On the hydrography of the Northeast Water ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH,

VOL. 100, NO. C3, PAGES 4287-4299, MARCH 15, 1995

On the hydrography of the Northeast Water Polynya Gereon

Bud6us

Alfred-Wegener-Institut ffir Polar- und Meeresforschung, Bremerhaven, Germany

Wolfgang Schneider Max-Planck-Institut f/lr Meteorologie, Hamburg, Germany Abstract.

The water

masses and circulation

in the area of the Northeast

Water

Polynya, located on the East Greenland Shelf north of 79øN, are described on the basis

of an R/V Polarstern cruise during spring and summer 1993. The baroclinic flow shows northward componentsclose to the East Greenland coast and eastward componentsat the northern limit of the polynya. An anticyclonic half circle is formed by this and the southward flowing East Greenland Current. In the south the circle is not closed. The upper water column, occupied by Polar Water, is affected by this circulation pattern, while deeper waters in the trough system of the area seem to spread independently. Different types of deep waters are found in different troughs, all being of Atlantic origin, though they seem not to be directly connected to Return Atlantic Water. It is shown that what is called Polar Water must be formed, at least partly, on the Greenland Shelf and that deepwater formation does not occur in the investigatedarea. Introduction

Although about half of Fram Strait's width is covered by the East Greenland Shelf, a considerably greater amount of work has been devoted to the investigation of its deeper parts. This is, of course, partly due to the major role Fram Strait plays in the exchangeof deepwater massesbetween the Polar Arctic and the Greenland Sea (GS), but other reasons might also contribute. The waters on the shelf have only a small volume compared with those of the deeper parts, their influence on the conditions in the central Greenland Sea across the East Greenland Polar Front (EGPF) is thought to be small [Foldvik et al., 1988], and, finally, the area is not easily accessible.For the shelf regions of the GS, Kiilerich [1945] has been a main source of information on circulation patterns until the mid-1980s. The Arlis (drifting ice floe) and R/V Edisto data [Aagaard and Coachman, 1968] still represent the bulk of our knowledge about the seasonal changes of water properties in the area. With the advent of the Marginal Ice Zone Experiment studies, conducted throughout the summer and winter seasons during the 1980s [Johnson et al., 1985; Johannessen, 1987; Muench et al., 1991], the EGPF has been determined to extend only to about 150 m depth, when it is defined as the transition between Polar Water (PW) and warmer waters of Atlantic origin. Underneath the PW, warmer waters are observed over the entire shelf [Bourke et al., 1987;Bud•us et al., 1993]. The term PW is commonly used to summarize all waters with salinity (S) < 34.4 and temperature (T) < 0øC. This water occupies the upper water column between East Greenland and the EGPF, and most authors agree that it issuesmainly from the Arctic Ocean [Bourke et al., 1987]. A subdivisionof PW is suggestedby Paquette et al. [ 1985] and

Bourke et al. [1987], with the introduction of the so-called knee water (KW), named after its properties in the TS space. Interest in the processesoccurring on the East Greenland shelf has increased, largely due to the climatic relevance of polynyas, one of which appearsregularly on the East Greenland Shelf at about 80øN, the Northeast Water (NEW). The major task of this paper is to give a detailed description of the hydrographic conditions of NEW's subsurface layers. This includes refinements of water mass differentiations, as well as considerationsof their respective origins and spreadings. The generation process of the NEW and ice coverage development are not considered in this paper, as they do not appear to be affected by the water mass distribution of the waters present below the surface layer [see Schneider and Bud•us, this issue].

Material

and Methods

The results presentedin this paper originate from the ARK IX cruise of R/V Polarstern

to the NEW

area in 1993. All

Copyright 1995 by the American Geophysical Union.

data were sampled during legs 2 and 3 of the cruise (May 16 to June 24 and June 25 to August 4, respectively). Conductivity-temperature-depth (CTD) profiler casts were conducted with a Sea-Bird 911+, equipped with a standard sensorset which includes a pumped temperature-conductivity duct for optimized spike reduction by defined time alignment between temperature and conductivity measurements. The CTD was calibrated in the laboratory before the cruise. Additional checks were performed at a calibration station in the central Greenland Sea (75øN, 3øW), at about 3000 m depth on May 23, June 22 and 27, and August 2, as the area under considerationis too variable in its properties for any attempt to calibrate the CTD sensorsby comparative temperature measurements and water sampling. Evaluation of the calibration casts yielded an accuracy of better than 5 mK for temperature and 0.006 for salinity during the entire

Paper number 94JC02024.

duration

0148-0227/95/94 JC-02024 $05.00

temperature scale to be consistent with the algorithms for 4287

of the cruise.

We note that Sea-Bird

uses the 1968

BUD]•USAND SCHNEIDER:HYDROGRAPHY OF THE NORTHEASTWATERPOLYNYA

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Figure1. Positions of conductivity-temperature-depth (CTD) stations performed duringR/V Polarstern cruise(a) ARK IX, leg2 and(b) ARK IX, leg3. For orientation purposes thetroughsystemhasbeen

indicated bythe300-mand500-misobaths (digitized byAlfred-Wegener-Institut Bathymetric Groupfrom

Perryet al. [1986]).Fasticeat Norske0 (NO) hasbeenhatched,andthepositionof thecoastaround81øN

hasbeencorrected basedonmodern navigation data.In Figurelb, transects referredto in Figures3 and 4barelabeled A toF. Also,somestations areindicated bytheirthree-digit identification numbers. Squares denote stationsused for Figure 9.

computingdensity (International Equation of State of Sea-

stalled behind an acoustic window made from 81-mm thick

water 1980[UNESCO, 1983])andsalinity(PracticalSalinity low density polyethylene (LDP) to allow measurementsin Scale1978[UNESCO, 1983].No conversion hasbeenap- ice-coveredareas.Only bottomdepthdata are usedquantipliedhereto the 1990temperature scale.Salinities aregiven tatively in this paper. accordingto the Practical Salinity Scale 1978without units [UNESCO, 1986].

