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JOURNALOF GEOPHYSICALRESEARCH,VOL. 106,NO. C5,PAGES9075-9092, MAY 15,2001 River runoff, seaice meltwater,and Pacificwater distribution and mean residence times in the Arctic Ocean Brenda Ekwurzel, • PeterSchlosser, 2'3Richard A. Mortlock, andRichard G.Fairbanks 2 Lamont-Doherty EarthObservatory of Columbia University, Palisades, NewYork James H. Swift Scripps Institution ofOceanography, University ofCalifornia, SanDiego, LaJolla, California Abstract.Hydrographic andtracerdatacollected duringARKIV/3 (FSPolarstern in 1987), ARCTIC91(IB Oden),andAOS94(CCGSLouisS.St-Laurent) expeditions revealtheevolution of thenear-surface watersin theArcticOceanduringthelate 1980sandearly1990s.Salinity, nutrients, dissolved oxygen, and;5•80dataareusedtoquantify thecomponents of Arctic freshwater: riverrunoff,seaicemeltwater,andPacificwater. The calculated riverrunoff fractions suggest thatin 1994a largeportion ofwaterfromthePechora, Ob,Yenisey, Kotuy,and Lena Riversdid not flow off the shelfclosestto their river deltas,but remainedon the shelfand traveled viacyclonic circulation intotheLaptevandEastSiberian Seas.Riverrunoffflowedoff theshelfattheLomonosov RidgeandmostlefttheshelfattheMendeleyev Ridge.ARCTIC91 andAOS94Pacificwaterfractionestimates of UpperHaloclineWater,thetraditionally defined coreof thePacificwatermass,document a decrease in extentcompared to historicaldata. The front betweenAtlantic water andPacificwater shiftedfrom the LomonosovRidge locationin 1991totheMendeleyev Ridgein 1994.Therelative agestructure of theupperwaters is described byusing the3H-3He age.Themean 3H-3He agemeasured inthehalocline withinthe salinity surface of33.1+_0.3is4.3_+1.7years andthatforthe34.2_+0.2salinity surface is9.6+_ 4.6years.Lateral variations intherelative agestructure withinthehalocline andAtlantic water support thewell-known cyclonic boundary current circulation. and GreenlandBasins and increasedcirculation of upper layer 1. Introduction Arcticwaterin theNorwegianSea. Thusfluctuations in outflow Oneof themostimportant features of theArcticOceanis its well-developed halocline. The surfacemixed layer and the of Arctic Ocean freshwater fractions combined with NAO atmospheric forcingwill leadto variabilityin therateandvolume highlystratified halocline forma low-salinity layerof waterwith of convectionin theseregions. Salinityhasa profoundinfluenceon thermohaline circulation. temperatures closetothefreezing point,which"blankets" a much thicker,moresaline,andwarmerlayerof Atlanticorigin. These The Arctic Ocean model resultsof H•ikkinen [1993] supportthe by Aagaardand Carmack[1989] that anomalously low-salinitysurfacewatersinsulatethe seaice from the heat hypothesis storedin the Atlantic water, which, if it were to reachthe surface, large ice exportin 1968 causeda fresheningof the northern couldpotentially meltor at leastreduce theseaicecover.Three North Atlantic known as the "Great Salinity Anomaly" (GSA) freshwater sources maintain the halocline: river runoff, sea ice [Dickson et al., 1988]. Schlosser et al. [1991] document a meltwater, andthelow-salinityPacificwaterenteringtheArctic reduction during the 1980s of Greenland Sea Deep Water Oceanthrough theBeringStrait.Thelargestorage of freshwater formation that may be linked to the return of the GSA to the in the Arctic Ocean (around 100,000 km3; [Aagaardand Greenland Sea. Steele and Boyd [1998] demonstrated a Carmack,1989]) feedsfreshwateroutflowsthroughFram Strait significantdecreasein the extentof the Cold Halocline Layer in andtheCanadianArchipelago.Blindheimet al. [2000]link the the EurasianBasin duringthe springof 1995. In this contextit is winter North Atlantic Oscillation(NAO) index of wind forcingto reducedcirculationof Atlantic water into the westernNorwegian • NowatDepartment of Hydrology andWaterResources, University of Arizona, Tucson, Arizona. 