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Abstract Two sets of Synthetic Aperture Radar (SAR) images were collected, as part of the Labrador Ice Margin Experiment (LIMEX), over the Newfoundland Shelf on consecutive days in April 1990. Ice movement is detected from the displacement of ice floes between the two images sets and compared with ice drift data from six satellite‐tracked beacons and in situ CTD data. The ice velocity data derived from the SAR images and the beacons are used to generate a map of ice velocity vectors. A streamfunction map of ocean currents is produced by removing the direct wind‐driven component in the ice movement data, and by using an objective analysis method. The resulting flow pattern contains the offshore branch of the Labrador Current with a speed of 30 to 50 cm s?1. The current closely follows the shelf break topography from north to south through the study area (47–50.5°N) as a continuous flow. In comparison, if the wind effect was not removed from the ice velocity data, the calculated Labrador Current north of 50°N would stray from the shelf break. The position of the current axis and the current speed derived from the ice movement data are in good agreement with the geostrophic current computed from the CTD data. 相似文献
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Abstract Analysis of 39 satellite‐tracked drifter records from the Newfoundland Grand Banks region has allowed maps of the mean and variable flows to be drawn. The variable currents are particularly large relative to the mean for the shelf, Flemish Cap and in the Newfoundland Basin. The ratio of the mean to variable flow is largest along the path of the Labrador Current. Drifters that either have been released on or migrate onto the Grand Banks remain therefor an average of 71 d. A statistical study of the effect of wind on drifter motion has shown that winds can only account for about 10% of current variability. This result is examined with consideration given to data noise, aliasing and non‐stationary conditions. Some drifters that were deployed in the Labrador Current moved onto the shelf and vice versa. These observations have been used to estimate the rate of exchange between the Current and the Grand Banks. Using this exchange rate in a box model, it is calculated that, over the iceberg season, 30% of the bergs will be in the Avalon Channel, 20% on the Grand Banks and 50% in the Labrador Current, in good agreement with the observed distribution. An alternative model based solely on advection is considered as well. The exchange model is also applied to the salinity budget for the Labrador Current with some success. 相似文献
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Abstract An examination of the temperature and current measurements from the NE Newfoundland Shelf indicates significant frontal variability at about a 7‐day period during the months of June‐September 1989. The oscillations recorded in 1989 appeared to have propagated into the region from the Labrador Shelf. Significant variability in the position of the shelf water/slope water front in the Bonavista area is also found between years and within the same year. Time series measurements also indicated that the transition from winter to summer conditions in the inshore region may be occurring during late July to early August. 相似文献
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Abstract Anemometer‐measured winds for the period 5–13 March 1994 were used to study the coherence of observed and forecast coastal winds along the mid‐Labrador shelf. The reliability of these variables in predicting the response of the ocean and ice to wind forcing is an important issue for ice forecasting in this area. Two anemometer‐equipped 2‐m ice beacons were deployed on pack ice north of Wolf Island and a third beacon was deployed on Grady Island. The results indicate that due to the influence of local topography, 10‐m winds observed at the meteorological station in Cartwright, Labrador provide a poor estimate (r2 = 0.2) of wind conditions over the offshore sea‐ice. In contrast, the σ = 1 level (～10 m) winds from the Canadian Meteorological Centre's Regional Finite Element (RFE) model provided a better correlation with anemometer beacon winds (0.90 for the 6‐hour forecast down to 0.45 at 36 hours). However, the RFE model overestimates the magnitude of the winds by 10–40%. The response of the ocean and ice cover to wind forcing was measured by an ocean bottom‐mounted acoustic Doppler current proþler (ADCP). Relative to the 2‐m beacon winds, the ice moved at 2.5% of wind magnitude and turned 0.6° to the left of the wind. The ocean response decreased with depth until it reached a constant value of 0.9% of the wind speed. The turning angle increased from 0.3° to the right of the wind at 3.5 m to 50° at the lowest level measured by the ADCP (73 m depth). Approximately 57% of the variance in the ocean currents at 3 m below the surface can be attributed to the 2‐m winds; at 73 m the explained variance decreases to 27%. 相似文献