The datahavebeenprocessed by standard methods using Sea-Birdsoftwareand are averagedover 1 dbar in the final

Observations

Bathymetry

dataset.A totalof 257CTD stationsweresampled during The bathymetryof the NEW areais of primeimportance bothlegsof ARK IX. Theirlocationsaredepictedin Figures to the spreadingof the deeperwater masses.It is characterla and lb. We note that the positionsof coastalboundaries ized by a trough systemsurroundingBelgicaBank which

in available digitaldatasetsaresomewhat erroneous, espe- includesBelgica Trough, Norske Trough, and Westwind cially in the northernpart of the NEW area, and a better Trough. Considerableenhancementof the bathymetry's approximationof the coastlineposition, stemmingfrom knowledgewas establishedby Bourke et al. [1987]; see shipborne radarobservations, thanwasgivenby the World Figure2. This figureis derivedfrom the mapof Perry et al. Data Bank II is indicatedin Figure l a. The fast ice area at 79øN has also been marked.

[1980], updated by measurementsmade at CTD stations during two cruises in 1979 and 1984. We add information on

Bottomdepthdata shownin this paperare from acoustic bathymetryby a number of sectionsacrossthe troughs Dopplercurrentprofiler(ADCP) measurements workingin (Figure 3). From the CTD stations, which are marked in bottomtrackmode.Thesemeasurements wereobtained by Figure3, it is evidentthata moredetailedbathymetrycannot a vessel-mounted RD Instruments 150-kHz instrument in-

be based on our CTD casts.

BUDI•US AND SCHNEIDER: HYDROGRAPHY OF THE NORTHEAST WATER POLYNYA

81

4289

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Figure2. Bathymetry andtopographic features of theNortheast Water(NEW)area[fromBourkeet al., 1987].Henrik-Kr0yerIslandsare abbreviatedHKI. In an area coveredby fast ice, depthcontoursare dashed.

400-500 m. Its lengthis approximately250 km and width is

Troughseemsto be of more uniformdepthof about300 m. The lengthandwidthof WestwindTroughare about110km and 20 km, respectively.In the entire NEW area, isolated

about 40 km. Parallel to the East Greenland Coast, the

underwater features seemto exist (see, for example, transect

The southernpart of the troughsystem,BelgicaTrough,is located between 77øN and 78øN and shows depths of about

trough systemis continuedby Norske Trough that extends F, Figure 3), but knowledgeabout this is incompleteat from 78øN to 80ø30'N with varying depth and shape. Its pre,sent. The shallowest areas of the NEW include western Belgica bathymetryis partly uncertainbecauseof persistentfast ice Bank between 78øN and 80øN at about 15øW. Depths of coveragearound79øN. North of this location,the NEW is

situated,thuscoveringonly part of BelgicaBank and the

slightly lessthan40 m havebeenmeasured duringARK IX

B andD). However,it i• associatedtrough system.The accessiblenorthernpart of inthisarea(seeFigure3, transects thatminimumdepthsarenotdetected by theARK NorskeTroughis somewhatshallowerthan BelgicaTrough. probable At some locations, cross sectionsof the trough show maxi- IX cruise of R/V Polarstern, since the ship's ability to mum depths approaching300 m; at other sites, depths approachshallowareasis limited. In other parts, Belgica exceed400 m. The deepestpart of our transectC (see Figure Bank showswater depths of 50 to 100 m with a slope to 3) correspondsto the Norske Trough depth maximumof greaterdepthseastof 15øW.A secondshallowpart of the morethan 400 m in Figure 2 (about80ø06'N, 15ø40'W).Deep NEW is Ob Bank, definingthe northern boundary of the waters of this area will be of particular interest. Westwind NEW, also partly showingdepthsof less than 50 m.

4290

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Figure3. Depth soundings on transects(labeledA-F, seeFigure1) acrossthe NEW troughsystem. CTD stations areindicated by shortverticallines.Notethatat transectF anisolateddeepholehasbeen met, while on transectC the deepestpartsof the troughhavebeenmissedduringARK IX.

BUDI•US AND SCHNEIDER: HYDROGRAPHY OF THE NORTHEAST WATER POLYNYA

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Figure 4. (a) Geopotentialanomaliesin the NEW area derived from ARK IX, leg 3 CTD stationsrelative to 200 dbar in joules per kilogram. The dashedline indicatesthe 500-m isobath at the shelf break. Stations

referred to in Figure 4b are marked by squares.(b) Geostrophicspeedsof selectedstationpairs in the deepestparts of the trough systemrelative to their deepestcommon pressurelevel for transects B, C, D, and F. The calculated flow is perpendicular to the direction of the transects; for transects B, C, and D, positive sign is defined as flowing to the northeast, for transect F, to the east.

