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.     

冬季北极海冰长期变化对华北降水的可能影响

分析表明,从长期趋势来看,冬季喀拉海、巴伦支海海冰可以影响８月份海河、辽河匠降水;而冬季巴芬湾、戴维斯海峡海冰对７月黄河中上游流和水有有影响,海冰与降水变化均呈相反的变化趋势     

The formation and maintenance of the North Water Polynya

Abstract

This paper investigates the formation and maintenance of the North Water Polynya, Baffin Bay in winter using a multi‐category sea‐ice model coupled with the Princeton ocean model. Monthly climatological atmospheric data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis provides the forcing. An objectively‐analysed climatology provides the initial ocean temperature and salinity. Wind stress drives the ice in a cyclonic gyre around northern Baffin Bay. Localized regions of thin ice form where wind drives ice away from coastlines or fast ice. The regions of thin ice are characterized by enhanced ice growth, exceeding 1.2 m mo^?1. In the regions of thin ice, surface ocean heat flux is also enhanced and is between 30–60 W m^?2. Surface heat flux is, in part, attributable to convective mixing and entrainment driven by ice growth. The surface heat flux reflects advection of the warm West Greenland Current. Heat and salt balances show that horizontal advective exchange counterbalances surface fluxes of heat and salt.     

Diagnostic simulations of the summer circulation in the Canadian arctic archipelago

Abstract

Gridded fields of potential temperature and salinity, interpolated to the time of minimal ice coverage, are constructed for the Canadian Arctic Archipelago based on archived data. In order to overcome the large variations in the horizontal coverage of the observations, the gridding is performed in an iterative procedure where the horizontal correlation scales depend on the data coverage as well as on the flow field. The mean flow corresponding to the temperature and salinity fields are calculated with a diagnostic numerical ocean model. The simulations show that the relative flow through the different straits depends on the elevation difference from the Arctic Ocean to Baffin Bay, and on the density distribution and baroclinic pressure gradients. A 5‐cm increase in the Arctic‐Baffin elevation difference can double the transport. Mean values of the summer flow are a total transport of 0.9 Sv, with 34% flowing through Barrow Strait, 20% through Jones Sound, and 46% through Nares Strait.     

Interannual variability of sea‐ice cover in Hudson bay,Baffin bay and the Labrador sea

Abstract

The spatial and temporal relationships between subarctic Canadian sea‐ice cover and atmospheric forcing are investigated by analysing sea‐ice concentration, sea‐level pressure and surface air temperature data from 1953 to 1988. The sea‐ice anomalies in Hudson Bay, Baffin Bay and the Labrador Sea are found to be related to the North Atlantic Oscillation (NAO) and the Southern Oscillation (SO). Through a spatial Student's i‐test and a Monte Carlo simulation, it is found that sea‐ice cover in both Hudson Bay and the Baffin Bay‐Labrador Sea region responds to a Low/Wet episode of the SO (defined as the period when the SO index becomes negative) mainly in summer. In this case, the sea‐ice cover has a large positive anomaly that starts in summer and continues through to autumn. The ice anomaly is attributed to the negative anomalies in the regional surface air temperature record during the summer and autumn when the Low/Wet episode is developing. During strong winter westerly wind events of the NAO, the Baffin Bay‐Labrador Sea ice cover in winter and spring has a positive anomaly due to the associated negative anomaly in surface air temperature. During the years in which strong westerly NAO and Low/Wet SO events occur simultaneously (as in 1972/73 and 1982/83), the sea ice is found to have large positive anomalies in the study region; in particular, such anomalies occurred for a major portion of one of the two years. A spectral analysis shows that sea‐ice fluctuations in the Baffin Bay‐Labrador Sea region respond to the SO and surface air temperature at about 1.7‐, 5‐ and 10‐year periods. In addition, a noticeable sea‐ice change was found (i.e. more polynyas occurred) around the time of the so‐called “climate jump” during the early 1960s. Data on ice thickness and on ice‐melt dates from Hudson Bay are also used to verify some of the above findings.     

