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1.
Observations of internal tide generation over continental slopes in a laboratory experiment have been carried out, with the objectives of making comparisons with linear generation theory and investigating its limitations. Both continuous and layered stratification have been considered. A measure of the amplitude of the barotropic tidal forcing (and hence of non-linearity) is given by the Froude Number F = usb/cw, where usb is the maximum barotropic tidal velocity at the shelf break, and cw is the long-wave speed of the lowest internal mode.For continuous stratification, good agreement was obtained for “steep” slopes (α/c > 1), where α is the slope at the continental slope and c is the slope of the internal wave rays of tidal frequency), even for quite large amplitude motions (F < 1.6), and the upper limit of its quantitative usefulness was not reached. For “flat” slopes (α/c < 1) reasonable agreement was also obtained, even up to quite amplitudes (F < 3.1), although some departure from linear theory was apparent.For two-layer flows the applicability of linear theory was much more restricted. For F 0.5 there was only qualitative agreement and for larger F (>1) significant differences were observed. The latter were due to the substantial advection and associated hydraulic jumps which occured seaward of the shelf-break during the ebb-phase of the barotropic tide. Shelf-break values of F > 1 are common in the ocean.  相似文献   

2.
Abstract

Half‐hourly measurements of soil surface heat flux density (G0 ), solar irradiance (S), and the surface energy balance components were made at Agassiz, b.c., in the spring and early summer of 1978 at two adjacent bare‐soil sites, one of which was culti‐packed while the other was disc‐harrowed. G0 was calculated using the null‐alignment procedure from half‐hourly measurements of soil temperature at 30 depths down to 1 m, and volumetric soil heat capacity calculated from measurements of bulk density, organic matter fraction, and moisture content. The latent and sensible heat flux densities were measured using the energy balance/Bowen ratio technique.

It was found that both the daily averages and diurnal variations of Go at each site were not affected as the soil surface dried, despite reductions in evaporation rate of as much as 50% at the culti‐packed site and 75% at the disc‐harrowed site on the clear dry‐soil days. Diurnal variations of G0 at the disc‐harrowed site were about 25% less than at the culti‐packed site, although daily averages were similar at both sites. Daily and daytime averages of G0 at each site were linear functions of S alone, or functions of net radiation and some measure of near‐surface soil water content. Night‐time averages of G0 at each site were linear functions of a cloudiness ratio equal to the fraction received of the clear‐day S.  相似文献   

3.
Abstract

We examine the residual (non‐tidal) flow in Juan de Fuca Strait on the west coast of Canada using current and bottom pressure data collected on cross‐channel sections in the summers of 1975 and 1984. A positive estuarine circulation was evident in both sections but was better defined at the mid‐strait section than at the outer section near the mouth of the strait. Magnitudes of the volume transports in both the upper and lower layers of the channel ranged from ～ 0.25 X 10 m s at the mid‐strait section to ～ 0.15 X 10 m s～ at the outer section. The method of geostrophic levelling (Garrett and Pétrie, 1981; Pétrie et al, 1988) is used to determine the relationship between the daily averaged long‐strait velocity component and the cross‐strait pressure difference. A statistically significant relationship, consistent with a cross‐strait geostrophic balance, is obtained between the time series of shallow currents and shallow bottom pressures at the mid‐strait section. The deep currents and cross‐strait pressure differences were correlated at both sections but, because of the placement of the pressure gauges and/or ageostrophic effects such as bottom friction, were not related through a simple geostrophic balance. Cross‐spectral analysis and the calculation of the current amplitude ratios (square root of the energy ratio between the deep and shallow currents) are used to examine the baroclinicity of the flow as a function of frequency. Results suggest that flow variability in Juan de Fuca Strait is strongly baroclinic and has marked cross‐channel structure throughout the low‐frequency band.  相似文献   

