. The stratification of the upper layers is observed to decrease rapidly landward of the 40-m isobath, a possible signature of enhanced diapycnal mixing in the shallower reaches towards the loch head.Responsible Editor: Jens Kappenberg     

Tidal and low-frequency currents off the Jaguaribe River estuary (4° S, 37° 4′ W), northeastern Brazil

This article characterizes the spatial and temporal current variations, in the subtidal and tidal ranges, during the rainy and dry seasons, at the continental shelf off the Jaguaribe River, through measurements of continuous current field data from an acoustic Doppler current profiler (ADCP) mooring during 124 days, from June 12 to October 14, 2009. To support this dataset, we collected corresponding data from a meteorological station located at the estuary. The spatial variation showed that highest current speeds occur near the coast, with an offset of a NNW coastal jet, decreasing intensity, monotonically, towards offshore up to 0.1 ms^?1. In the rainy season, small inversions of the wind field were observed, lasting 2 to 3 days on average and were accompanied by the direction of surface currents only. In the dry season, the period of reversal of wind fields and currents lasted 14 and 35 h, respectively. The analysis of empirical orthogonal functions in rainy and dry seasons showed that the continental shelf is predominantly barotropic, where the second and third modes explained only 7% of the total variance, during the dry season. The tidal currents are more intense in the direction normal to the coast, showing a semidiurnal tidal regime. Energy distribution between tidal currents and currents of longer periods showed that for the component parallel to bathymetry, subtidal frequency currents are dominant, contributing to more than 70% of the variance. For the normal component to the coastline bathymetry, there is a significant increase of power concerning tidal currents, at all depths, so they contribute with about 55% of the total variance.     

Taiwan strait current in winter

Four bottom-mounted current profilers were deployed across the Taiwan Strait from September 28 to December 14 of 1999 to monitor the current velocity when the northeast monsoon was strong. Results indicate both diurnal and semidiurnal tidal currents were primarily barotropic. The barotropic diurnal tide might be explained by a single Kelvin wave propagating along the Mainland China coast from north to south. However, the barotropic semidiurnal tide manifested as a more complicated form in the Taiwan Strait.The subtidal current generally fluctuated with the northeast winds. When the northeast wind was weak, the along- and cross-strait subtidal current flowed primarily against the wind and toward Taiwan, respectively. As the northeast wind intensified, the along-strait current flowed downwind, brought the cold China coastal water southward, and formed a baroclinic velocity front in the western portion of the Taiwan Strait. The Ekman effect forced the cross-strait current toward Mainland China in the upper water column and toward Taiwan in the lower water column, respectively. The along-strait volume transport, estimated from interpolated current velocity, varied from −5 to 2 Sv with a mean value of 0.12±0.33 Sv. Similar transport was also estimated from the sea level difference across the Taiwan Strait.Although the local wind played a dominant role for the fluctuations of current velocity and transport in the Taiwan Strait, it could be not the only important factor. The current or transport directed frequently against the wind could be related to the northward current, which was consistently observed in the Penghu Channel.     

Performance of a nowcast/forecast system for Prince William Sound,Alaska

A coastal ocean extended Prince William Sound nowcast/forecast system (EPWS/NFS) has been running semi-automatically for an extended domain of Prince William Sound (PWS), Alaska for 2 years. To determine the performance of this modeling system, an assessment is conducted. EPWS/NFS and PWS/NFS (viz., its predecessor) nowcasts are compared with observed time series of sea surface temperature (SST) and coastal sea level (CSL) at a few stations, and to velocity profiles from a moored ADCP. With the extension of the model domain to include the continental shelf outside PWS and forced by an operational global ocean model (Global-Navy Coastal Ocean Model (Global-NCOM)) and a 2D tidal model at the open boundary, EPWS/NFS has achieved significant improvement over PWS/NFS, which covered only PWS per se, for most of the predicted variables in this study. In both magnitude and phase, EPWS/NFS accurately predicts the coastal tide fluctuations, as well as M2 tidal currents in Central Sound, although significant errors in coastal tides exist during some spring and neap tide cycles. Other than for the tidal motions, EPWS/NFS generally produces less energetic CSL and velocity variations than those observed. In comparison, although PWS/NFS well predicts the coastal tides, it suffers from the absence of low-frequency CSL variations, as well as misprediction of M2 tidal currents in Central Sound. For 40 h low-passed PWS/NFS and EPWS/NFS velocities, significant phase error occurs during the model–date comparison period, while EPWS/NFS nowcasts generally produce less root-mean-square-error (rmse) and smaller correlations with the observations than PWS/NFS does. Both observations and EPWS/NFS have similar vertical profiles of baroclinic velocity standard deviations, but some substantial discrepancies occur in the velocity direction. Also, in the Central Sound, EPWS/NFS predicts well the SST seasonal cycle and a major cooling event during the summer 2005. However, for periods shorter than 1 week, both PWS/NFS and EPWS/NFS SST underestimated the observed fluctuations by an order of magnitude.     