Circulation

The general circulation in the NEW area is basically inferred from geostrophic calculations based on the CTD measurementsduring ARK IX. Geopotential anomalies derived from leg 3 CTD data are shownin Figure 4a, relative to a pressurelevel of 200 dbar. This reference pressurerepresents a compromisebetween areal coverage and deepest possible reference level. No attempt has been made to includestationsof smallerdepths.Figure 4a is presentedto illustrate the pattern of the general circulation. In order to estimatethe highestoccurringvelocities,geostrophicspeeds are calculatedseparatelyfor selectedstationpairs locatedin the troughsystemrelative to their deepestcommonpressure level (Figure 4b). In the center of the NEW area a geopotentialanomaly maximum is observed, located at about 79ø30'N and 11øW. West of the maximum, northward currents are indicated, flowing parallel to the Greenland coast. In the northern NEW area, eastward motion connects the northward coastal current and the southward flowing East Greenland Current

(EGC). This pattern suggeststhat part of an anticyclonic gyre is formed which follows the topographicstructureof the northern trough system. At the eastern boundary of the NEW, high horizontal gradients are associated with the

baroclinic jet of the EGC at the EGPF. The isolines of geopotential anomaly do not run along the shelf edge but cross the shelf diagonally from the northeast to the southwest.

We

note

that

north-south

distances

between

zonal

transects are large, and details of the current pattern between 79øN and 77øN might be missed. The surface dynamic topography from 1984 data, presented by Bourke et al. [1987], indicates in this area an excursion of the southwest-

ward flow toward the shelf edge and back. The anticyclonic half circle in the northern NEW and the diagonal crossingof the shelf are common to both data sets of 1984 and 1993. The

situation at the southern limit of Belgica Bank is less clear, in part due to a scarcity of stations in that area. Our sole transect along Belgica Trough shows southwestward current components almost over its entire extent. Northward velocities are confined to a small area close to the coast between

about 15øWand •le de France. The boundarybetween southward and northward flow is found in 1993 at exactly the same position as it was observed by .Bourke et al. [1987] in 1984. Other indications, like water mass spreading, will be used to further complete the picture of the general circulation in the NEW

area.

By the calculations relative to deeper pressure levels,

speedsof above10 cm s-• are revealedfor the northward

4292

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Figure 5. (a) Thickness(in meters)of the PolarWater layerin the NEW for ARK IX, leg 3 (indicatedby the depthof the salinity(S) = 33.0isohaline).Maximumthicknessis observedat the centerof the anticyclonichalf circle. The dashedline indicatesthe 500-m isobathat the shelfbreak. (b) Developmentof the meltwater layer in the central NEW for a stationon transectE (80ø39'N, 13ø23'W)from June 6, 1993(ARK IX, leg 2), to July 11, 1993(ARK IX, leg 3). The PW layer between50 and 100m remainscloseto the freezing point. coastal flow at transects B to D, as well as for the eastward

flow at transect F (Figure 4b). It can also be seenfrom Figure 4b that the vertical shear of geostrophic speedsis concentrated between 100 and 150 m, i.e., in the main pycnocline that separatesPW from the underlying waters. This indicates mechanicaldecouplingbetween these two layers and illustrates that a general circulation pattern cannot be inferred from stations shallower than about 150 m. It is suggested

layered [Aagaard and Coachman, 1968]. The upper layer is occupied by PW, generally specified by S < 34.4 and T < 0øC at its deeper boundary. In this paper we specify the salinity limit of PW to 33.0 for reasons given below. The observedthicknessof the PW layer is largest in the center of the NEW (about 140 m; see Figure 5a), diminishing toward its coastal boundaries, toward Ob Bank, and toward the EGPF.

This distribution

of PW thickness can be attributed to

from Figure 4b that the deeper layers of the troughsshow geostrophicadjustmentand is therefore not a direct measure little movementor that possiblemovementsare barotropic. of the strength of erosional effects from below. In June, Calculations for transect A give results similar to those at temperaturesare fairly uniform throughoutthis layer, being transect F. Preliminary evaluations of ADCP measurements close to the freezing point, while salinities increase slightly over 1 to 3 hours on some stationsin Belgica Trough indicate with depth. Later in summer, the surface of the PW layer maximumspeedsof up to 10cm s-1 for thisdeeperlayer, becomes warmer and fresher because it is influenced by sea with varying directions and small vertical shears. Whether they are attributed to tidal movements or to mean flow ice melting and land runoff of snow thaw waters (see Figure 5b). Temperature is modified mainly by heat gain from solar cannot be decided at present. radiation

Water

Masses

Simplifiedto someextent, the verticalstructureof the water mass compositionon the East Greenland Shelf is two

which is the main constituent

of the surface heat

balance in summer [Schneider and Bud•us, this issue]. Radiative heat input is particularly effective in areas of low ice concentration. All possible combinations of freshening

BUD15.USAND SCHNEIDER: HYDROGRAPHY 31

32

33

34

35

OF THE NORTHEAST WATER POLYNYA 31

32

33

34

4293 35

RAW ; '

, / ,

o

1 ,

,

b)

a) 31

32

34

33

35

SAL

33

SAL

Figure 6. (a) TS diagramfor all stationsof ARK IX, leg 3, except the stationsnorth of Ob Bank. (b) Same as Figure 6a, but for the stations north of Ob Bank.