2AlsoatDepartment of EarthandEnvironmental Sciences, Columbia University,New York. 3AlsoatDepartment of EarthandEnvironmental Engineering, important to assess the volume, circulation pattern, and timescales for each of the freshwater componentscritical to maintaining propertiesof the Arctic Ocean mixed layer and halocline. The approachof our investigationis to quantifyshifts in the Arctic Ocean freshwater componentsand check for possiblelinksto proposedforcingmechanisms. This studybuilds on previouswork [Schlosseret al., 1994; Bauch et al., 1995; Schlosser et al., 1995] by combining nutrients,dissolvedoxygen,/5t80,tritium,andheliumisotope ColumbiaUniversity,New York. data for determining the freshwater components and mean residencetimes of the upperwatersin the Arctic Ocean. In this Copyright2001 by the AmericanGeophysical Union contribution, we usePO4* (= PO43' + O2/175-1.95 •mol kg't Papernumber1999JC000024. [Broeckeret al., 1985, 1998]) in combinationwith /5t80 and salinityto separatethe Pacificwaterfractionfrom the river runoff 0148-0227/01/1999JC000024509.00 9075 9076 EKWURZEL o ET AL.: FRESHWATER East Siberian Sea DISTRIBUTION IN THE ARCTIC OCEAN i• Sea 1• Laptev . Novos• 41• Kara S'ea Chukchi Sea Makarov Basin 150øW Canada Basin 30øE 0 o Figure1. Station locations forARKIV/3 (circle), ARCTIC91 (triangle), andAOS94(square). CAPis an abbreviation for ChukchiAbyssalPlain. and sea ice meltwater fractions contained in the Arctic al. [1989, 1994], and Swift et al. [1997]. Tritium, helium freshwater. Theresidence timesof theupperwaters in theArctic isotopes, andsome•80 samples fromARK IV/3 weremeasured Ocean aredetermined fromthe3H-3He agedistribution. at the Institut far Umweltphysikat the Universityof Heidelberg following the methodsdescribedby Roether [1970] and Bayer et 2. Methods al. [1989]. All othertritium,heliumisotopes, and•80 samples 2.1. SampleCollection were measuredat the Lamont-DohertyEarth Observatoryusing proceduresdescribed by Fairbanks [1982] and Ludin et al. measurements of the H2180/H2160 Salinity,nutrients, dissolved oxygen,•5•80,tritium,andhelium [1998]. Massspectrometric ratio were performedafter water sampleequilibrationwith CO2 sampleswere collectedduringicebreakerexpeditionsto the central Arctic Ocean. In summer 1987 the German icebreakerFS [Epstein and Mayeda, 1953; Roether, 1970; Fairbanks, 1982]. Oxygenisotoperatiosare reportedas •5180or the per mil Polarsterncrossed theNansenBasin(ARK IV/3 expedition). deviation of theH2180/H2160 ratioof thesamplefromthatof The Swedishicebreaker IB Odenoccupied Nansen,Amundsen, Standard Mean Ocean Water (SMOW) [Craig, 1961; anda few MakarovBasinstations duringthesummer of 1991 Gonfiantini,1981]. Measurementprecisionwas aboutñ0.05(ARCTIC91expedition).Duringthe 1994summer theCanadian 0.07%o for a smallportionof the ARK IV/3 •5180samples and icebreaker CCGS Louis S. St-Laurent crossedthe southwestern about_+0.03%o fortheremaining •5•80data. boundary of theCanadaBasin,surveyed theMakarovBasin,and Heliumisotoperatiosare reportedas •53Heor the percent occupied a fewEurasian Basinstations (AOS94expedition). We deviation of the3He/4He ratioof a measured sample fromthatof collected watersamples usinga rosette system equipped with10 an air standard(Rat m = 1.386 x 10'6 [Clarke et al., 1976]). L Niskin bottles during each cruise. The combineddata set Precision of the3He/4He ratioswasaboutñ 2%o.Tritium(3H) yieldsa comprehensive collectionof tritium,helium,and•5•80for the halocline waters over all basins between Fram Strait and BeringStrait(Figure1). 