Boundary Undercurrent and Water Mass Changes in the Lincoln Sea

Home , Eurasian Basin, Lincoln Sea, Lomonosov Ridge

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. C2, PAGES 3393-3403, FEBRUARY 15, 1997

Boundary undercurrent and water mass changes in the Lincoln Sea

John L. Newton Polar Associates, Inc., Santa Barbara, California

Barbara J. Sotirin Research,Development, Test and Evaluation Division, Naval Command Control and Ocean SystemsCenter, San Diego, California

Abstract. Oceanographicmeasurements taken between 1989 and 1994 in the Lincoln Sea describethe currentsand water massstructure along the continentalslope of the Arctic Basinbetween the Canadianand EurasianBasins. The measurementsincluded periodic conductivityand temperatureversus depth (CTD) profilesfrom ice camps,CTD cross sectionsnormal to the slope, and year-round current measurements.Analysis of this data describesthe hydrographicstructure of the waters over the continentalslope and identifies somesignificant interannual variations. An undercurrent,confined to the continental slope,with a widthof about50 km andspeeds of 5-9 cms -1 is shownin geostrophic current crosssections. The presenceof this easterlyflow at depth is confirmedby current meter measurements, and its existence has also been observed in the Beaufort Sea and the BarentsSea [Aagaard,1989]. The waterswithin this undercurrentexhibit temperature- salinity(TS) characteristicssimilar to CanadianBasin waters, suggesting a boundary current systemwhich is continuousalong the continentalslope north of Alaska and Canada.Significant interannual variations in temperatureand salinityprofiles may be related to variationsin the large-scalecirculation of the Arctic. Examinationof the upper pycnoclinewaters over the slopebetween 1991 and 1994 indicateda relative temperature maximumoverlying a minimum, TS characteristicssimilar to waters attributed to Bering Sea origin found in the CanadianBasin. These characteristicswere not seen, however, during 1989 and 1990, suggestinga recent increasein the transportof surfacewaters from the Canadian Basin into the Eurasian Basin.

1. Introduction between the Canadian and Eurasian Basins.Aagaard [1989] proposesthat importantlarge-scale advection within the Arctic The Lincoln Sea lies within the Eurasian Basin of the Arctic Ocean occursin narrowboundary currents along the continen- Ocean,just north of Greenlandand EllesmereIsland. Because tal marginsof the basins.These boundarycurrents are a major of extremely harsh ice conditionsyear-round, oceanographic feature of Arctic circulationand shouldbe present along the measurementsover the continentalslope of the Lincoln Sea Lincoln Sea continentalslope. Anderson et al. [1994] and Ru- are relativelysparse. Prior to the late 1980s,observations were delset al. [1994] suggestthat the return flow of Atlantic Water obtained either from ice islands,which drifted over the deep from the Canadian Basin into the Eurasian Basin occursalong watersof the EurasianBasin north of the Lincoln Sea slope,or the Greenland Sea continentalslope. This return flow, if a from icebreakersand aircraft operatingin the relatively shal- permanent large-scalefeature, should be evident along the low, nearshore areas of the continental shelf and in the waters of Lincoln Sea shelf. the CanadianArchipelago [c.f. Seibert, 1968; Melling et al., 1984]. Followinga brief overviewof the ICESHELF field trials in During Project Spinnaker'sICESHELF field experiments, the Lincoln Sea, this paper will describethe water massstruc- 1989-1994, instrumentation deployed from ice camps col- ture over the shelf and slope within the context of present lected temperatureand salinityprofiles and short- and long- knowledgeof Arctic Oceanwater masses.Next the circulation term current measurementsover the continentalslope of the along the slope will be discussed.Finally, some long-term Lincoln Sea (Figure 1). These observationswere locatedjust changesin the water massproperties, including temperature east of the intersectionof the LomonosovRidge with the and salinitystructure of the surfaceand haloclinewaters, will North American continentand provide a detailed description be described. of the hydrographiccharacteristics and current structureover this part of the Arctic Ocean continentalshelf and slope. Par- ticular large-scaleprocesses important to our understandingof 2. ICESHELF Oceanographic Field Program Arctic circulation addressedby these measurementsinclude in the Lincoln Sea the presence of boundary undercurrents and the exchange 2.1. Lincoln Sea Setting Copyright1997 by the American GeophysicalUnion. The Lincoln Sea is boundedon the southby the land masses Paper number 96JC03441. of EllesmereIsland, a part of the CanadianArchipelago, and 0148-0227/97/96J C-03441 $09.00 northwest Greenland (Figure 1). Between these two land

3393 3394 NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT

90 •

Sea '?n

Eurasian Basin

Fram Strait

•1- Benng St.

60 W

Figure 1. Ovelwiewof Arctic Ocean and ICESHELF area. Inset showslocation of ICESHELF long-term currentmeter mooringsand CTD stationlines. Soundings are in meters.Major bathymetricfeatures are based on National Oceanicand AtmosphericAdministration National GeophysicalData Center ETOPO5 gridded elevation data and ICESHELF measurements.