冬春季节北极海冰的年际和年代际变化

利用1953～1990年海冰密集度资料,研究了冬、春季节北极海冰的时空变化特征.结果表明:冬,春季节海冰变率大的海区主要有巴伦支海、格陵兰海、巴芬湾、戴维斯海峡以及白令海;在巴芬湾、戴维斯海峡和白令海海区,冬季海冰变率比春季的大;冬、春季节喀拉海、巴伦支海海冰面积均与春季白令海海冰面积呈反向变化关系,与巴芬湾、戴维斯海峡海冰面积也存在相反的变化趋势.分析还表明:北极海冰面积还表现出年代际时间尺度变化,尤其在冬季.春季格陵兰海海冰明显存在12年变化周期,而在冬、春季节,喀拉海、巴伦支海海冰存在l0年变化周期.     

The baroclinic circulation in Hudson strait

Abstract

The baroclinic circulation in the mouth of Hudson Strait is modelled using general results for nearly geostrophic flow along an indented coastline. A simple T‐junction model is first discussed, followed by a somewhat more faithful idealization that includes the sharp northern tip of Labrador, the southwest tip of Baffin Island and part of Ungava Bay. The results show that the mouth of Hudson Strait does not present a significant obstacle to baroclinic flow in and out of it. We thus conclude that the observed recirculation must be due to other effects.     

Physical properties and isotopic characteristics of landfast sea ice around the North Water (NOW) Polynya region

Abstract

An ice core sampling program was conducted during the North Water (NOW) Polynya Project 1998 Experiment in northern Baffin Bay during April‐May 1998. The physical properties of snow and sea ice as well as the microstructure and stable isotopic composition of first‐year landfast sea ice near the polynya were investigated. The thickness of sea ice at the sampling sites ranged between 147 and 194 cm with thinner snow cover during the period between mid‐April and late May. The ice was characterized as typical first‐year landfast sea ice, being composed of a thin granular ice layer at the top and an underlying columnar ice layer towards the bottom of the ice. The samples obtained at a site closer to the ice edge of the polynya contained a thin granular ice layer originating from frazil ice near the ice bottom. Formation of frazil ice was considered to be caused by turbulent processes induced by winds, waves and currents forced from the polynya and also mixing with water masses produced at the polynya.     

Investigation of summer synoptic climate controls on the mass balance of Meighen Ice Cap

Abstract

The Meighen Ice Cap synoptic climate classification system, developed from the study of six years of summer meteorological and glaciological observations, appears to account for significant variations in the energy‐ and mass‐balance climates of the ice cap. In relating the summer frequency of the three synoptic types to fourteen years of mass‐balance measurements, it was found that variations in surface conditions, solar angle and type of precipitation could be accounted for by the relative sequence of synoptic types. Further it was shown that the types could be represented by the position of the dominant 500‐mb cold Low influencing Meighen Island, thus providing a link between the mass balance and the general circulation.

Dominance of the winter pattern of a 500‐mb Low in the Hudson Bay –Baffin Island region throughout the summer season is capable of maintaining Meighen Ice Cap at its present size. A shift of the 500‐mb Low from the winter position directly to the Beaufort Sea or adjoining Polar Ocean area is capable of increasing the size of the ice cap. On the other hand, a shift of the 500‐mb vortex to the Asiatic side of the Polar Ocean before taking up position in the Beaufort Sea – Polar Ocean area produces negative mass‐balance conditions. When the 500‐mb Low remains on the Asiatic side of the Polar Ocean during most of the summer season the slow accumulation of two decades of Polar Ocean years is destroyed.     

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.     