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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. 相似文献
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An examination of current‐meter data gathered in 1967/68 on the continental shelf and slope off Nova Scotia has shown that meteorological forcing is an important source of energy. The response of currents to wind forcing is concentrated in a frequency band of 2.5 to 7 days. Daily mean currents of up to 25 cm/s appear to be associated with wind‐stress events. The highest correlations between wind and current are for the alongshore components of these variables. Wind‐induced currents may have been responsible for an intrusion of slope water onto the shelf which was observed in hydrographic sections from October and December 1968. Long data series (up to 167 days) formed by patching together shorter records demonstrate the existence of distinct low‐frequency variability at periods greater than 10 days. Some aspects of these motions suggest the presence of topographic Rossby waves on the shelf and slope. However, spatial and temporal coverage of data are not sufficient to define the sources of this variability. 相似文献
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Abstract A coupled ice‐ocean dynamical model is applied to the simulation of sea‐ice motion and distribution off Newfoundland during the Labrador Ice Margin Experiment (LIMEX), March 1987. In the model, the ice is coupled to a barotropic ocean through an Ekman layer that deepens with increasing wind speed. A 6‐hourly gridded wind dataset was used as input to drive the ice and the ocean. The results show that ice velocities with ice‐ocean coupling are appreciably higher than those without coupling because of the generation of wind‐driven coastal currents. This suggests that coupled ice‐ocean dynamics should always be considered in short‐term sea‐ice models. The model gives reasonable agreement with the observed ice edge except in the southern boundary where ice‐melt has a strong influence on the ice‐edge position. Ocean currents, sea level and ice velocities computed from the model are in qualitative agreement with limited current‐meter, tide‐gauge, and ice drifter trajectory data. 相似文献
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Paul H. LeBlond 《大气与海洋》2013,51(2):129-142
Abstract Analysis of visual images of the offshore ice margin of the Labrador Coast, taken on four consecutive days from the NOAA‐5 satellite, reveals horizontal oscillations with a mean wavelength of 75 km and amplitude of 15 km. The oscillations travel downstream, with the Labrador Current, at a speed C ≈ 0.2 m s‐1. Oscillations of similar periods are seen in moored current meter records. An examination of available models of baro tropic and baro‐clinic instability shows that the latter mechanism could account for the generation of the observed oscillations from the shear in the Labrador Current. 相似文献
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William R. Crawford Michael J. Woodward Michael G. G. Foreman Richard E. Thomson 《大气与海洋》2013,51(4):639-681
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. 相似文献
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Ingrid Peterson 《大气与海洋》2013,51(4):402-415
Abstract Ice floes along the Labrador Coast were tracked using visible NOAA satellite images on two consecutive days (26 and 27 April, 1984) when the ice‐pack extended beyond the Labrador Current, and winds were weak. The resulting “snapshot” of the velocity field reveals strong topographic steering of the Labrador Current, such that the current speed and width in different areas are dependent on the steepness of the continental slope, and the current deflects into and out of Hopedale Saddle. Between 55 and 58°N, the main core of the current is 60–90 km wide, with speeds of 30–55 cm s?1. The overall circulation pattern is in good agreement with historical water mass analyses over the shelf and slope, and with estimates of the speed of the Labrador Current obtained by other methods. 相似文献
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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. 相似文献
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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
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The forcing mechanisms for Antarctic coastal polynyas and the thermodynamic effects of existing polynyas are studied by means of an air-sea-ice interaction experiment in the Weddell Sea in October and November 1986.Coastal polynyas develop in close relationship to the ice motion and form most rapidly with offshore ice motion. Narrow polynyas occur frequently on the lee side of headlands and with strong curvature of the coastline. From the momentum balance of drifting sea ice, a forcing diagram is constructed, which relates ice motion to the surface-layer wind vector v