massesthe Lincoln Sea is connected to Bafi•n Bay via the on the western boundary of the Lincoln Sea between about passagesof Robeson Channel, Kennedy Channel, and Nares 60øWand 70øWlongitudes. Approaching the shelfbreak from Strait. The northernmost reaches of the Lincoln Sea coincide the north, the crestof the LomonosovRidge shoalsto depths with the deep waters of the Arctic Basin north of about 84øN. lessthan 500 m at 85ø15' N and then deepensto 1500 m before The eastern and western boundaries are considered here to be attaining shelf depths. This relatively deep saddle in the approximatelyalong longitudes of 40øWand 70øW,respectively. LomonosovRidge structure provides a deep-water channel The continental shelf break of the Lincoln Sea occurs at a through the Lincoln Sea connectingthe Canadian and Eur- relatively deep depth of about 300-400 m. The width of the asianBasins of the Arctic Ocean alongthe continentalmargin continental shelf is about 110 km at a longitude of 40øW, of the CanadianArchipelago. increases to about 220 km at 60øW, and reduces to about 110 km at 70øW. Water depthsincrease northward from 300-400 2.2. ICESHELF Field Measurements m at the shelf break to basin depths of greater than 1600 m Temperature and salinity versus depth profiles were ob- over distances of about 20-50 km. tained from the ICESHELF ice camps,located in the vicinity The LomonosovRidge extendstoward the continentalshelf of 84øN, 65øW and along three lines of stations extending NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT 3395 acrossthe shelf and slopewhich were occupiedby helicopter (Figure 1). The temperatureand salinitymeasurements were obtained using Ocean SensorsModel OS100 or OS200 conductivityand temperatureversus depth (CTD) instruments. The CTDs were calibrated to ñ0.02øC in temperature and 200 ñ0.02 mScm -• in conductivityby the manufacturerprior to eachexperiment. Instrument intercomparisons in the field and postcalibrationresults suggest that the final temperatureand 400 ...... ; ...... salinity measurementsare accurate to within ñ0.03øC and ñ0.03, respectively.The CTDs were typicallyset to record at ', about2 Hz, providinga verticalsampling resolution of 0.5 m at a droprate of 1 m s-•. Profileswere collected from the surface 600 , i , I , to the bottom over the shelfand to at least500 m in the deeper 30 31 32 33 34 waters. Salinity A few short currenttime seriesand severalcurrent profiles Figure 2b. Salinityversus depth profiles for the nine stations were collectedfrom the ice campsduring spring operations. along the 1991 CTD sectionover the shelf and slope of the Long-term current measurementswere obtained from two Lincoln Sea. Typical profiles (salinity offset by +2) for the mooringsindicated as M1 and M2 in Figure 1. Mooring M1 Canadianand EurasianBasins (MOODS) are shownfor com- was deployedfrom April 25, 1992, throughApril 12, 1993, at parison. 83057' N, 63ø05' W, where the water depthwas 580 m; mooring M2 operatedfrom April 26, 1993, throughApril 16, 1994, at 83ø38' N, 62ø49' W with a water depth of 306 m. The current the magneticheadings appeared stable over the period of the metersof the M1 mooringwere placed 1.5 m abovethe bot- measurements. tom, at a depth of 578 m, and approximately220 m abovethe bottom at a depth of 360 m. For mooring M2 the current 3. Water Mass Structure meterswere located near the bottom at a depth of 304 m and at a depth of 84 m. Both meters on the M1 mooring and the 3.1. Review of the Eurasian and Canadian Basin Water Mass Structure deep meter of the M2 mooring were set to record a 1-min averageof speedand directionevery 15 min. The upper meter The Lincoln Sea lies near the LomonosovRidge, which has of the M2 mooringrecorded a 1-min averageof current speed been thought to separate the intermediate layer circulation and direction hourly. All of the current measurementswere patternsand thusbound water massstructures characteristic of made using Interocean S-4 current meters. Accuraciesstated the western Arctic (Canadian Basin) and the eastern Arctic bythe manufacturer are 2% (or ñ 1 cms- •) for speedand ñ2 ø (Eurasian Basin) [Pounder,1986]. Thus the differencesin the for direction. Becauseof the high inclination of the magnetic temperature and salinitystructure of the two basins,shown in field at this location(about 88ø),current directionsshould be the offsetprofiles of Figures2a-2c, are an important aspectof viewedwith somecaution. In the springof 1993, severalmag- the hydrographyof the Lincoln Sea and are highlightedbelow. netic compasseswere located on the ice and the headings It is noted that recent measurements[McLaughlin et al., 1996] recorded for about 26 days [Sotirin and Newton, 1993]. The suggesta shift of this frontal boundaryinto the CanadianBasin magneticheadings displayed a periodic (24 hour) fluctuation to the Mendeleyev-Alpha Ridge. of 4ø-6ø . Allowing for ice flow rotation, confirmedby Sun lines, The watersof the Arctic Basinand the adjoiningGreenland, Iceland, and Norwegian Seas are made up of three major layers which include a surface layer, an intermediate layer

Canadian • Euras/an]

200 - - Shelf

North Stn 400 ......

Freezing 600 , • , I -2 -1 0 1 2 ...... Point Offset'+1 , tn Temperature (øC) 3O 31 32 33 34 35 Figure 2a. Temperatureversus depth profiles for the nine Salinity stationsalong the 1991 CTD sectionover the shelfand slopeof Figure 2c. Temperature versussalinity correlationsfor the the Lincoln Sea.Typical profiles (temperature offset by + iøC) nine stationsalong the 1991 CTD sectionover the shelf and for the Canadianand EurasianBasins, extracted from the Navy slopeof the Lincoln Sea.Typical TS correlations(temperature Master OceanographicObservation Data Set (MOODS) da- offset by +iøC) from the Canadian and Eurasian Basins tabase,are shownfor comparison. (MOODS) are shownfor comparison. 3396 NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT

than 0.7øC,with the highestvalues of about 3.0øCnear the sourcein Fram Strait and lowervalues along the Lomonosov

lOO Ridge [Treshnikov,1977]. In the CanadianBasin the maximum core temperaturesare more uniform and lower, rangingfrom 0.4ø to 0.5øC.Coachman and Aagaard [1974] note that the core 200 of theAW is relativelyshallow in theEurasian Basin (200-250 m) anddeeper in the CanadianBasin (400-500 m). 300 Water massdifferences between the Canadian(western Arc- tic) andEurasian (eastern Arctic) Basin waters can be easily 400 summarizedusing the temperature-salinity(TS) correlations (Figure2c). In the upperpart of the pycnoclinethe TS corre-