Simultaneous winter sea‐ice and atmospheric circulation anomaly patterns

Abstract

This study looks at simultaneous changes in atmospheric circulation and extremes in sea‐ice cover during winter. Thirty‐six years of ice‐cover data and 100‐kPa height and 50–100‐kPa thickness data are used. For the entire Arctic, the study found a general weakening of the Aleutian and Icelandic lows for heavy (i.e. severe) compared with light sea‐ice conditions suggesting reduced surface heating as a possible cause. The weakening of the two lows would also reduce meridional atmospheric circulation and poleward heat transport into the Arctic. The study also looks at three regions of high sea ice and atmospheric variability: the Bering Sea, the Davis Strait/Labrador Sea and the Greenland Sea. For the Bering Sea, heavy sea‐ice conditions were accompanied by weakening and westward displacement of the Aleutian Low again suggesting reduced surface heating and the formation of a secondary low in the Gulf of Alaska. This change in circulation is consistent with increased cold air advection over the Bering Sea and changes in storm tracks and meridional heat transport found in other studies. For the Davis Strait/Labrador Sea, heavy ice‐cover winters were accompanied by intensification of the Icelandic Low suggesting atmospheric temperature and wind advection and associated changes in ocean currents as the main cause of heavy ice. For the Greenland Sea no statistically significant difference was found. It is felt that this may be due to the important role that ice export through Fram Strait and ocean currents play in determining ice extent in this region.     

Intermittent ice sheet discharge events in northeastern North America during the last glacial period

The 3D ice sheet model of Marshall and Clarke, which includes both dynamics and thermodynamics, is used to successfully simulate millennial-scale oscillations within an ice sheet under steady external forcing. Such internal oscillations are theorized to be the main cause of quasi-periodic large-scale ice discharges known as Heinrich Events. An analysis of the mechanisms associated with multi-millennial oscillations of the Laurentide Ice Sheet, including the initiation and termination of sliding events, is performed. This analysis involves an examination of the various heat sources and sinks that affect the basal ice temperature, which in turn determines the nature of the ice sheet movement. The ice sheet thickness and surface slope, which affect the pressure-melting point and strain heating, respectively, are found to be critical for the formation and development of fast moving ice streams, which lead to large iceberg calving. Although the main provenance for Heinrich Events is thought to be from Hudson Bay and Hudson Strait, we show that the more northerly regions around Lancaster Strait and Baffin Island may also be important sources for ice discharges during the last glacial period. This paper is dedicated to the memory of Gerard C. Bond.     

On the relationship between spatial patterns of sea‐ice type and the mechanisms which create and maintain the North Water (NOW) polynya

Abstract

Polynyas represent polar oceanic areas with anomalous low sea‐ice concentrations. The North Water (NOW) Polynya refers to a region at the northern end of Baffin Bay which encompasses three separate polynyas. This paper examines the spatial patterns of sea‐ice cover within the NOW region during the winter, spring and fall of 1998 in the context of polynya formation and maintenance mechanisms. To accomplish this a sea‐ice classification scheme for RADARSAT‐1 ScanSAR imagery, obtained between 21 January and 7 December 1998, was developed and implemented within a Geographic Information System (GIS).

The results identify a clear and consistent spatial structure of sea‐ice cover throughout the winter, spring and fall of 1998. Temporally, the polynya opened southward along the Canadian coast and westward away from the Greenland coast. Comparison with parallel oceanographic, atmospheric and ice motion studies suggested that the polynya was primarily controlled by a latent heat mechanism with the exception of the west Greenland coast between Whale Sound and Cape York. The underlying mechanism used to explain the polynya's occurrence along this location is delayed ice formation during freeze‐up and a resultant thinner winter ice cover causing earlier spring ablation than surrounding areas. Arguments for oceanic and/or atmospheric sensible heat contributions are made.     

Winter‐time near‐surface currents in the strait of Juan de Fuca

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.     