z
and to the geostrophic ocean current vector c
g
. In agreement with the data, wind forcing dominates when the wind speed at a height of 3 m exceeds the geostrophic current velocity by a factor of at least 33. This condition within the ocean regime of the Antarctic coastal current usually is fulfilled for wind speeds above 5 m/s at a height of 3 m.Based on a nonlinear parameter estimation technique, optimum parameters for free ice drift are calculated. Including a drift dependent geostrophic current in the ice/water drag yields a maximum of explained variance (91%) of ice velocity.The turbulent heat exchange between sea ice and polynya surfaces is derived from surface-layer wind and temperature data, from temperature changes of the air mass along its trajectory and from an application of the resistance laws for the atmospheric PBL. The turbulent heat flux averaged over all randomly distributed observations in coastal polynyas is 143 W/m2. This value is significantly different over pack ice and shelf ice surfaces, where downward fluxes prevail. The large variances of turbulent fluxes can be explained by variable wind speeds and air temperatures. The heat fluxes are also affected by cloud feedback processes and vary in time due to the formation of new ice at the polynya surface.Maximum turbulent fluxes of more than 400 W/m2 result from strong winds and low air temperatures. The heat exchange is similarly intense in a narrow zone close to the ice front, when under weak wind conditions, a local circulation develops and cold air associated with strong surface inversions over the shelf ice is heated above the open water. 相似文献
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Abstract Analysis of current measurements taken between June and October 1984 at four moorings in Trinity Bay, Newfoundland, is discussed. The alongshore component of current exhibits baroclinic fluctuations coherent with the along‐bay component of wind stress at periods between 3 and 7 days. A two‐layer model of internal Kelvin waves propagating around the perimeter of an elongated bay and forced by a spatially uniform wind stress is presented. The observations support several features of the model response to wind forcing. Along the side of the bay on which Kelvin waves are incoming, the amplitude of the response increases into the bay and decreases with increasing frequency. Along the outgoing side of the bay the amplitude of the response generally shows a maximum at a frequency between 0.2 and 0.3 cpd. The phase lag between current and wind is consistent with a forced response. An example is given of upwelling and downwelling on opposite sides of the bay in agreement with the model behaviour. 相似文献
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A study of the circulation in the northern South China Sea （SCS） is carried out with the aid of a three-dimensional, high-resolution regional ocean model. One control and two sensitivity experiments are performed to qualitatively investigate the effects of surface wind forcing, Kuroshio intrusion, and bottom topographic influence on the circulation in the northern SCS. The model results show that a branch of the Kuroshio in the upper layer can intrude into the SCS and have direct influence on the circulation over the continental shelf break in the northern SCS. There are strong southward pressure gradients along a zonal belt largely seaward of the continental slope. The pressure gradients are opposite in the southern and northern parts of the Luzon Strait, indicating inflow and outflow through the strait, respectively. The sensitivity experiments suggest that the Kuroshio intrusion is responsible for generating the imposed pressure head along the shelf break and has no obvious seasonal variations. The lateral forcing through the Luzon Strait and Taiwan Strait can induce the southwestward slope current and the northeastward SCS Warm Current in the northern SCS. Without the lateral forcing, there is the continental slope. The wind forcing mainly causes the The wind-induced water pile-up results in the southward no high-pressure-gradient zonal belt seaward of seasonal variation of the circulation in the SCS. high pressure gradient along the northwestern boundary of the basin. Without the blocking of the plateau around Dongsha Islands, the intruded Kuroshio tends to extend northwest and the SCS branch of the Kuroshio becomes wider and stronger. The analyses presented here are qualitative in nature but should lead to a better understanding of the oceanic responses in the northern SCS to these external influence factors. 相似文献