and warming of the surface layer occur in the NEW, as can be seen from Figure 6a. As the modifications of the surface layer are interrelated with the establishment and sustainment of the NEW, we do not treat them here but discuss them elsewhere [Schneider and Budgus, this issue]. Below the

surfacelayera coreof watersat or nearthefreezingpointTf persistsin somewhatgreater depths, rangingfrom 50 to more than 80 m, throughout the summer (Figure 5b). The coldest temperatures in the PW layer are associatedwith salinities of about S = 32.4. North of Ob Bank, outside the NEW, minimum temperatures are associatedwith different salinities and the deviations from freezing temperatures show a maximum at salinities of about 32.4 (Figure 6b). Below the deeper boundary of the PW layer and the main pycnocline, waters with gradually increasing temperatures and salinities are observed in the NEW. The warmest deeper waters

in the area are those of the Return

Atlantic

Current

(RAC) with temperatures of up to 3øC and salinities greater than 34.9. These waters are observed just off the East Greenland

Shelf

but

not

in the

two

east-west

oriented

troughs. Here waters with temperatures of 0.5øC to 1.0øC and salinities of less than 34.9 are found (the transition between RAW and waters in the troughs is described later). The connection between the warm deeper waters and the PW in the TS space varies at different locations. The extreme cases are represented, on one hand, by an almost straight line between them, indicating direct mixing without other involved water masses, and, on the other hand, by a

connection withanangleto aboutS = 34.0andT = Tf. The latter case suggeststhe introduction of a third water body, named Knee Water (KW) by Bourke et al. [1987], and thus a subdivision of the rather general term of PW. In order to distinguishKW from PW, we restrict the use of the term PW to waters with S < 33.0. KW is found always below the Polar Water layer that contains the upper temperature minimum.

KW and PW are separated by an intermediate temperature increase which occasionally is as small as 0.1øC, while salinity increasescontinuously with depth. This layering can be recognized from Figure 6a. Consequently, KW is located in depths below those shown in Figure 5a. KW can gradually be mixed into the waters that result in the straight mixing line between return Atlantic water (RAW) and PW, causing a more or less curved shape of a station's individual TS diagram. However, there seems to exist a distinct limit of the KW's occurrence, indicated by the gap in the TS scatter plot at about S - 33.9 and T = - 1.2øC.In fact, high concentrations of KW are only found in certain parts of the NEW area. The varying maximum concentration of KW present at each station of ARK IX, leg 3 is shown in Figure 7. The values are calculated according to Mamayev [1975] from three reference points in the TS space (see Table 1). Highest percentages of KW content are observed to the northeast of the NEW, off the shelf edge, and in the eastern part of Belgica Trough, up to 14øW. The isoline of 0.5 KW content crossesthe shelf diagonally, from the shelf edge at 80øN and 6øW toward a point near the coast

atthelatitudeof •le deFrance.In theNEW, percentages are generally small, with highest values at the stations in the trough system. The highest values in Norske Trough are significantly smaller (below 0.4) than those found at the head of Belgica Trough (above 0.5). In the trough system, no horizontal gradient could be established that would suggesta direction of spreadingalong the axis of either Norske Trough or Westwind Trough. The lower PW layer and the KW are part of the main pycnocline in the NEW. Thus in the trough system the deeper waters below the pycnocline occupy depths below 150 to 200 m. RAW (S > 34.9, T > 0øC; see Hopkins [1991]) is found in front of the seaward entrance of Belgica Trough in depths of about 250 m. Three stations at the shelf edge

4294

BUDI•US AND SCHNEIDER: HYDROGRAPHY OF THE NORTHEAST WATER POLYNYA

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82 J

I

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'. . . .. '•0'75

Trough (maximum Cr t about27.96kgm-3) andaresomewhat

0 82

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8o "'L "0•25 "• -8O

lower in Norske Trough that connects the other two troughs. In contrast to Belgica Trough, both Norske Trough and Westwind Trough do not exhibit a consistent intermediate temperature maximum. Only occasionally is a bottom temperature minimum observedin northern Norske Trough, but these occasionsappear as isolated spots. Two examples of such stations are included in Figure 8c. Discussion

To elaborate further on indications of the general circulation pattern of the upper water layer in the NEW area, it is useful

79

to discuss the distribution

of KW.

We

exclude

its

formation by double diffusion between RAW and PW, as did Bourke et al. [1987] and Paquette et al. [1985], who discuss