2.2. Measurement has a half-life of 12.43 years [Unterwegeret al., 1980]. After sufficientlylongstorage(frozenat -25 øC)of thedegassed water in flame-sealed glassbulbs,tritium valueswere determined by massspectrometric measurement of the decayproduct3He. Tritium measurementshad a precision of about ñ2% and a Temperature, salinity,oxygen,and nutrientsampleswere detectionlimit of 0.03 TU, where 1 TU represents a tritiumto analyzed oneachshipusingtechniques reported byAnderson et hydrogen ratioof 10'18.In ordertocompare datacollected during EKWURZEL ET AL.: FRESHWATER DISTRIBUTION differentyearsall tritium datawere decay-corrected to January1, 1991, or TU91 units. This date is between the first and last cruise discussedin this study,and it follows the conceptintroducedby OstlundandHut [1984],whoreporttritiumdatain TU81 units. The 3H-3Heage or the time elapsedsincethe waterleft the surface is 1l 3Hetri t r=-•ln1+ 3H IN THE ARCTIC OCEAN 9077 characteristics, shadedareason Figure2 are usedto represent FSBW andBSBW insteadof exactranges. Coachman and Barnes [1961] find that the "Eurasian Basin Surface Water" has a wide range of salinities (28-33.5) and closely follows the freezing line. The CanadianBasin surface waters, however, exhibit temperaturefluctuationswith a local potentialtemperaturemaximum(-1.55 < © < -0.65 øC at 31.6 < S (1) < 32.4) and a local © minimum (-1.5 < E} < -1.25 øC at 32.4 < S < where z is the age (years), 3Hetriis the tritiogenic3H e concentration (TU), 3His thetritiumconcentration (TU), and)• is the3Hdecayconstant. 33.2). Coachmanand Barnes [1962] call these"SummerBering Sea Water" and "Winter Bering Sea Water," respectively. This classificationimplies not only the geographiclocation but also the season in which these water masses form. Jones and Anderson [1986] noted the occurrence of a nutrient maximum and oxygen minimum near the 33.1 isohaline and referred to this water massas "Upper Halocline Water" (UHW). Previousinvestigatorshave provided various classifications They observed that a minimum in NO (NO = 9NO3' + O2 and namesfor portionsof the water column surface,halocline, [Broecker,1974]) occurrednear a salinity of 34.2, which they and Atlantic water (Figure 2). Carmack [1990] synthesizesthe defined as "Lower Halocline Water" (LHW). Broecker [1974] Arctic water massdefinitionsof Swift and Aagaard [1981] and definesNO to accountfor the ratio of decompositionof organic matter in the nutrient balance and thereby identified a Aagaard et al. [1985] in potential temperatureO and salinity S spaceas follows: (1) Polar Water: near-freezingsurfaceand main conservative tracer for water isolated from the atmosphere. halocline water (O < 0øC; S < 34.4); (2) Polar Intermediate Rudels et al. [1996] envision the formation of LHW with low NO as a stagein the evolutionof inflowing Atlantic water as it travels Water, or arctic thermoclinewater: O < 0øC; 34.4 < S < 34.7; (3) into the NansenBasin toward the Laptev Sea througha seriesof UpperArctic IntermediateWater: the coreof the Atlantic layer in melt, freeze, and winter convectioncycles. Salmon and McRoy the Arctic Ocean (O < 2øC; 34.7 < S < 34.9); (4) Lower Arctic [1994] suggest that the LHW advects into the Canada Basin Intermediate Water (0 < O< 3øC; 34.9 < S < 35.1); and (5) where interleaving with shelf-derived water occurs, thereby Atlanticwater:inflowingNorwegian-Atlanticcurrentwater (O > increasing the nutrient content. They define this water as 3 øC; S > 34.9). The Atlantic water divides into two branchesas it flows into the Arctic: (1) warm, saline water flows through "Canada Lower Halocline Water." McLaughlin et al. [1996] recognized the difficulty with the traditional definitions and Fram Strait known as the Fram Strait Branch Water (FSBW) and (2) water diverted onto the Barentsshelf southof Svalbardcools instead defined individual stationsas belonging to a "Western and entrains shelf waters to become the colder and fresher Arctic Assembly,"recognizedby the presenceof Pacific water, or an "Eastern Arctic Assembly," with no Pacific water. The Barents Sea Branch Water (BSBW) [Schauer et al., 1997]. terms UHW and LHW will be used in this contribution as a Recognizing the variable nature of the Atlantic inflow convenientway to differentiatethe broad differencein thesetwo 1. Polar Water parts of the Arctic Ocean halocline,while recognizingthat each 2. Polar Intermediate Water stationrepresentsa uniquemixture of the abovementionedwater 3. Water Mass Definitions 3. Upper ArcticIntermediateWater 4. 