Sensitivity experiments for polar low forecasting with the CMC mesoscale finite‐element model

Abstract

High‐resolution versions of the Canadian operational regional finite‐element model (RFE) have been developed to assess their potential in simulating mesoscale, difficult‐to‐forecast and potentially dangerous weather systems commonly referred to as polar lows. The operational (1989) 100‐km version and a 50‐km version of the model have been run for two different polar low cases: one over Hudson Bay and one over Davis Strait. More integrations have also been performed on the Hudson Bay event both at 50 and 25 km to assess the model sensitivity to ice cover. As expected, the reduction in spatial truncation errors provided by the increase in resolution results in a better simulation of the systems. Moreover, when run at higher resolutions the model shows a significant sensitivity to ice cover. The results of the ice‐cover experiments also put into perspective the interaction between the heat and moisture fluxes at the surface, the low‐level wind structure, and the relation of these to the development of the polar low. This study suggests that the improved forecast accuracy obtained from increased resolution is limited by the correctness of the analysis of the ice cover, which acts as a stationary forcing for the entire forecast period.     

Coupled ice‐ocean variability in the Greenland Sea

Abstract

A major surface feature of the Greenland Sea during winter is the frequent eastward extension of sea ice south of 75°N and an associated embayment to the north. These features are nominally connected with the East Greenland Current, and both the promontory and the embayment are readily apparent on climatic ice charts. However, there are significant changes in these features on time‐scales as short as a few days. Using a combination of satellite microwave images (SSM/I) of ice cover, meteorological data and in situ velocity, temperature and salinity records, we relate the ice distribution and its changes to the developing structure and circulation of the upper ocean during winter 1988–1989. Our measurements illustrate the preconditioning that leads to convective overturn, which in turn brings warmer water to the surface and results in the rapid disappearance of ice. In particular, the surface was cooled to the freezing point by early December and the salinity then increased through ice formation (about 0.016 m d^‐1) and brine rejection. Once the vertical density gradient was sufficiently eroded, a period of high heat flux (>300 W m^‐2) in late January provided enough buoyancy loss to convectively mix the upper water column to at least 200 m. We estimate vertical velocities at about 3 cm s^‐1 downward during the initial sinking. The deepening of the thermocline raised surface temperatures by over 1°C resulting in nearly 1.5 × 10⁵ km² of ice‐melt within two days. Average rates of ice retreat are about 11 km d^‐1 southwestward, generally consistent with a wind‐driven flow. Comparison of hydrographic surveys from before and after the overturning indicate the fresh water was advected out of the area, possibly to the south and east of our moorings.     

A source of hail embryos

Abstract

The sensitivity of the annual cycle of ice cover in Baffin Bay to short‐wave radiation is investigated. The Princeton Ocean Model (POM) is used and is coupled with a multi‐category, dynamic‐thermodynamic sea‐ice model in which the surface energy balance governs the growth rates of ice of varying thickness. During spring and summer the short‐wave radiation flux dominates other surface heat fluxes and thus has the greatest effect on the ice melt. The sensitivity of model results to short‐wave radiation is tested using several, commonly used, shortwave parameterizations under climatological, as well as short‐term, atmospheric forcing. The focus of this paper is short‐term and annual variability. It is shown that simulated ice cover is sensitive to the short‐wave radiation formulation during the melting phase. For the Baffin Bay simulation, the differences in the resulting ice area and volume, integrated from May to November, can be as large as 45% and 70%, respectively. The parameterization of the effect of cloud cover on the short‐wave radiation can result in the sea‐ice area and volume changes reaching 20% and 30%, respectively. The variation of the cloud amount represents cloud data error, and has a relatively small effect (less then ±4%) on the simulated ice conditions. This is due to the fact that the effect of cloud cover on the short‐wave radiation flux is largely compensated for by its effect on the net near‐surface long‐wave radiation flux.     