4.
Abstract

As part of the Canadian Atlantic Storms Program (CASP), near‐bottom current velocity, pressure, light transmission (as a measure of suspended sediment concentration) and water temperature were recorded using a variety of instruments deployed in water depths of 20 to 37 m on the inner Scotian Shelf, during February and March 1986. Detailed mapping of a 12‐km2 area encompassing the instrument mooring sites revealed a variety of bottom types. These include sand and gravel (both forming ripples at various scales), cobble‐boulder lags, and bedrock, resulting in bottom roughness estimates that vary widely (10?4 m < k < 10° m) over short horizontal distances (of the order 102 m). The velocity data provided information on the near‐bottom current response to winter storms anda basis for computations of sediment load and transport rates. The near‐bottom mean flow showed distinct storm‐driven circulation patterns, with velocities roughly parallel to alongshore wind stress but opposed to shore‐normal wind. Wave‐induced oscillatory motions also showed marked increases during storms and frequently dominated the near‐bottom flow. Sediment load (depth‐integrated concentration) and transport were computed using a model in which the load is related to the excess normalized shear stress. The computed mean concentrations were compatible with the optical transmis someter data. These computations yielded estimates ranging up to 0.7 kg m?2 for the mean sediment load and 443 kg m?1 h?1 for the net transport. Hindcast scour rates, of the order of 1 mm h?1 under moderate storm conditions were generally compatible with depths of scour measured by divers.  相似文献   

5.
Abstract

Airborne measurements in the atmospheric boundary layer (ABL) above the marginal ice zone (MIZ) on the Newfoundland Shelf reveal strong lateral variations in mean wind, temperature and the vertical fluxes of heat and momentum under conditions of cold, off‐ice wind. Flux measurements in (and near) the surface layer indicate that the neutral 10‐m drag coefficient depends on ice concentration, ranging from 2 × 10‐3 at 10% coverage to 5 × 10‐3 at 90%. Furthermore, cross‐ice‐edge transects consistently show increasing wind speed, temperature and heat flux in the off‐ice direction, but the momentum flux may either increase or decrease, depending on the relative importance of surface buoyancy flux and roughness. For the conditions encountered in this experiment, it appears surface wave maturity does not have a significant influence on the drag coefficient in fetch‐limited regimes near the ice edge.  相似文献   

6.
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 (τx) 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 (τy) 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.  相似文献   

7.
Abstract

Estimates of the rate of dissipation of turbulent energy,?v, were made on the continental shelf off the southwest coast of Vancouver Island throughout the diurnal tidal cycle at six stations. If turbulent mixing takes place away from boundaries, and a constant fraction of the energy supplied to turbulence is converted into potential energy, this dissipation rate per unit volume, ?v, is shown to be proportional to the rate of turbulent mass and nutrient flux across the isopycnals.

We compute these fluxes to determine the source of nutrients in the upper mixed layer on the shelf. It is found that tidal mixing on the shelf throughout the water column contributes less than 10% of the flux of nutrients supplied by the estuarine outflow out of Juan de Fuca Strait. Strong winds during upwelling events supply nutrients at rates greater than those due to tidal mixing, but at rates that are likely smaller than the Juan de Fuca source. Therefore, the nutrient‐rich waters observed in the euphotic zone in spring and summer on this shelf derive mainly from Juan de Fuca Strait. Nutrients in bottom water derive from upwelling along shelf‐break canyons.  相似文献   

8.
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.  相似文献   

9.
Abstract

A reanalysis of 4 current‐meter records taken from Knight Inlet, B.C., is presented. The data indicate that the internal tide in Knight Inlet at M2 frequency undergoes a substantial reflection near a sharp bend in the channel. This result conflicts directly with a result obtained recently by Blackford and presented in this issue.  相似文献   

10.
Abstract

The wind generation of transverse thermocline motions in an infinitely long two‐layer channel is studied theoretically. Winds both along and across the channel are considered. Horizontal momentum transfer is parameterized by a constant coefficient of eddy viscosity. For a channel narrow compared with its internal Rossby radius of deformation, the transverse motions are uni‐modal and generated most efficiently by long‐shore winds. Theoretical results in this narrow channel limit agree well with observations made at Babine Lake, British Columbia. For a channel wide compared with its Rossby radius, the response is multi‐modal, especially for cross‐channel winds. For a model Lake Michigan, computed interfacial displacements due to a steady wind either across or along the channel are small compared with the observed displacements. However, a semi‐diurnal wind (in either direction) is in near resonance with the seventh transverse mode. Thus multi‐modal displacements as large as those observed could possibly be generated by semi‐diurnal winds across or along the channel.  相似文献   