5. 6. Lower Arctic Intermediate Water Atlantic Water Canadian Sector Surface Water 7. 8. 9. UHWend member LHWend member Eurasian Basin Surface water masses. 4. Results and Discussion 4.1. Tracer •5•O 10. BeringSea SummerWater 11. BeringSea Winter Water Fractionationbetweenthe heavierand lighteroxygenisotopes due to temperatureand distancefrom the water vapor sourcearea 12. Fram Strait Branch Water 13. Barehis Sea Branch Water resultsin significant depletionof •80 in Arcticrunoff(relativeto 13 SMOW) and the Arctic Ocean into which these rivers flow (Figure3). Hence•5•80is a naturaltracerof riverrunoffwithin the Arctic Ocean. Most of the samplesin Figure4 with salinities •34.1 fall alonga mixinglinebetweenAtlanticwater(•5•80:0.3 _+0.1%•) andArcticriverrunoff(weightedaverage•5•80:-18%o; -2 30 31 32 33 34 35 Salinity Figure2. Selected watermassdefinitions, providedin the literature:1-5 [Carmack, 1990]; 6-8 [Jones and Anderson,1986];9 [Coachmanand Barnes,1962]; 10 and 11 [Coachmanand Barnes, 1961]; and 12 and 13 36 Figure 3). There is a sharpbreak at salinity 34.1-34.2 between higher-salinitywatersfalling on or abovethe Atlantic water-river mixing line and lower salinity waters falling below the mixing line. This break coincideswith the salinity of LHW as defined by Jones and Anderson [1986]. Water sampleswith salinities •34.1 may also include waters of Pacific origin. The Pacific inflow through Bering Strait (Figure 4c) has a larger salinity range [Roachet al., 1995] than the Atlantic inflow and has lower •5•80values(~-1%•)becauseof the largeinfluenceof Alaskan samplescollectedin the upper300 m duringARK and Siberian runoff to the Bering Sea [Grebmeier et al., 1990; Kipphut, 1990; Cooper et al., 1997]. Hence, for many samples, mixing is evident between Atlantic water, LHW, UHW, or IV/3, ARCTIC91, and AOS94. Pacific inflow. [Schauer et al., 1997]. The dots are bottle data for (a) 15 River Greenland 0o Canada 10 runoff inventory 5 height 0 (meters) 150 ø Russia (b) Greenland 0o Pacific Canada 40 water inventory 20 height (meters) 0 -150 ø Russia Plate 1. (a) River runoff water columninventoryexpressedas heightin metersfor the integratedwater columnbetweenthe surfaceand 300 m (or bottom measurementif shallowerthan 300 m) depth at each station. Also shown are the inventories Frank [1996] calculated for the 1993 ARK IX/4 and 1995 ARK XI/1 stationsusing a three-componentmassbalance with slightly different end-memberconcentrations. (b) Pacific water columninventoryexpressedas heightin metersfor the integratedwater columnbetween the surfaceand300 m (or bottommeasurement if shallowerthan300 m) depthat eachstation. EKWURZEL ET AL.' FRESHWATER DISTRIBUTION IN THE ARCTIC OCEAN 9079 meltwater. Sampleswith salinitieslower than that of the LHW 180 ø have 15•80values that reflect a combination of sea ice formation, mixingwith river runoff,and,in manysamples,mixingwith a fraction of Pacific inflow waters. 