The circulation and hydrography of conception bay,Newfoundland

Abstract

The hydrography and circulation of Conception Bay (Newfoundland) are described based on hydrographic, current‐meter and drifter data collected over four years (1988–1991). The seasonal cycles of temperature (‐1.6 to 13–17°C) and salinity (31–32.5) in the bay closely follow those on the adjacent shelf. Exchange of bottom water was observed in April 1989. Deepwater exchange was observed from late fall to early winter of 1989–90. Tidal currents are weak, 1–2 cm s^‐1 for the M₂ and K₁ constituents. Observed Eulerian mean currents (<3 cm s^‐1) are smaller than the standard deviation (1–11 cm s^‐1); however, there is a persistent outflowing current of 10 to 20 cm s^‐1 within 2 km of the shoreline on the eastern side of the outer bay. The Lagrangian correlation length scale is from 4 to 10 km, in agreement with the weak coherence squared (≤0.4) found between the fixed current‐meter sites separated by greater than 4–5 km. The variable currents (up to 20 cm s^‐1) tend to be cyclonic. Cyclonic eddies were observed near the mouth on the eastern side of the bay, adjacent to the outflow. A simplified fractal dispersion model gives residence times of 42 d similar to those obtained from a scaling analysis (30–40 d) and a diagnostic numerical model (30 d).     

Comparison of Canadian daily ice charts with surface observations off Newfoundland,winter 1992

Abstract

Forecast ice drift rates and thicknesses displayed on daily ice charts and forecast winds for the Canadian east coast are compared to on‐ice observations made during the second Canadian Atlantic Storm Program (CASP II) of March 1992. Observed and 24‐hour forecasts of daily ice drift rates were weakly correlated even though long‐term means closely matched observations. Daily drift rates have an RMS error of 13 cm s^‐1 relative to a 15 cm s^‐1 mean in addition to an RMS direction error of 50 degrees. Contributions towards daily drift uncertainties were: the estimation of winds, unmodelled physics of ocean and ice cover processes; and the inconsistency in the methods used by the ice forecaster. Correlation coefficients between forecast winds and on‐ice observed winds decreased from 0.8 at 0‐hour to 0.7 for the 30‐hour forecast. Similar results were found between ice drift rates from forecast winds. Histograms of ice thicknesses observed along narrow swaths using a helicopter‐towed electromagnetic sensor compared well with undeformed ice thicknesses representing large areas on ice charts, with differences mainly caused by difference in ice type representation and by co‐registering the two data sets.     

The transport of water,heat and salt through the strait of Belle Isle

Abstract

We present an analysis of current‐meter, sea‐level and hydrographic data collected in the Strait of Belle Isle and the northeastern Gulf of St Lawrence. From an array of moorings in the Strait from July to October 1980, we calculate a net transport into the Gulf of 0.13 × 10⁶ m³ s^?1 and show that the mean and eddy fluxes of heat through the Strait represented a net loss of heat to the northeastern Gulf. The estimated rate of loss of heat is less than the long‐term mean computed by Bugden (1981) but becomes comparable if adjusted for interannual changes of transport and water temperature. Moreover, the 1980 data permit the permanent tide‐gauge stations in the Strait at West Ste Modeste and Savage Cove to be levelled relative to one another, thus allowing surface currents to be calculated from sea‐level alone. Hence the long‐term wintertime transport into the Gulf can be calculated after fractional effects on the vertical structure of the flow are considered. During an average winter it appears that advection through the Strait can account for about 35% of the Gulf Intermediate Layer. A multiple regression involving average Intermediate Layer temperatures over 9 years suggests that winter air temperature in the Gulf, representative of atmospheric cooling, and sea‐level difference across the Strait, representative of advection, are equally important variables and together account for 50% of the Layer's temperature variability. Analysis of current‐meter, sea‐level and hydrographic data collected in 1975 supports earlier hypotheses that the strongest inflow of water with ? < 0° C and salinity between 32 and 3 3 should occur in winter. It appears that during the 1975 field program the inflow was about 0.6 × 10⁶ m³ s^?1, which is about twice the long‐term average for January to May.