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Abstract The action of tides on density‐driven circulation, internal gravity waves, and mixing was investigated in the St. Lawrence Estuary between Rimouski and Québec City. Time‐varying fields of water level, currents and density were computed under typical summer conditions using a three‐dimensional hydrostatic coastal ocean model that incorporates a second order turbulence closure submodel. These results are compared with current meter records and other observations. The model and the observations reveal buoyancy effects produced by tidal forcing. The semi‐diurnal tide raises the isopycnals over the sills at the head of the Laurentian Trough and English Bank, producing internal tides radiating seaward. Relatively dense intermediate waters rise from below 75‐m depth to the near surface over the sills, setting up gravity currents on the inner slopes. Internal hydraulic controls develop over the outer sills; during flood, surface flow separation occurs at the entrances of the Saguenay Fjord and the upper estuary west of Ilet Rouge Bank. Early during ebb flow (restratification), the surface layer deepens to encompass the tops of the sills. As the ebb current intensifies, the model predicts the formation of seaward internal jumps over the outer sills, which were confirmed from acoustic reflection observations. As the internal Froude number increases further, flow separation migrates up to sill height. As a result of these transitions, internal bores emanate from the head region one to two hours before low water. We find that the mixing of oceanic and surface waters near the sills is driven by the vertical shear produced during ebb in the channel south of Ilet Rouge, the shear produced in the bottom gravity flood currents, and, to a lesser extent, the processes over the sills. 相似文献
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Abstract During November 1976 to February 1977 near‐surface wind, current and temperature measurements were made at three sites along the Strait of Juan de Fuca. Strong tidal currents and major intrusions of warmer, fresher offshore coastal water were superimposed upon the estuarine circulation of near‐surface seaward flow. The r.m.s. amplitudes of the diurnal and semidiurnal tidal currents were ～30 cms‐1 and 30–47 cm s‐1, respectively. The vector‐mean flow at 4 m‐depth was seaward and decreased in speed from 28 cm s‐1 at 74 km from the entrance to 9 cm s‐1 at 11 km from the entrance. On five occasions intrusions of 1–3 C warmer northeast Pacific coastal water occurred for durations of 1–10 days. The 25 cm s‐1 up‐strait speed of the intrusive lens agreed to within 20% of the gravity current speed computed from Benjamin's (1968) hydraulic model. The near‐surface currents associated with the intrusions and the southerly coastal winds were significantly correlated, indicating that the intrusions were initiated when shoreward Ekman currents advected Pacific coastal water into the Strait. The reversals were not significantly coherent with the along‐strait sea surface slope measured along the north side of the Strait nor were they strongly related to local wind forcing. 相似文献
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The Oregon State University coupled upper ocean-atmosphere GCM is evaluated in terms of the simulated winds, ocean currents and thermocline depth variations. Although the zonal wind velocities in the model are underestimated by a factor of about three and the zonal current velocities are underestimated by a factor of about five, the model is seen to qualitatively simulate the major features of the gyral scale currents, and the phases of the seasonal variation of the principal equatorial currents are in reasonable agreement with observations. The simulated tropical currents are dominated by Ekman transport and the eastern boundary currents do not penetrate far enough equatorward, while the western boundary currents do not penetrate far enough poleward. The subtropical trade wind belt and the mid-latitude westerlies are displaced equatorward of observations; hence, the mid-latitude eastward currents, principally the Kuroshio-North Pacific Drift and the Gulf Stream-North Atlantic Current are displaced equatorward. In spite of these shortcomings the surface current simulation of this two-layer upper ocean model is comparable with that of other ocean GCMs of coarse resolution. The coupled model successfully simulates the deepening of the thermocline westward across Pacific as a consequence of the prevailing Walker circulation. The region of most intense simulated surface forcing is located in the western Pacific due to a southwestward displacement of the northeast trade winds relative to observations; hence the equatorial Pacific is dominated by eastward propagation of thermocline depth variations. The excessively strong Ekman divergence and upwelling in the western Pacific cools the local warm pool, while incorrectly simulated westerlies in the eastern Pacific suppress upwelling and inhibit cooling from below. These features reduce the simulated trans-Pacific sea-surface temperature gradient, weakening the Walker circulation and the anomalies associated with the simulated Southern Oscillation.
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