5OO lation for stationsin the CanadianBasin reflects the Bering 84-30 84-00 83-30 83-00 SeaWater (BSW) inflowas a relativetemperature maximum associatedwith salinitiesof about31.9-32.7 overlyinga rela- Latitude (N) tive temperature minimum associatedwith salinities in the Figure 3a. Temperature crosssection over the Lincoln Sea range of 32.7-33.5 [Coachmanand Barnes, 1961; Carmack, shelf and slopebased on 1991 CTD stations. 1990].Below the relativetemperature minimum the Canadian BasinTS correlationfollows a positiveslope to the AW temperature maximum at about 0.5øC,34.8. In contrast,the TS formed by the influx of warm, salinewater from the Atlantic, correlationfrom the EurasianBasin follows a nearly linear and a deep layer conditionedby convection.The convention relationshipfrom the surfacelayer to a temperatureof about usedby Carmack[1990], Swift and Aagaard[1981], and Aa- -1.5øC,at a salinityof about34.1, and then continues along a gaard et al. [1985] is followed for these definitionsand the nearly constantslope to the AW temperaturemaximum at descriptionof the regionalwater masscharacteristics as they 0.5ø-1.0øC, 34.8-34.9. pertain to the Lincoln Sea. The surfacelayer extendsto a depth of about 200 m and 3.2. New Data: Temperature-SalinityStructure consistsof PolarWater (PW), a watermass with coldtemper- Over the Lincoln Sea Continental Slope atures,usually less than 0øC,and relativelylow salinities,less A sectionof nineCTD stationsobtained during the springof than 34.4. The upper part of the surfacelayer is a seasonal 1991along the 65øWmeridian from 83004 ' N to 84024' N (see mixedlayer whichtends to be homogeneousin temperature Figure1 for location)will be usedto describethe hydrographic and salinityduring wintertime but may be salt stratifiedin the structureover the continentalshelf and slope.From southto summer as a result of melting ice. north this line of stations crosses the continental shelf with Regionalvariation of the PW is summarizedby Carmack water depthsof 250-300 m, intersectsthe shelf break at about [1990].Surface salinities are highestin the EurasianBasin near 83050' N, andextends over the continentalslope where sound- Svalbardand generallydecrease in a counterclockwisesense ings increase to about 1600 m at the northern end of the aroundthe basin.Surface salinities are relativelylow in the section.The verticalprofiles of temperatureand salinity along southernBeaufort Sea and in the areajust northof the Cana- thisline are plottedin Figures2a and2b; Figure2c showsthe dian Archipelago[Coachman and Aagaard, 1974]. Within the correspondingTS correlations.Profiles corresponding to the EurasianBasin, salinity increases rapidly with depthattaining innershelf (south of about83036 ' N), the regionover the slope, about34.9 at about200 m. The temperatureremains low, less andthe northernmost station are indicated in Figures2a-2c by than -1.5øC, to depthsof 150 m beforeincreasing. Tempera- the notationsshelf, slope, and north station,which are usedin tures in the EurasianBasin typically increase monotonically the followingdiscussion. The temperatureand salinitycross with depthfrom the base of the mixedlayer to thetemperature sectionsalong this line are presentedas Figures3a and 3b. maximumin the intermediatelayer. Within the CanadianBa- At the shelf stations,temperature and salinityincreased sin, salinityincreases more slowlywith depth resultingin a deeperhalocline. PW in the CanadianBasin typically exhibits a relativetemperature maximum overlying a relativetemper- 0 atureminimum in the upperpart of the halocline(Figures 2a Depthof MixedLayer and2c) whichresults from the inflowof Pacificwaters through .32.5 •-.• 100 __.33.0 the BeringStrait [Coachmanand Barnes,1961]. Cold saline _•33.5• watersformed on the Arctic shelvesplay a role in maintaining •34.0__--.---•- 34•--. the temperature minimum [Aagaardet al., 1981; Carmack, 1990;Melling and Moore, 1995]. The temperatureand salinitycharacteristics of the interme- 300 diate layer of the Arctic Basinare stronglyinfluenced by the Atlantic Water (AW) mass which enters the Arctic Basin throughFram Strait andvia the BarentsSea. As the AW enters the basin, it is cooled and diluted by mixingwith adjacent 400I waters.Additional decreases in temperatureand salinityresult 5001 from meltingas the AW encountersthe seaice. The core of the 84-30 84-00 83-30 83-00 AW is evidentthroughout the Arctic Basinas a temperature Latitude (N) maximumat depthsbetween 200 and 800 m. Figure 3b. Salinity crosssection over the Lincoln Sea shelf AW core temperaturesin the EurasianBasin are greater and slopebased on 1991 CTD stations. NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT 3397 monotonicallywith depth from the base of the mixed layer to ature maximumof the AW coreis relativelylow (<0.45øC) and the bottom. The TS correlationfor the shelf stations(Figure the temperatureprofile is smooth. 