0.75

this mechanism

but favor advection

of KW

from the Arctic

Basin. Since the vertical gradients of temperature and salinity in the consideredarea would imply double diffusionin the / -78 78'-I diffusive sense, homogeneoussublayers would be a precondition to establish vertical exchange rates above molecular values [Huppert, 1971]. Such sublayers have not been obI served. In addition, the KW temperatures are occasionally / colder than those of the overlying PW (see Figure 6a), which -77 77/ / is possible because of the higher salinities of KW. Thus / contact to atmosphere or ice must be assumed to cool these '. waters to their observed temperatures. Moore and Wallace / [1988] showed that Atlantic Water (AW) indeed can attain 76 76 i i i the properties of KW when melting sea ice in the Arctic. -10 -20 -15 -5 Evidently, the formation of KW does not take place in the Longitude NEW area, since here KW is separatedfrom the surface by Figure 7. Maximum content of "Knee Water" in the water the PW layer. If KW would be formed on the East Greenland column (for specifications, see text) of each station of ARK Shelf, then it shouldbe found in high percentagesthroughout IX, leg 3. The dashed line indicates the 500-m isobath at the the NEW, which is not the case. Its origin from the Arctic shelf break. Basin is supported by the observations during Polarstern cruise ARK IV, leg 3 [see Koltermann and Lfithie, 1989], which show that this type of water is ubiquitous in the area (stations 109, 110, and 111) show the presence of this water around 85øN and 20øE, where it is the lightest water, and, mass. West of these stations, there is an abrupt change in consequently, is found in the ocean surface layer. When KW water properties at this depth level. Maximum temperatures is approachingthe NEW area from the northeast, its higher drop from about 2øC at station 111 to about IøC at the density (in comparison with PW) forces it below the PW to adjacent station 112 which are 22 km apart. At the latter the observed depths of 100 to 150 m. However, KW is still station, maximum salinitiesare slightly lessthan 34.9. From lighter than the AW recirculating directly in Fram Strait and the entrance of Belgica Trough to its head, a temperature also than the warmer waters in the trough system of the maximum layer is found that shows little variation in its NEW area. Thus PW and modified warm AW have no water properties over the distance of 250 km (8øW to 17øW). common interface at places where high percentagesof KW Temperatures in this layer remain at about IøC, salinities at are found. This is the case for the eastern part of Belgica about 34.86. The thickness of the temperature maximum layer is roughly 80 m. It is found at a depth of about 300 m. Figures 8a and 8b show TS characteristics of selected Table 1. Definition of Tie Points for the Calculation of stations of a transect along Belgica Trough, illustrating the Maximum Knee Water Percentage range of TS variations in the deeper waters (Figure 8b) and Temperature, the differences to RAW (Figure 8a). Water Type øC Salinity In the accessible northern part of Norske Trough, maxiPW - 1.80 32.40 mum temperatures have decreased to about 0.7øC (Figure KW -1.83 34.15 8c), and another decrease can be recognized toward AIW 0.5 34.80 Westwind Trough, where maximum temperatures are about 0.4øC (Figure 8c). Bottom salinities show the same trend See Figure 7 for Knee Water content diagram. Abbreviations are (decrease)from Belgica to Norske Trough but increaseagain PW, Polar Water; KW, Knee Water; and AIW, Arctic Intermediate /

/

/

toward WestwindTrough. Here the TS characteristics differ from those in Norske Trough, with several stationsshowing very homogeneoustemperatures in the deeper layers. Bottom densities are similar in Westwind Trough and Belgica

Water. The values for AIW are specifiedaccordingto its appearance at the shelf edge and differ from those of the Return Atlantic Water. The tie points are chosento form a triangle which includesalmost all measured TS points and are not centered at the measured mean values for the water masses.

BUDI•US AND SCHNEIDER: HYDROGRAPHY OF THE NORTHEAST WATER POLYNYA

34.5

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4295 35.0 3.0 _

_

_

_

b

_

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-2.5

-2.5

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_

-2.0

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-

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-

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/

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0.0 34.5

34.6

34.7

34.8

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34.6

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34.5

34.6

34.7

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0.0

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34.8

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34.8

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34.8

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34.5

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I I I I I I I I I. I I • • • • I I I I [ I

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34.7

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Figure 8. TS relations of NEW bottom waters. Dots are plotted in 5-dbar intervals. (a) Stations 109 to 111 off the mouth of Belgica Trough containing Return Atlantic Water. (b) Stations 112 and 113 at the mouth and three stationsfrom transectA at the western limit of Belgica Trough. (c) Seven stationsat the transition between Norske Trough (NT) and Westwind Trough (WT). For comparison a station from Belgica Trough (BT) is repeated from Figure 8b. From Norske Trough, two stations are included which give an example of a cold bottom spot (see Discussionsectionin text). (d) Three stationsfrom Westwind Trough in comparisonto two stationsnorth of Ob Bank (n of OB). Note that the temperature range in Figures 8c and 8d is different from 8a and 8b.

Trough. After subductingunder the PW, the spreadingof KW can be restricted by shoals.In particular, it cannot pass Belgica Bank and could enter Norske Trough only through the other parts of the trough system. The lack of KW in the central parts of the NEW trough system therefore contradicts the concept of a closed, anticyclonic gyre for the upper 200 m of the NEW area that

do not enter the NEW. Thus the flow pattern seems to be asymmetrical with a half-gyrelike structure in the northern NEW but with no equivalent at Belgica Trough. The conclusion that the waters flowing southward diagonally over the shelf are not or are only weakly connected to the northward coastal current confirms the suggestionof Kiilerich [1945], who proposed a northward flowing coastal extendsin the southto [le de France[Bourkeet al., 1987]. current along East Greenland that begins at about Koldewey The waters with high percentagesof KW (observed values 0 at 76øN. As an implicationof this circulationscheme,the exceed 0.5 as far west as 15øWin Belgica Trough) apparently PW in the NEW cannot be imported entirely from the Arctic

4296

BUDI•US AND SCHNEIDER: HYDROGRAPHY OF THE NORTHEAST WATER POLYNYA

but, rather, must be formed on the Greenland Shelf itself. The difference in deviation of the temperature minimum from the freezing point between the NEW area and the region north of Ob Bank further supportsthe idea that the waters of these two regions undergo different processesand that exchange across Ob Bank is presumably small. If the PW present in the NEW area can be regarded as a local