11.
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.  相似文献   

12.
Abstract

Mixing near the sloping boundaries of oceans or lakes may be a significant mechanism of diapycnal transport. The basic physics of this is reviewed, with emphasis on the reduction of the effectiveness of the process due to both reduced stratification and the restratifying secondary circulation driven by buoyancy forces. This re stratification is shown to reduce the effectiveness of intermittent mixing events as well as steady mixing. It is argued that for boundary mixing to be effective in the abyssal ocean it must extend sufficiently far from the boundary that the stratification can be maintained; this may be true for breaking bottom‐reflected internal waves. The alongslope flow implied by steady‐state boundary mixing theories is downwelling‐favourable and has a magnitude related to the thickness and other properties of the boundary layer. Mixing near a boundary may thus tend to drive a downwelling‐favourable mean circulation in the interior. If the interior circulation is imposed by other forces, the bottom boundary layer may evolve to a steady state if the interior flow is downwelling‐favourable, but if it is upwelling‐favourable initially a steady state seems unlikely and the downwelling‐favourable alongslope flow induced by the boundary mixing will tend to diffuse slowly into the interior. The nature of the solution in all these cases is sensitive to the Burger number, N2 sin2 θ/f2, where θ is the bottom slope, and to the eddy Prandtl number.  相似文献   

13.
Abstract

Inertial oscillations in current records collected from May to September, 1977, at ten mooring sites 20–300 km apart in the semi‐enclosed sea off northwest British Columbia are analysed. Near‐surface oscillations were wind‐driven, clockwise rotary and circularly polarized; near‐bottom oscillations at depths of 155–330 m were clockwise rotary, less than 10% of near‐surface amplitudes, highly elliptical and poorly correlated with surface winds. In the open southwest sector of the region, near‐surface spectra possessed well‐defined peaks centred roughly 3.5% above the local inertial frequency (f), whereas spectra for the semi‐enclosed northern sector had broad peaks centred at f. The peak spectral frequency at the southeast corner of the mooring array was 6.5% below f and is linked to a Doppler shift by mean flow advection of comparatively high wavenumber inertial oscillations. A particularly vigorous wind‐generated surface “event” in mid‐June was coherent to 99% confidence over a distance of 300 km and persisted for more than 8 days at most locations and 11 days at a mooring at the edge of the continental shelf. (Typical durations for single wave groups were ～2 1/2 days.) This event, together with a similar less energetic event in August, was due to quasi‐resonant forcing by frontal winds associated with sequences of regularly spaced, eastward travelling extratropical cyclones. Estimated inertial wavelengths ranged from 300–700 km over the main portion of the sea to 85–95 km in the southeast corner.  相似文献   

14.
Abstract

We analyse time series records of isopleth depths derived from two extended sequences of hourly and bi‐hourly sampled profiles taken at Ocean Weather Station P during the summers of 1961 and 1969. Vertical displacements to 240‐m depth are mainly of semidiurnal frequency with r.m.s. amplitudes of 1–4 m. Displacements at diurnal and near‐inertial frequencies are typically less than a metre and have little statistical significance. Within the semidiurnal band, motions appear to be predominantly at the principal solar (S2) rather than the principal lunar (M2) semidiurnal frequency. The phase of the M2 baroclinic tide is roughly equal to that of the M2 barotropic tide (as extrapolated from coastal and seamount observations); phases of other constituents differ appreciably from barotropic values.  相似文献   

15.
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.  相似文献   

16.
Abstract

Winter‐time (1985–86) observations of sea‐level, surface waves, currents, bottom pressure, and water properties were made on the Scotian Shelf as part of the Canadian Atlantic Storms Program (CASP). The purpose of the field program and the locations, instrument systems, and sampling schemes are described.

Statistics on the observed currents are presented, and monthly estimates of the longshore transport of the Nova Scotian Current are given for four months spanning the winter season (December to March). The 1985–86 transports are compared with previous estimates of the baroclinic transport made using the dynamic method.

Cross‐shelf temperature and salinity distributions corresponding to the beginning and end of the field program are described and compared with the climatological means.