2c) had a positiveslope and did not indicatethe temperature At the north station, below depths of about 150 m and at maximum/minimum characteristic of BSW. The water mass salinitiesabove 33.6, there is a changein water masscharac- characteristicsover the shelf are similar to thoseobserved by teristics compared to the slope stations.The waters in the Seibert[1968] and describedby Mellinget al. [1984]for the area lower part of the pycnoclineare colder than at equivalent of the Lincoln Sea. depthsand salinitiesat the slopestations. The AW core tem- Vertical temperatureprofiles at the slopestations evidenced peraturesare higher and are located at shallowerdepths. In a relative temperature maximum at 75-100 m and a relative summary,at the north stationthe characteristicsof the waters temperatureminimum at 80-120 m (Figure 2a). The salinities below 150 m are transitioningtoward those of the Eurasian associatedwith the relative temperaturemaximum and mini- Basin (Figure 2c). mum were 32.4-32.8 and 33.0-33.3, respectively(Figure 2c). Below the relative temperature maxima/minima features, 4. Circulation through the lower pycnoclineand into the Atlantic Water 4.1. Overview of Arctic Circulation layer, temperaturesat the slopestations increased with depth to a maximumof about 0.4øCat depthsof about 450-550 m. The long-term average, surface circulation of the Arctic Thesetemperature profiles were relativelysmooth through the Ocean [Coachmanand Aagaard, 1974] featuresan anticyclonic AW layer and exhibiteda rather low temperaturemaximum at gyre (the Beaufort Gyre) in the CanadianBasin and a general relativelydeep depthssimilar to the descriptionof the Cana- flow acrossthe LomonosovRidge and then southtoward Fram dian Basin AW. Strait (the TranspolarDrift Stream). The Lincoln Sea lies at At the north stationof this sectionthe temperaturestructure the boundaryof thesemajor circulationpatterns in an area of and the TS correlationwere the sameas for the slopestations poorly defined but probablylow mean surfacecurrents. The circulation of the intermediate waters within the inte- throughthe upper 150 m. However, through the lower pycnocline, below 150 m and at salinitiesof greater than about 33.6, rior of the Arctic Basin is defined in general terms by tracing temperaturesat the north station were cooler than at slope the characteristicsof the AW mass.AW circulatesalong the stations.The TS correlationdisplayed a significantchange in continentalmargin of the EurasianBasin in a cyclonicdirec- slopeat about -1.3øC, 34.1. The temperaturemaximum of the tion [Coachman and Barnes, 1963]. A portion of the AW AW was somewhathigher, greaterthan 0.5øC,at the northern crossesthe LomonosovRidge into the Canadian Basin along the Siberian continentalslope and circulatesthrough the Ca- stationthan at the slopestations, and the temperatureprofile nadian Basin.There is evidence[Anderson et al., 1994;Rudels displayedsome structure between 300 and 400 m. The TS et al., 1994; •uadfasel et al., 1993] that a portion of the AW characteristicsof the north stationappeared similar to thoseof the Eurasian Basin. entering the Arctic may recirculatewithin the Eurasian Basin and move directly back toward Fram Strait along the Nansen- A surfacemixed layer, homogeneousin temperature and Gakkel Ridge. salinitywith temperaturesat the freezing point for the ob- In the interior regionsof the deep Arctic Basin the average servedsalinity, was continuousacross the section(Figure 3b). motionof the Atlantic Water [CoachmanandAagaard, 1974] is The mixedlayer attained depthsof 50-60 m over the shelf and believedto be veryslow (less than 1-2 cms-1). In otherArctic deepenedto 75-85 m over the slope. Neither the isotherms regions,such as the Beaufort Sea,along the LomonosovRidge, nor the isohalinesshowed a consistentor significantslope at and north of the Siberian Shelf, there are indicationsthat along the shelfstations, south of about83ø36 ' N (Figures3a and 3b). the shelf break, currents may increasewith depth [Aagaard, However,farther north, over the slope,both the temperature 1989].The resultis a relativelystrong boundary current flowing and salinitysurfaces deepen by about 40 m. The densitysur- along the shelf break near the bottom. Current speedscan faces,which correspondto the salinity surfacesat these low attain30 cms- • or morefor periodsof severaldays [Aagaard, temperaturesand small temperature range, also slope down- 1984]. The direction of the current is generallycounter to the ward to the north acrossthe continentalslope. The tempera- local surface flow. ture maximum/minimumfeature in the upper pycnoclinefol- lows the downward sloping salinity (density) surfacesand 4.2. Current Structure Over the Lincoln Sea extends from the outer shelf at about 83ø36 ' N northward to at Continental Slope least the end of the sectionat 84ø24' N. The geostrophiccur- 4.2.1. General current structure. The general nature of rent associatedwith this densitystructure, oriented normal to the currentsover the Lincoln Sea slope,based on short-term the section line in the along-slopedirection, would increase time seriesmeasurements from the ICESHELF ice camps,can toward the east and decreasetoward the west with increasing be describedas a tidal circulationof about5-10 cm s-• super- depth. imposedon a relativelylow mean flow of a few centimetersper second.Vertical current profiles from the ice campsshowed 3.3. Relation to Canadian and Eurasian Basin Water the mean current speedto increasewith depth. The distribu- Masses tion of the U (positive,east; negative, west) and V (positive, At the slopestations the TS characteristicsof the lower part north; negative,south) current components versus depth from of the PW massand the AW massesare very similar to water current profiles obtained at the 1991 and 1992 ice camps is massesof the Canadian Basin. For example, the water mass summarizedin Figure 4. Note that the U componentis aligned occupyingthe upper part of the pycnocline,from the base of approximatelyalong slope and the V componentis oriented in the surfacemixed layer to depthsof 150 m or salinitiesof about the across-slopesense. 33.6, shows the characteristictemperature maxima/minima The averagecurrent was low, essentiallyzero, at the under- structureattributed to the contributionof BSW. The temper- ice surface. The mean U current increased toward the east 3398 NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT

ø1 ', Y'i!•"v",v v'....v •"v -1SD'•"I.I ",•+1 SO ':'+1SO 200'

M;'•n % Mean

400. • 400

6001 • i • 600/ -5 0 5 10 -5 0 5 u Component (cm s'l) V Component (cm s-1) Figure 4. Mean and 1 standarddeviation envelopes of the U (positive,east; negative, west) and F (positive, north; negative,south) current components versus depth over the Lincoln Sea slope.Based on five profilesin April 1991(84006 ' N, 62003' W, waterdepth of about1000 m) and 35 profilesin April 1992(83057 ' N, 63005' W, water depth of 580 m). relativelyrapidly from the baseof the mixedlayer (about80 m) are oriented north-south,the directionof the computedcur- through the pycnoclineto a depth of about 200 m and more rents are east-west,approximately along the continentalslope. slowly from 200 m to near the bottom. The mean V current Positive values indicate that the current is toward the east componentremained small throughoutthe water column.The relative to the surface flow. variation of the current, indicatedby the 1 standarddeviation The distributionof the geostrophiccurrent acrossthe shelf envelope,is due to the tidal fluctuationssuperimposed on the and slopewas similar for all 3 years.The currentrelative to the mean current profile. surfacewas generallypositive, toward the east,throughout the 4.2.2. Geostrophic currents. Geostrophic currents were sections.The magnitudeof the geostrophiccurrent was low, computedfrom the densitydistribution along the 1991-1993 lessthan 2 cm s-• relativeto the surface,down to a depthof CTD stationlines. The currentswere computedfrom adjacent about 100 m. North of the shelfbreak, the geostrophiccurrent stationpairs, and the resultsare contouredin Figures5a-5c. increasedsignificantly below a depth of about 150 m. At the The contoursof Figure 5 indicate the magnitudeof the dy- northernend of each sectionthe magnitudeof the geostrophic namicallycomputed current componentnormal to the section current was reduced to less than 1-2 cm s- • from the surface line assumingno motion at the ocean surface,an assumption down to depthsof at least 500 m. supportedby the measurements(Figure 4). As the stationlines The dominantfeature of the geostrophiccurrent crosssec-

2O0 2OO

400 400

6O0 6O0 84-36 84-12 83-48 83-24 83-00 84 -36 84-12 83-48 83-24 83-00 Latitude(Deg) Latitude(Deg) Figure 5a. Geostrophic current (centimeters per second) Figure 5b. Geostrophic current (centimeters per second) normal to the 1991 sectionassuming no motion at the ocean normal to the 1992 sectionassuming no motion at the ocean surface. Positive values of current indicate the relative current surface. Positive values of current indicate the relative current is directed toward the east. is directed toward the east. NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT 3399

M2 ' 93 •' 5 cm s '1

, 200" 200 ......

400• 400•-

600 600 84-36 84-12 83-48 83-24 83-00 200 160 80 40 0 Latitude(Deg) Distance(km) Figure 5c. Oeostrophiccurrent (centimeters per second) Figure 6. Positionsof the current meters of mooringsM1 normal to the 1993 sectionassuming no motion at the ocean and M2 relativeto the high currentcore (denotedby the 5 cm surface. Positive values of current indicate the relative current s-• isotach)based on the 1991-1993geostrophic current sec- is directed toward the east. tions (Figures5a-5c). There were somevariations of the ba- thymetry along the three cross sectionsdue to their exact locationand orientationso the bathymetryprofile is typicalof the shelf break and slope along the 65øW meridian. The cur- tions is a core of relativelyhigh currentsover the continental rent meter mooringsare placed on the appropriateisobaths. slopeextending from a depthof 150m to the bottom.The high current region is laterally boundedby the shelf break to the south and extends to the north a distance of about 50 km. This riods,current meter depths,and the mean and varianceof the feature was observedin the dynamicsof all three sections current components. (1991-1993) with essentiallythe same characteristics.This The currentmoorings, designated as M1 and M2 in Figure1, geostrophiccurrent is flowingalong the continentalslope, sim- werepositioned on the 580-mand 306-misobaths, respectively. ilar to a boundaryundercurrent in the Beaufort Sea described To assist in the correlation of the current records and the by Aagaard [1984]. The region of the significantundercurrent, geostrophiccurrent computations,Figure 6 locatesthe moor- denotedby the 5 cm s-1 shearisotach in the figures,extends ings and current meters relative to the dynamiccurrent cross horizontallyfrom near shelf break at about 83ø48' northward sectionsof Figures5a-5c. Both of the current meters of the M1 to about 84ø18' N. Maximum easterlycurrent componentsat mooringwere positionedwell within the region of the signifi- the center of the core were about 5-9 cm s-• cant eastwardgeostrophic currents. The lower meter of the M2 The vertical current profilessummarized in Figure 4 were mooringis on the southernedge of the strongcurrent region; obtainedwithin the region of the significantundercurrent, at the upper meter of the M2 mooring at a depth of 84 m is latitudesof about84ø06 ' N (1991) and 83ø57' N (1992). There located in the surface layer above and to the south of the is goodagreement between the dynamiccalculations (Figures region of significantcurrents. 5a-5c) and the measured U componentcurrent difference The annualmean magnitude of the U component(Table 1), from the surface to 600 m of about 4-5 cm s-1. directedslightly north of due east,was in goodagreement with 4.2.3. Long-term current measurements. The year-long that predictedfrom the geostrophiccalculations. The mooring current records of speed and direction were converted to U M1 resultsof 5-6 cms-• at bothmeters are representative of (positive,east; negative, west) and l/(positive,north; negative, their position within the core of the undercurrent.The lower south) components,which are oriented approximatelyalong meter of mooringM2, about 2 cm s-1, is consistentwith a and acrossisobaths, respectively, in this area. Energy density reduction in current speed near a lateral boundary of the spectrawere computedfor each current componentrecord. undercurrent.The positionof the M2 upper meter is outside The current componentswere then filtered with a 25-hour the boundarycurrent and near the surface,supporting its low runningmean to removesignificant tidal energyand provide (<1 cm s-1) annualmean. daily averagecurrents. Table 1 summarizesthe recordingpe- Spectracomputed for the 4-year-longcurrent recordswere

Table 1. Long-Term Current Moorings, Summaryand Statistics

Current Meter Depth, U Component V Component Mean Speed, Mean Direction, Meter RecordDates m Mean (Variance) Mean (Variance) cm s-1 oT