These considerationscan be used to infer the trough depth

below the fast ice betweenBelgicaTrough and Norske 0. Historical shipbornedepth soundingsexist for considerable

areas which have been covered by fast ice during the past few years, since the size of the persistentfast ice feature at 79øN undergoes substantial variations [see Schneider and Bud•us, this issue]. These soundingsare enlisted in older sea product and not as imported water, its salinitieswould be charts[ServiceHydrographiquede la Marine, 1953], and for determined by a long-term balance of freshwater input in the covered area, no obstacle to the northward coastal flow summer, from both land runoff and ice melting, salt input by in the trough is shown. A region of uncertain bathymetry brine release in winter, and mixing with higher-salinity remains, however, under the core area of the persistent fast seawater. The salinity of the PW would not necessarily be ice feature at 79øN that we will call Norske 0 Ice Barrier constant over many years, and a change of about AS = 1.0, (NOIB). Hydrographic evidence is given that the trough like that observed by Bourke et al. [1987] between 1979 and continuesbelow NOIB with a shallowest depth along the 1984, could be explained. This formation process would troughaxis of about 250 m. A CTD sectionalongthe troughs question the use of the term Polar Water. Greenland Shelf (Figure 9) showsthe samewater structure in Belgica Trough Water would possiblybe a more adequatecharacterization. and Norske Trough above this depth level, while waters in Indeed, Killerich [ 1945]used the term Polar Water for waters Westwind Trough are clearly different. At 250 m depth we with salinities of about 34.0 that are called KW in the recent observe approximately 0.6øC in Norske Trough, while in literature and also in this paper. Westwind Trough, temperaturesdo not exceed 0.4øC. Thus

The lowesttemperatures andhighestsalinities in the PW

layer must then be establishedlocally by surfaceconvection during the times of winter cooling and ice formation combined with brine release. To result in efficient convection,

the salt input from brine release first has to compensatefor the summertime surface freshwater input and is able to penetrate to greater depthsonly subsequently.In principle, it is possible to form dense bottom waters by this process [Melling and Lewis, 1982], but the strong vertical density gradients make the NEW not a well-suited place for the generation of dense bottom waters. Indeed, no evidence for bottom water formation is found during ARK IX. Dense and cool bottom waters could not completely escape from the NEW area owing to the presence of isolated deep holes and could clearly be recognized in the relatively warm deep waters in the trough system. An interesting observation in this context is the possibility of tracing the warm waters from Belgica Trough to the northern NEW without any indication of winter erosion. Furthermore, the clear separation of PW and KW by warmer waters seemsto indicate that winter salt input does not even sufficein producingsalinities of about S = 34.0. Using a July profile to calculate the thickness of ice formation necessary to attain this salinity reveals, in fact, the unrealistic ice thickness of more than 4 m. We therefore exclude the NEW area as a place of bottom water formation, particularly with respectto the neighboring

with the absence of other heat sources, waters of Norske Trough have to be supplied from Belgica Trough south of

NOIB where higher temperaturesare found. Below about 250 m, salinitiesand temperaturesnorth of NOIB are reducedin comparisonwith BelgicaTrough, hereby indicating the shallowest trough depth below the fast ice. If using only the temperature distribution of the bottom waters as the indicator, an anticyclonic spreading of the deeper waters in the troughs would be suggested. The gradualdecreasein temperaturefrom slightlyabove 1.0øCin Belgica Trough, through about 0.7øC in Norske Trough, to only 0.4øCin Westwind Trough would be due to increasingly higher ridges eventually combined with cooling. While the

reducedbottomdensities of •r = 27.85k gm-3 in northern

Norske Trough are consistentwith the idea of a sill with bottom depths of about 250 m between Belgica Trough and the NEW Polynya, our transect along the connection between Norske Trough and Westwind Trough showed no sill between them that could explain a further temperature decrease.Also, the bottom salinity increasein that direction is in contrast to an anticyclonic bottom water circulation in the northern part of the trough system. We therefore conclude that waters below the main pycnocline spreadfrom Belgica Trough into Norske Trough, but different waters occupy Westwind Trough. This meansthat Greenland Sea. there is no one directional flushing of the deeper trough The properties of bottom waters in the NEW trough waters, and their renewal time might accordingly be very system are greatly controlled by its bathymetry. Waters at large. Possible mechanismsfor water renewal would be depth levels below about 150 m can enter the troughs only entrainment of deeper waters into the upper layer and via the mouths of Belgica Trough and Westwind Trough. subsequentexport from the NEW area or differential flow The water types present there thus depend on the waters acrossthe trough sections.A stagnantpoint for the inflows found at their entrances and on their respective spreading. into Belgica Trough and Westwind Trough seems to be The latter is sensitive to ridges in the troughs or changesin located at the connection between Westwind Trough and bottom depth. When meeting a ridge or a location of shal- Norske Trough, i.e., close to the Henrik Kr0yer Islands. At lower depth, only the lighter, upper part of the water column this meetingpoint a horizontaldensitygradientis established can continue its spreading, and downstream bottom densi- in the deeper waters by the density contrast between waters ties will be reduced if there is no other bottom water supply. in Norske Trough and Westwind Trough. The higher densiAs downstream bottomtemperatures andsaliniti•s will be ties in Westwind Trough are due to the fact that deepwater close to those of the waters at the bottom depth of the spreadingis not limited by ridges there. The only small difference of about0.1 kg m-3 canbe explained by obstacle, the vertical water mass structure of the NEW area density leads to reduced bottom temperatures and lower salinities the only slightly greater depth of Westwind Trough (300 m) comparedwith the indicatedbottomdepthbelowNOIB (250 behind a ridge.