4.2. PO4* Nutrient-richPacific water flows throughthe shallowBering Strait onto the Chukchishelf where it is modifiedby a variety of shelfprocesses. BeringStraitfreshwater inflowis estimated to be from one half of [Aagaardand Carmack,1989] to equal to [Becker,1995] the total Arctic river runoff. Pacific inflow thereforeis a significantfreshwatercomponent of the Arctic surfaceand haloclinewaters,and its high level of nutrientsis a Figure 3. Mean annualdischargeand 8180data available in the literature for Arctic Rivers. keyto calculating itsfractionin theArcticfreshwater balance. Cooperet al. [1997]hypothesize thatthehigh-nutrient Bering Sea and Anadyr waters flowing throughBering Strait are depletedin nutrientson the Chukchishelfduringsummerby biological growthor production andarenotthewatersthatrenew the nutrientmaximumlayer of the haloclineknown as UHW. However,duringwinter, as first suggested by Coachmanand Barnes[1961] and confirmedby morerecentstudies[Roachet al., 1995;Cooperet al., 1997;Weingartneret al., 1998],there River discharge (km3yr'•) is proportional to trianglesizeand is listed in parenthesiswithin or near the triangle symbol for the river [Becker, 1995; Parlor et al., 1996]. The negative numberswithin or near the triangle symbol for eachriverarethe15•80 (%0)values [Macdonaldet al., 1989; Ldtolleet al., 1993; Ekwurzel, (a)0.5 0 ,0,5 -1 -1.5 1998]. -2'.5 -3 Net seaice melting and formationis anotherprocessrecorded in the deviationof the samplesfrom Atlantic water-river runoff -3.5 mixinglinein 15180 versussalinityplots(seearrowsof Figure4). During sea ice formation, changes in salinity dominate. However,thereis also a small enrichmentof ]80 relativeto the (c)o5 seawater from which itformed. The 8180 equilibrium -osJ•,•,t ,Fre._•ieze / fractionation value(2.9%o) measured forfreshwater icein laboratory experiments [Lehmann and Siegenthaler, 1991] is o ooI -0.s sea ice formed from seawater under slow growth conditions. higherthanthatmeasuredbyCra for Macdonald et al. [1995] determined from Arctic field of2.7%0 found byEicken [1998] intheWeddell Sea. Inthe -3.5 ••"='" measurements the8•80fractionation ofseaicetobe2.6+ [., 11•'./• ' 0MBOl { '-2. -- 0.1%o, -2.5 a valuesimilar tothemaximum equilibrium fractionation factor -3 30 will producea rangeof 15180 fractionation factors.Conditions presentin the Arctic could theoreticallyproduce15t80 stationsfrom (a) NansenBasin, (b) Gakkel Ridge and Amundsen Basin, (c) Canadian Basin, and (d) end members within the data range for Atlantic water, fractionationsbetween1.5 and 2.7%o. Without knowing the exact dynamicfractionationfactorwe used2.6%ofor our calculations, recognizingthat the averagemay well be closerto that reported by Melling and Moore [1995]. Sensitivitytests[Ekwurzel,1998] suggestthat this uncertaintyhas little impacton the freshwater inventorycalculations. The sea ice fractionationeffect can be seen in Figure 4a for 31 32 xCB 94 I -3.5• "A'CS+CAP94/ -4 Beaufort Sea, Melling and Moore [1995] measured a mean fractionationfactor of 2.1%o. Eicken [1998] stressesthat rapid sea ice growth and different effectiveboundarylayer thickness Salinity Salinity Figure 4. The•5•80valuesversussalinityplotsfor Pacific water, LHW, and UHW. Arrows indicate how oceanwater concentrationchangeswhen water freezes to form sea ice or When sea ice melts. The dashed line representsmixing betweenAtlantic water and river water with -18%o8t80 concentration. Legend abbreviations are as follows: AB, Amundsen Basin; the southernNansen Basin (ARK IV/3 stations269, 285, 287, and 310; ARCTIC91 stations4, 8,9, and 61; and AOS94 station CAP, Chukchi AbyssalPlain; CB, CanadaBasin; CS, ChukchiSea;GR, GakkelRidge;MB, MakarovBasin; MJP, Morris JesupPlateau; MR, Mendeleyev Ridge; 37). Thesesamplesare the symbolsthatplot abovethe Atlantic water-river mixing line reflecting the addition of sea ice NB, Nansen Basin; 87, ARK IV/3; 91, ARCTIC91; and 94, AOS94. 