M1 April 25 1992, 360 5.0 cm s-• 0.6 cms -• 5.0 083 Upper to April 12, 1993 (14.2cm 2 s-2) (16.6cm2 s -2 ) M1 April 25, 1992, 578 5.6 cm s-• 0.9 cm s-• 5.7 081 Lower to April 12, 1993 (10.8cm 2 s-2) (19.6cm 2 s- 2) M2 April 26, 1993, 84 0.2 cm s-• -0.7 cm s- • 0.7 167 Upper to April 16, 1994 (12.5cm 2 s-2) (19.6cm 2 s-2) M2 April 26, 1993, 304 2.0 cm s-• 0.2cm s -1) 2.0 083 Lower to April 16, 1994 (20.0cm 2 s-2) (19.6cm 2 s-2) 3400 NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT

10000 slope(V component)currents were low at both depths(from 2 cms -• westto 3 cms -• east). The M2 mooringwas located near the southernboundary of the undercurrent(Figure 6). At the lower currentmeter, daily average currentswere typically directed east along the isobaths.The mean flow (Figure 8b) wasin the samedirection as at mooringM1, locatednear the core of the undercurrent,but lO the speedwas lower. There were a largernumber of exceptions to the typical easterlyflow comparedto the record at the M1 mooring. At the M2 upper meter, above the region of the undercurrent,the dailyaverage currents were typicallylow and variable.Exceptions were two periodswith rather strongcur- rents directed to the southeastand southwestduring April 1993 and August 1993, respectively.

O.Ol o.ool O.Ol o.1 1 5. Long-Term Changes in Water Mass Frequency(cph) Properties Figure 7. U and V current componentspectra from the up- During the 1989-1994 ICESHELF field measurements,sig- per meter of mooring M1. Fast Fourier transformcoeificients nificant changeswere noted in the structureof the waters of were computed from four nonoverlappingsegments of the the upper pycnocline.To assessthe year to year changesin the time serieswith Hanning window.Energy peaksin the diurnal water mass properties, a representativeprofile was selected and semidiurnalband are significantat the 90% confidence from each year from 1989 through 1994. These profiles lie level. within the area boundedby latitudes83052 ' and 84006' N and longitudes62003 ' and 65031' W and thus are representativeof conditionsover the continental slope near the axis of the generallysimilar. An examplefor the upper current meter of boundaryundercurrent. Figures 9a and 9b plot the tempera- mooring M1 is shownas Figure 7. Substantialenergy contri- ture profiles,on full and expandeddepth scales,and Figure 9c butions are evident in the diurnal and semidiurnal tidal bands. plots the salinityprofiles; Figure 9d is the correspondingTS Energy at the inertial period was not separable from the plot for these stations. nearby tidal components(S2 and K2) by this analysis. In 1989 and 1990, there was no distincttemperature maxi- Figures 8a and 8b show the daily averagesof the currents mum/minimumsignal which could be attributed to the pres- from the long-term mooringsM1 and M2. Each line segment ence of the BSW water mass(Figures 9b and 9d). An isother- in Figure 8 representsa daily averagecurrent, indicatingthe mal layer between 90 and 100 m may be a remnant of BSW direction by the orientation of the vector relative to the hori- water whose temperature maximum has been reduced over zontal, which is north, and the magnitudeby the length of the time by mixing. In 1991 a temperature maximum/minimum vector. Within the core of the undercurrent,typified by the feature appearedwith a relativelyweak temperaturemaximum measurementsat mooring M1, the generalnature of the daily of about -1.4øC and a smallvertical extent. During 1992 and mean currentsat middepth(360 m) and near the bottom (578 1993 the vertical extent of the temperature maximum/ m) was similar (Figure 8a). The daily currentswere nearly minimumfeature increasedand the value of the temperature alwaysdirected toward the east along the continentalslope. maximumwarmed progressivelyto -1.26øC in 1992 and then Minor exceptionsto the persistenteasterly flow occurredfor to -1.20øC in 1993. As the temperaturemaximum increases, short, 2- or 3-day periods,at the lower meter on November 8, the associatedsalinity range progressivelyextends to lower 1992, and at both meters on April 6 and 11, 1993. Daily aver- salinitiesand the salinityassociated with the peak temperature aged along-slope(U component)currents ranged from west- maximum decreasedslightly. The 1994 temperatureprofiles erly at about2 cm s-• to easterlyat 15 cm s-• at the upper and TS correlations(Figures 9b and 9d) indicatethat the value meterand from westerly at 5 cms-• to easterlyat 12cm s- • at of the temperaturemaximum may have decreasedslightly and the near-bottom current meter. The daily averaged across- the vertical extent of the maximum feature was reduced. There

:0 1:0 20• N UpperMeter / Depth=360rn Mooring M1/ 1992-93 cm/sec I

5/1/92 7/1/92 9/1/92 11/1/92 1/1/93 3/1/93 Figure 8a. Daily mean currentsat 360 m (upper meter) and near the bottom at 578 m (lower meter) from mooringM1, April 25, 1992, throughApril 12, 1993. NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT 3401

•.•• \ MooringM2 / 1993-94 :ocm/sec 10 20•- - N

LowerMe[er/Depth=304m ' XSN, , ' ,Xx.... • , [,, , , , 5/1/92 7/1/92 9/1/92 11/1/92 1/1/93 3/1/93 Figure 8b. Daily meancurrents at 84 m (uppermeter) and near the bottomat 304 m (lowermeter) from mooringM2, April 26, 1993, throughApril 16, 1994.