BUDI•US AND SCHNEIDER: HYDROGRAPI-tY OF THE NORTHEAST WATER POLYNYA

- BT

•1-•

NT

1.5

-1.5

•1
0øCand S

> 34.9;seeHopkins[1991].)However, thereis evidencethat the warm waters in Belgica Trough are not originating directly from RAW. TS diagramsof stationscontaining RAW showhigh fluctuationsat the temperaturemaximum,

4298

BUDl•US AND SCHNEIDER: HYDROGRAPHY

OF THE NORTHEAST WATER POLYNYA

Table 2. Silicate Concentrationsfor Water Masses in the NEW Troughs, North of Ob Bank, and of the Return Atlantic Waters (RAW) From ARK IX Data (G. Kattner, personal communication)

Location/Water Mass RAW

at 81ø20'N

Pressure,

Temperature,

dbar

øC

Silicate,

Salinity

/xmolL -1

200

2.5

34.89

5.5

340

1.1

34.89

7.3-9.0

maximum layer Westwind Trough, deep waters

300

0.4

34.84

8.6-9.6

North North

400 300

0.4-0.5 0.47

34.85 34.82

8.2-8.3 8.1

Belgica Trough, temperature

of Ob Bank of Ob Bank

Silicate data are from G. Kattner (personal communication, 1994).

which is not colder than 2øC. In contrast, stations in Belgica Trough show a smooth turn in the TS relationship at their temperature maximum of about IøC. This could possiblybe explained by vertical exchange and long resident times, but this would require vertical mixing under conditions of low

vertical velocity shear and considerable vertical density

gradients (dp/dzbeingalmost constant between 100and300 m andamounting to about0.4 k gm-3 per 100m). Also,a gradual decrease of the maximum water temperature with increasing distance from the RAW source at the mouth of Belgica Trough would result from vertical mixing. We observe, however, vertical temperature maxima remaining at the same temperature over the entire length of Belgica Trough (see Figure 8b). A difference between RAW and deep waters in the NEW troughs is further supported by nutrient data (Table 2) (G. Kattner, Alfred-Wegener-Institut ffir Polar- und Meeresforschung, personal communication, 1994). In the RAW, silicate concentrations are about 5 to 6

/zmolL -1 (seealsoKoltermann andLiithje[1989]),whereas in BelgicaTrough,7.3 to 9 /zmolL -1 areobserved in the temperature maximum layer. Deep waters in Westwind Trough suggesta connectionto RAW

to a much lesser extent

because of their lower

tem-

peratures. The temperature contrast between waters in Westwind Trough and in Belgica Trough is explained by Bourke et al. [1987] by the different distances between the trough entrances and the EGPF at which RAW is found. During ARK IX, indeed, no AW was observed in front of

Westwind Trough(stations around 5øW,79øto 80øN).However, whether or not this observation can be generalized remains doubtful, since recirculating AW is found sometimes at even higher latitudes, as is the casecloseto 81ø20'N during ARK IX (stations213 and 214). If AW recirculatesby forming eddies which move across Fram Strait, rather than by fixed circulation branches [Gascard et al., 1988], then the presence of RAW in front of the troughs could vary with

that a greater variety of distinguishablewater massesthan is reflected by the term RAW might be present close together in Fram Strait, which change their properties only slowly. As indicated by their temperatures, both water types in the deeper parts of the NEW troughs must originally be stemming from AW. Admittedly, a complete concept of these waters' modifications cannot be derived from our present data set, and only larger-scale surveys can solve this problem.

Conclusions

We summarize our results in a schematic picture of the different water massespresent in the NEW area and their respective spreadings(Figure 10). Among the most important results of this study is the independence of spreading paths of waters at different depth levels that is due to little vertical mechanical coupling and to the bathymetric structure of the area. In the lower level, RAW flows southward alongthe shelf edge without penetrating into the troughsand two different types of modified AW fill the deepest parts of Belgica Trough and Westwind Trough. Their depth level is slightly deeper than that of the RAW. In the upper level, KW, presumably also of Atlantic origin, spreadsdiagonally over the shelf. It does not enter the NEW, and upper water layers (including KW) do not form a closed gyre at the southernboundary of the Belgica Bank trough system. PW seems to be formed locally on the Greenland shelf, rather than being established in the Arctic and imported into the NEW

area. No indications

be detected

on the northern

for bottom water formation East Greenland

could

Shelf. Convec-

tion in wintertime seems to penetrate to only small depths not affecting waters with salinities above 34.0. The increased knowledge gained by recent fieldwork should not, however, suggest that the hydrographic processesin the NEW area are perfectly understood. In raising time. Apart from the idea that RAW, dilutedto different a few unresolved questions, we would mention, above all, extents, is present in the NEW trough system there is a the role of tidal movements (which appear to be at least of possibility that both water types, in Belgica Trough, as well the order of average circulation speeds) and perhaps tidal as in Westwind Trough, attain their characteristicsat other mixing (of which we could find only a little indication, even places in the Arctic and are imported into the troughs. While in shallow areas like Ob Bank). Another important aspect waters of the Belgica Trough type have not been observed that could not be treated satisfactorily is the exchange outside the NEW during ARK IX, waters resembling the between waters north and south of Ob Bank. Whether a characteristics of deeper waters in Westwind Trough have northward flow of NEW surface waters exists that could been found north of Ob Bank. Their properties are similar extend the polynya to the North Greenland Shelf or if such not only in temperature and salinity (see Figure 8d), but also observed extensions are initialized locally cannot be stated in silicate concentration (see Table 2). This supportsthe idea at present. Already mentioned is the need to further inves-

BUDI•US AND SCHNEIDER:

HYDROGRAPHY

OF THE NORTHEAST

WATER POLYNYA

4299

Ob Bank )?