The influence of wind forcing on across-shelf transport in the Florida Keys

Acoustic Doppler current profiles and current meter data are combined with wind observations to describe the transport of water leaving Florida Bay and moving onto the inner shelf on the Atlantic side of the Florida Keys. A 275-day study in the Long Key Channel reveals strong tidal exchanges, but the average ebb tide volume leaving Florida Bay is 19% greater than the average flood tide volume entering the bay. The long-term net outflow averages 472 m³ s⁻¹. Two studies in shelf waters describe the response to wind forcing during spring and summer months in 2004 and during fall and winter months in 2004–2005. During the spring–summer study, southeasterly winds have a distinct shoreward component, and a two-layer pattern appears. Surface layers move shoreward while near-bottom layers move seaward. During the winter study, the resultant wind direction is parallel to the Keys and to the local isobaths. The entire water column moves in a nearly downwind direction, and across-shelf transport is relatively small. During the summer wet season, Florida Bay water should be warmer, fresher, and thus less dense than Atlantic shelf waters. Ebbing bay water should move onto the shelf as a buoyant plume and be held close to the Keys by southeasterly winds. During the winter dry season, colder and saltier Florida Bay water should leave the tidal channels with relatively high density and be concentrated in the near-bottom layers. But little across-shelf flow occurs with northeasterly winds. The study suggests that seasonally changing wind forcing and hydrographic conditions serve to insulate the reef tract from the impact of low-quality bay water.     

Subtidal flow and its variability at the entrance to a subtropical lagoon

Spatial and temporal variability of the subtidal exchange flow at West Pass, an inlet at the entrance to a subtropical lagoon (St. Andrew Bay, Florida), was studied using moored and towed current velocity profiles and hydrographic data. Towed and hydrographic measurements were captured over one diurnal tidal cycle to determine intratidal and spatial changes in flow. Hydrographic profiles over the tidal cycle showed that tidal straining modified density stratification asymmetrically, thus setting up the observed mean flow within the inlet. During the towed survey, the inlet's mean flow had a two-layer exchange structure that was moderately frictional and weakly influenced by Coriolis accelerations. Moored current profiles revealed the additional contribution to the dynamics from centrifugal accelerations. Along channel residual flows changed between unidirectional and exchange flow, depending on the forcing from the along-estuary wind stress and, to a lesser extent, the spring–neap tidal cycle. Increases in vertical shear in the along channel subtidal flow coincided with neap tides and rain pulses. Lateral subtidal flows showed the influence on the dynamics of centrifugal accelerations through a well-developed two-layer structure modulated in magnitude by the spring–neap tidal cycle.     

Internal tide modelling and the influence of wind effects

Initially the development of shallow sea three-dimensional barotropic tidal models is briefly reviewed with a view to determining what were the key measurements that allowed progress in this field and rigorous model validation. Subsequently this is extended to a brief review of baroclinic tidal models to try to determine a “way forward” for baroclinic model development. The difficulty of high spatial variability, and wind influence are identified as possibly important issues that must be considered in validating baroclinic tidal models. These are examined using a three-dimensional unstructured grid model of the M₂ internal tide on the shelf edge region off the west coast of Scotland. The model is used to investigate the spatial variability of the M₂ internal tide, and associated turbulence energy and mixing in the region. Initial calculations are performed with tidal forcing only, with subsequent calculations briefly examining how the tidal distribution is modified by down-welling and up-welling favourable winds. Calculations with tidal forcing only, show that there is significant spatial variability in the internal tide and associated mixing in the region. In addition, these are influenced by wind effects which may have to be taken into account in any model validation exercise. The paper ends by discussing the comprehensive nature of data sets that need to be collected to validate internal tidal models to the same level currently attained with three dimensional barotropic tidal models.     

Numerical study of three-dimensional shelf circulation on the Scotian Shelf using a shelf circulation model

A numerical shelf circulation model was developed for the Scotian Shelf, using a nested-grid setup consisting of a three-dimensional baroclinic inner model embedded inside a two-dimensional barotropic outer model. The shelf circulation model is based on the Princeton Ocean Model and driven by three-hourly atmospheric forcing provided by a numerical weather forecast model and by tidal forcing specified at the inner model's open boundaries based on pre-calculated tidal harmonic constants. The outer model simulates the depth-mean circulation forced by wind and atmospheric pressure fields over the northwest Atlantic Ocean with a horizontal resolution of 1/12°. The inner model simulates the three-dimensional circulation over the Gulf of St. Lawrence, the Scotian Shelf, and the adjacent slope with a horizontal resolution of 1/16°. The performance of the shelf circulation model is assessed by comparing model results with oceanographic observations made along the Atlantic coast of Nova Scotia and in the vicinity of Sable Island (on the Scotian Shelf) during two periods: October 2000–March 2001 and April–June 2002. Analysis of model results on Sable Island Bank indicates that tidal currents account for as much as ∼80% of the total variance of near-bottom currents, and currents driven by local winds account for ∼30% of the variance of the non-tidal near-bottom currents. Shelf waves generated remotely by winds and propagating into the region also play an important role in the near-bottom circulation on the bank.     

Modelling Internal Solitary Waves in the Coastal Ocean

In the coastal oceans, the interaction of currents (such as the barotropic tide) with topography can generate large-amplitude, horizontally propagating internal solitary waves. These waves often occur in regions where the waveguide properties vary in the direction of propagation. We consider the modelling of these waves by nonlinear evolution equations of the Korteweg–de Vries type with variable coefficients, and we describe how these models are used to describe the shoaling of internal solitary waves over the continental shelf and slope. The theories are compared with various numerical simulations.