is someindication (Figure 9c) that the temperaturemaximum 6. Discussion and Summary may have startederoding in the lower salinityrange, perhaps The Lincoln Sea is positionedbetween the Canadian and by mixing with the surface waters. The TS correlations at EurasianBasins near the intersectionof the LomonosovRidge salinitiesgreater than 33.6, equivalentto depthsbelow about with the North American continent.A relativelydeep water 150 m, remainedsimilar, suggesting that there were no signif- (greaterthan 1500m) path existsalong the slopeconnecting icant changeswithin the intermediate(AW) layer. the Canadianand EurasianBasins. The ICESHELF Project The increasedtemperature maximum of the BSW in region oceanographicmeasurements describe the currents and water overthe LincolnSea slope from 1991to 1993was accompanied massstructure along the continentalslope of the Arctic Basin by a reductionin the thicknessof the surfacemixed layer, just to the eastof the interconnectionbetween the two major shownmost clearly in the salinityprofiles of Figure 9c, from basins of the Arctic. about80 m to 20 m, and a loweringof the surfacelayer salinity The structure of the water masses of the Lincoln Sea shelf 32.25 to 30.80. During this time, there did not appear to be and slopefell into three distinctivecategories. These included significantchanges in the structure of the AW water mass waters over the inner part of the shelf with temperatureand (Figures9a and 9d). The north south extent of the BSW in the Lincoln Sea can be salinityincreasing monotonically from the surfaceto the bottom, waters over the outer shelf and slopewith overall char- estimatedfrom the CTD profilesalong the 1991-1993 station acteristicssimilar to those found in the Canadian Basin, and lines (Figure 1). During all 3 yearsthe BSW water masswas evident at the northernmost extent of the measurements which watersnorth of the slopewhich appearedto begin a transition to characteristics of the Eurasian Basin. rangefrom 84ø24' to 84ø33' N. When the relativetemperature A boundaryundercurrent is positionedover the continental maximum first appeared in 1991, the southernextent of the slope between the shelf break and the base of the slope at signalwas at about 83ø36'N.In 1992 the temperaturemaxi- about 1600 m. The width of the current is about 50 km, and it mum increasedin strengthand extendedfarther southto about extendsfrom the base of the mixed layer (30-75 m) to the 83ø27'N.In 1993 the temperaturemaximum extended south to bottomin water depthsof at least600 m. The typicalstrength at least 83ø25'N, the southernmost station obtained on the shelf. of the currentnear the core was about 5-9 cm s-], basedon dynamiccalculations, and 5-6 cms-1 fromthe long-termcur- rent measurements.Assuming that the boundaryundercurrent has a mean strengthof about 4 cm s-] over a horizontal dimension of about 50 km and vertical extent from 100 m to lIlk 1991 1•992 1994 ••,• 1993 1991 1992 200 989-90.....• . .• 1994

._. 1 19 ......

200 ......

600 -1 0 Temperature (Deg C)

Figure 9a. 300 Temperature versus depth profiles from 1989 -2 -1 0 1 through 1994 ICESHELF ice camps.Profile locationsare be- Temperature (Deg C) tween latitudes83o52 ' and 84ø06'Nand longitudes62o03 ' and 65ø3I'W, over the continentalslope near the axisof the bound- Figure 9b. Expandeddepth scaletemperature versus depth ary undercurrent. profiles, 1989-1994. 3402 NEWTON AND SOTIRIN: LINCOLN SEA BOUNDARY UNDERCURRENT

1993•ff•••L -1991 Changesin the water masscharacteristics and structureof the upper layer were alsonoted in the ICESHELF measure- 1994• 1989.••••••.•. • ments.In the springof 1989 and 1990, temperatureprofiles over the Lincoln Sea continentalslope increasedmonotoni- 200 cally, exceptfor an isothermallayer between90 and 110 m, from the base of the mixed layer to the AW temperature maximum.During 1991through 1994 the characteristicsof the upper waters changedsignificantly. Starting in 1991, a water