The autospectra of the observed currents exhibit energy concentrations at frequencies of 0.2–0.5, 1.0, 1.4 and 2.0 cycles per day (cpd). Coastal‐trapped wave motions account for significant portions of the energy in the two lowest frequency bands, forced by wind stress and the diurnal tide, respectively. Inertial oscillations generated by wind stress events predominate at 1.4 cpd, and the semidiurnal tide, at 2 cpd. These motions are described separately, and references to more detailed discussions are given.  相似文献   

17.
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.  相似文献   

18.
Application of linear baroclinic instability theory to the observed distributions of velocity, stratification, and potential vorticity in the Gulf Stream near 74° W is successful in predicting the time and length scales of the most rapidly growing disturbances. A continuously-stratified, one-dimensional model with realistic bottom slope predicts propagation speeds of 10–50 cm s−1 associated with two regimes of rapid temporal growth centered at periods of 28 days and 5–7 days. This prediction is consistent with observations of the propagation and growth of Gulf Stream meanders derived from inverted echo sounder measurements in this region. The instability model also predicts that for realistic bottom slopes the baroclinic energy transfer should be weakly negative (eddy-to-mean) in deep water, but for low-frequency waves should change to significant positive (mean-to-eddy) transfer above depths of 1500 m, consistent with observations.  相似文献   

19.
《大气与海洋》2013,51(4):391-404
Abstract

An experimental study was conducted to investigate the transition of two‐layer stratified flow from the slope of bottom topography to a horizontal channel. Three experiments, with a reduced gravity of g’ = 1.64, 6.47 and 18.0 cm s?2, were performed. Particle image velocimetry and planar laser‐induced fluorescence were used to obtain the measurements of velocity and concentration fields. The flow rate, obtained from the measured velocity field, increases significantly toward the toe of the topography by almost 40% from that at the sill crest due to the interfacial wave activities. In the horizontal channel, however, the flow rate only increases marginally. Estimates of the composite Froude number indicate that the supercritical flow on the slope of the topography goes through the transition to the subcritical flow in the horizontal channel. The transition is mainly due to the increase in the lower‐layer thickness because of increasing interfacial friction caused by the breaking of interfacial waves, and no internal hydraulic jumps are observed. The measured mean concentration field showed the formation of an intermediate layer of medium density, which increased its thickness with g’ and helped to suppress turbulence. Spectral analysis of the density interfacial fluctuations indicated that the interfacial waves that developed on the slope of the topography broke up downstream of the toe into smaller amplitude waves at larger frequencies. The waves at several channel cross‐sections were also examined.  相似文献   

20.
《大气与海洋》2013,51(4):415-427
Abstract

An Mw = 7.2 earthquake occurred on 15 June 2005 (utc) seaward of northern California off the west coast of North America. Based on the earthquake location and source parameters, the West Coast and Alaska Tsunami Warning Center issued a tsunami warning for the region extending from the California‐Mexico border to northern Vancouver Island, British Columbia (the first tsunami warning for this region since the 1994 Mw = 8.2 Shikotan earthquake). Six tide gauges on the west coast recorded tsunami waves from this event, with a maximum trough‐to‐crest wave height of 27.7 cm observed at Crescent City, California. Waves of 2.5 to 6.5 cm were measured at the five other sites: Port Orford (Oregon), North Spit and Arena Cove (California), and Tofino and Bamfield (British Columbia). The open‐ocean Deep‐ocean Assessment and Reporting of Tsunami (DART) buoys, 46404 and 46405, recorded tsunami waves of 0.5 and 1.5 cm, respectively, closely matching wave heights derived from numerical models. Incoming tsunami wave energy was mainly at periods of 10 to 40 min. The observed tsunami wave field is interpreted in terms of edge (trapped) and leaky (non‐trapped) waves and a “trapping coefficient” is introduced to estimate the relative contribution of these two wave types. Due to the high (3000 m) water depth in the source area, approximately two‐thirds of the total tsunami energy went to leaky wave modes and only one‐third to edge wave modes. The improved response to and preparedness for the 2005 California tsunami compared to the 1994 Shikotan tsunami is attributable, in part, to the operational capability provided by the open‐ocean bottom‐pressure recorder (DART) system, higher quality coastal tide gauges, and the effective use of numerical models to simulate real‐time tsunamis.  相似文献   

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