Figure 10. Sketch summarizingthe waters observed in the NEW area and their supposedspreading. Abbreviations are PW, Polar Water; KW, Knee Water; and RAW, Return Atlantic Water. Deep waters of the troughs (modified AW) and their possible source north of Ob Bank are labeled 1 and 2.

tigate the origins and paths of the modified AW types found in Belgica and Westwind Troughs.

References Aagaard, K., and L. K. Coachman, The East Greenland Current North of Denmark Strait, II, Arctic, 2•(4), 267-290, 1968. Bourke, R. H., J. L. Newton, R. G. Paquette, and M.D. Tunnicliffe, Circulation and water masses of the East Greenland Shelf, J. Geophys. Res., 92, 6729-6740, 1987. Bud•us, G., A. Maul, and G. Krause, Variability in the Greenland Sea as revealed by a repeated high spatialresolutionconductivitytemperature-depth survey, J. Geophys. Res., 98, 9985-10,000, 1993.

Foldvik, A., K. Aagaard, and R. T0rresen, On the velocity field of the East Greenland Current, Deep Sea Res., Part A, 35, 13351354, 1988.

Gascard, J. C., C. Kergomard, P. F. Jeannin, and M. Fily, Diagnostic study of the Fram Strait marginal ice zone during summer from 1983 and 1984 Marginal Ice Zone Experiment Lagrangian observations, J. Geophys. Res., 93, 3613-3641, 1988. Hopkins, T. S., The GIN Sea--A synthesisof its physical oceanography and literature review 1972-1985, Earth Sci. Rev., 30, 175-318, 1991.

Huppert, H. E., On the stability of a seriesof double diffusive layers, Deep Sea Res., 18, 1005-1021, 1971. Johannessen, O. M., Introduction: Summer Marginal Ice Zone Experiments during 1983 and 1984 in Fram Strait and the Greenland Sea, J. Geophys. Res., 92, 6716-6717, 1987. Johnson, G. L., D. A. Horn, O. M. Johannessen,S. Martin, and R. D. Muench, MIZEX (Marginal Ice Zone Experiment), Sea Technol., 26, 18-22, 1985.

Kiilerich, A., On the hydrography of the Greenland Sea, Medd. Groent., 144, 1-63, 1945. Koltermann, K. P. and H. L•ithje, Hydrographischer Atlas der Gr6nland- und n6rdlichen Norwegischen See (1979-1987), Dtsch. Hydrogr. Inst., Hamburg, Germany, 1989. Mamayev, O. I., Temperature-Salinity Analysis of World Ocean Waters, 374 pp., Elsevier, New York, 1975.

Melling, H., and E. L. Lewis, Shelf drainage flows in the Beaufort Sea and their effect on the Arctic Ocean pycnocline, Deep Sea Res., Part A, 29, 967-985, 1982.

Moore, R. M. and D. W. R. Wallace, A relationship between heat transfer to sea ice and temperature-salinity properties of Arctic Ocean waters, J. Geophys. Res., 93, 565-571, 1988. Muench, R. D., K. Jezek, and L. Kantha, Introduction: Third marginal ice zone research collection, J. Geophys. Res., 96, 4529-4530, 1991.

Paquette, R. G., R. H. Bourke, J. F. Newton, and W. F. Perdue, The east Greenland polar front in autumn, J. Geophys. Res., 90, 4866-4882, 1985. Perry, R. K., H. S. Fleming, N. Z. Cherkis, R. H. Feden, and P. R. Vogt, Bathymetry of the Norwegian-Greenland and western Barents Seas, chart, Acoust. Div., Environ. Sci. Branch, U.S. Nav. Res. Lab., Washington, D.C., 1980. Perry, R. K., H. S. Fleming, J. R. Weber, Y. Kristoffersen, J. K. Hall, A. Grantz, G. L. Johnson, N. Z. Cherkis, and B. Larsen, Bathymetry of the Arctic Ocean, Map and Chart Ser., MC-56, Geol. Soc. Am., Boulder, Colo., 1986. Schneider, W., and G. Bud•us, On the generation of the Northeast Water Polynya, J. Geophys. Res., this issue. Service Hydrographique de la Marine, Mers de Norv•ge et du Groenland de la terre Peary au Scoresby Sound et de Trondheim au Cap du Nord d'apr•s les documents les plus r•cent, chart, Paris, 1953. UNESCO, Algorithms for computation of fundamental properties of seawater, Tech. Pap. Mar. Sci., 44, 53 pp., Paris, 1983. UNESCO, Progress on oceanographic tables and standards 19831986, Tech. Pap. Mar. Sci., 50, 59 pp., Paris, 1986.

G. Bud6us and W. Schneider, Alfred-Wegener Institut fiir Polarund Meeresforschung, Columbusstra/3e, D-27568 Bremerhaven, Germany.

(Received January 18, 1994; revised July 15, 1994; acceptedJuly 26, 1994.)