.... 400 masscharacteristic of the CanadianBasin, consistingof a relative temperaturemaximum overlying a temperatureminimum, began to be observed.The magnitudeof the relative temperaturemaximum and its vertical extent increasedpro- 600 30 31 32 33 34 35 gressivelythrough 1993. This featurewas present in 1994with Salinity a slightlyreduced maximum temperature. These changes were accompaniedby a correspondingreduction in the verticalex- Figure 9c. Salinityversus depth profiles,1989-1994. tent of the surfacemixed layer by a factor of 4 and a decrease in the salinityof the mixed surfacelayer from 32.25 to 30.80. Thesechanges in water masscharacteristics and structureof the bottom at an averagedepth of about 1000 m acrossthe the upperlayer couldbe explainedby an increasein the trans- slope,the transportwould be about2 Sverdrups(1 Sverdrup= 106m 3 s- 1). port of surfaceand upperhalocline waters from the Canadian Basin into the Eurasian Basin along the continental slope. The crosssections and long-term time seriesmeasurements These waterswould showthe characteristicBSW temperature over the Lincoln Sea slopedescribe a boundaryundercurrent maximum/minimum feature and would have a thinner surface similar to that of the Beaufort Sea [Aagaard,1984]. Aagaard mixedlayer with a lower salinity.This movementof the upper [1989] proposeda synthesisof the circulationof the Arctic Ocean which included a series of undercurrents along the layer watersprobably started between the springof 1990 and the springof 1991.The eventcontinued through 1994 although boundaryof the Arctic Basin.The boundaryundercurrents are some evidencesuggests the maximum effect may have been directedalong the continentalslope and are responsiblefor observedduring spring 1993. McLaughlin et al. [1996]describe large-scaleadvection in the Arctic. The undercurrentde- significantdisplacements of the BSW in the CanadianBasin. scribedby Aagaard[1984] in the BeaufortSea had a width of 60-70 km and a vertical extent from 40 m to bottom and was Thesechanges in the BSW distributionmay be related to the 1991-1994movement of upper layerwaters into the Eurasian directedalong isobaths between the shelfbreak and the baseof theslope with a meanstrength of about10 cm s -1. Thechar- Basinalong the Lincoln Sea continentalshelf describedhere. acteristicsof the undercurrent in the Lincoln Sea are compa- rable to the undercurrent in the Beaufort. The water mass Acknowledgments.Funding for this work has comefrom the U.S. below 150 m, and within the Lincoln Sea undercurrent, was Navy Spaceand Warfare SystemsCommand and the Officeof Naval similar for each of the spring1989-1994 measurementsand Research. Paul Bucca of the Naval Research Laboratory, Stennis was characteristic of the Canadian Basin waters. Thus the SpaceCenter, Mississippi, collected the 1989and 1990data. R. Meridith of Naval ResearchLaboratory, Stennis, and T. Wen of APL, Univer- undercurrent in the Lincoln Sea is a consistent,long-term sityof Washington,assisted with the measurementsin 1991.It is always feature and may be part of a boundaryundercurrent system a pleasureto work with R. Fisher of Naval ResearchLaboratory, which is continuousalong the continental slope of Alaska, StennisSpace Center, who hasbeen an integralpart of the ICESHELF Canada,and Greenland.As suggestedbyAnderson et al. [1994] environmentalmeasurements from 1989 through1994. SandraHall of ORINCON, SanDiego, assistedwith the figuresand manuscriptprep- andRudels et al. [1994]this boundary undercurrent may be the aration. majorpath for the returnflow of AW from the CanadianBasin into the Eurasian Basin. References Aagaard,K., The Beaufortundercurrent, in TheAlaskan Beaufort Sea, editedby P. W. Barnes,D. M. Schell,and E. R½imnitz,pp. 47-71, Academic, San Diego, Calif., 1984. Aagaard,K., A synthesisof the Arctic Oceancirculation, Rapp P. • Reun. Cons.Int. Explor.Mer, 188, 11-22, 1989. Aagaard,K., L. K. Coachman,and E. C. Carmack,On the haloclineof the Arctic Ocean,Deep SeaRes., Part A, 28, 529-545, 1981. Aagaard,K., J. H. Swift,and E. C. Carmack,Thcrmohalin½ circulation in the Arctic mediterraneanseas, J. Geophys.Res., 90, 4833-4846, 1994 1985. o.'- 19931992 .••,•. Anderson,L. G., G. Bjork, O. Holby, E. P. Jones,G. Kattncr, K. P. • 19 Koltcrman, B. Liljcblad, R. Lindcgrcn,B. Rud½ls,and J. Swift, Water masses and circulation in the Eurasian Basin: Results from Froozin• v/•• the Odcn-91 expedition,J. Geophys.Res., 99, 3273-3283, 1994. -----Point __ • 1989-90 Carmack,E. C., Large-scalephysical oceanography of polar oceans,in PolarOceanography Part A: PhysicalScience, edited by W. O. Smith, 30 31 32 33 34 35 chap.4, pp. 171-222,Academic, San Diego, Calif., 1990. Salinity Coachman,L. K., and K. 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Coachman,L. K., and C. A. Barnes,The contribution of Bering Sea Ser.,vol. 85, edited by O. M. Johannessen,R. D. Muench, and J. E. water to the Arctic Ocean,Arctic, •4(3), 146-161, 1961. Overland, pp. 33-46, AGU, Washington,D.C., 1994. Coachman, L. K., and C. A. Barnes, The movement of Atlantic water Seibert, G. H., Oceanographicobservations in the Lincoln Sea--June in the Arctic Ocean,Arctic, 16(1), 8-16, 1963. 1967,BaJ]in Bay-North WaterProj. Rep. 2, 23 pp., Arctic Inst. of N. McLaughlin, F. A., E. C. Carmack, R. W. Macdonald, and J. K. B. Am., Univ. of Calgary, Calgary,Alberta, Canada, 1968. Bishop, Physical and geochemicalproperties acrossthe Atlantic/ Sotirin, B. J., and J. L. Newton, Horizontal magneticfield fluctuations Pacificwater massfront in the southernCanadian Basin, J. Geophys. measuredin the Lincoln Sea, in ProceedingsIEEE Oceans'93, vol. 2, Res., 101, 1183-1197, 1996. pp. 30-34, Inst. of Elec. and Electron.Eng., Piscataway,N.J., 1993. Melling, H., and R. M. Moore, Modification of haloclinesource waters Swift, J. H., and K. Aagaard, Seasonaltransitions and water mass during freezing on the Beaufort Sea shelf: Evidence from oxygen formation in the Iceland and Greenland Seas,Deep SeaRes., Part A, isotopesand dissolvednutrients, Cont. Shelf Res., 15(1), 89-113, 28, 1107-1129, 1981. 1995. Treshnikov, A. F., Water massesof the Arctic Ocean, in Polar Oceans, Melling, H., R. A. Lake, D. R. Topham, and D. B. Fissel, Oceanic edited by M. Dunbar, pp. 17-31, Arctic Inst. of N. Am., Calgary, thermal structurein the westernCanadian Arctic, Cont. ShelfRes.,3, Alb., Canada, 1977. 233-258, 1984. Pounder,E. R., Physicaloceanography near the north pole,J. Geophys. J. L. Newton, Polar Associates, Inc., 100 Burns Place, Goleta, CA Res., 91, 11,763-11,773, 1986. 93117. (e-mail: [email protected]) Quadfasel,D., A. Sy, and B. Rudels,A ship of opportunitysection to B. J. Sotirin, Research,Development, Test and Evaluation Division, the North Pole: Upper ocean temperature observations,Deep Sea Naval Command Center and Ocean SystemsCenter, San Diego, CA Res., Part I, 40, 777-789, 1993. 92152. Rudels, B., E. P. Jones, L. G. Anderson, and G. Kattner, On the intermediate depth waters of the Arctic Ocean, in The Polar Oceans (Received September22, 1995; revisedAugust 20, 1996; and TheirRole in Shapingthe Global Environment,Geophys. Monogr. acceptedAugust 26, 1996.)