9080 EKWURZEL ET AL.' FRESHWATER DISTRIBUTION may be sufficient time for the nutrient-rich water to transit the shelf (i.e., when biologicalproductionis insignificant). In this scenario,winter transitis accompaniedby a sufficientincreasein salinitythroughbrine rejectionduringseaice formationto renew UHW (S - 33.1). These high-salinityshelf watersmay be further enrichedin nutrientsby being in contactwith shelf sedimentsthat release nutrients from decay of organic matter [Jones and Anderson,1986; Anderson, 1995]. Lower-salinity waters (S < 33.1) flowing through Bering Strait, predominantly Alaska Coastal Water, are more buoyant. They mix with Arctic river water and shelf waters influenced by sea ice meltwater and OCEAN (a) []•-. . '•'5NB87.1(b) NB 94 t + GR 87 n GR 91 DAB 91 Iii AB 94 & MJP 91 30 31 32 33 34 35 30 31 Salinity overlie the UHW [Coachman et al., 1975]. Silicate has been used in previous studies to estimate the Pacific componentcontributingto the Arctic Ocean freshwater balance. However, Arctic rivers have a wide range of initial IN THE ARCTIC 32 33 34 35 Salinity (c) silicateconcentrations (-25-160 /.tM kg'• [Macdonaldet al., 1989; L•tolle et al., 1993; Makkaveev and Stunzhas, 1995; Gordeev et al., 1996]) that are usually higher than the Pacific i.5• inflow concentration(-20 /.tM kg-• [Cooper et al., 1997]). Jones et al. [1998] also chose not to use silicate to trace Pacific water but insteadused the nitrate-phosphatedifference between Atlantic water and Pacific 0,5 LHW water end members to calculate Pacific o 30 waterfractionin the upper30 m. Arctic rivers have phosphate concentrationsclose to zero [Codispoti and Richards, 1968; Macdonald et al., 1987; Makkaveev and Stunzhas, 1995; Gordeev et al., 1996]. 31 32 33 Salin'• 34 35 30 31 32 "3•3 34 35 Salinity Figure 5. PO4*versussalinityplotsfor stations from In (a) Nansen Basin, (b) Gakkel Ridge and Amundsen Basin,(c) Canadian Basin,and(d) endmembers within the datarangefor Atlanticwater,Pacificwater,LHW, addition,Arctic river dischargeoccursmainlyduringthe summer months [Becker, 1995; Pavlov et al., 1996; Olsson, 1997]; hence biological productionduring summer takes up most of the and UHW and Mixed East Siberian and Chukchi Shelf nutrientsdelivered to the Arctic Ocean by rivers [Bj6rk, 1990]. Thereforewe chosephosphate to estimatethe contribution of the nutrient-richPacific inflow to the freshwaterbalanceof the upper water(MECS). Legendabbreviations are explainedin Figure4. waters in the Arctic Ocean. Recent refinement of the Redfield ratios [Takahashi et al., 1985; Andersonand Sarmiento, 1994] are usedto calculatePO4* as a quasi-conservativewater mass tracer that accounts for organic respiration [Broecker et al., 1985, 1995]. The range thanthe Pacificinflow andare unlikelyto havePacific-derived wateradvected to thislocation[Pfirmanet al., 1997]. Reyand Loeng [1985] postulateregeneratednutrientsto allow for the observed production duringmiddleto latesummeron theBarents betweenAtlanticwaterPO4* (-0.7 #mol kg'•) andPacificwater Seashelf. The remainingshelfstationsin thisdatasetare on the PO4* (-2 #mol kg'•) is significantlylarger than individual other side of the Arctic Ocean basin on the Chukchi shelf and measurement errorsfor PO43' andO2[cf. Swiftet al., 1997,Table havea widerangeof salinitieswith highPO4*valuesdueto the 1]. The one caveatis that Bering Strait inflow is variableand subjectto seasonalprocesses mentionedabove,and thereforethe dominance of BeringStraitinflowwaters(starsymbols, Figure 5c). PO4*, salinity,and•5•80endmemberconcentrations of Pacific NansenBasinandAmundsen Basinstations (excluding the water have higher uncertaintiesthat create larger errors in the calculated freshwater fractions (see section 4.5 for a discussion the sensitivitytestsregardingtheseend memberuncertainties). more westernstationssampledin 1991 betweenthe Lomonosov RidgeandtheMorrisJesup Plateau plottedastriangles onFigure 5b) fall alongtwo mixinglines:(1) Atlanticwatermixingwith The PO4* data are similarto •5•80datain that they both LHW and(2) LHW mixingwithmorebuoyant(S < 33.2)surface highlight the mixing between Atlantic water and LHW at all stations.Almost all stationsplottedin Figure5 havetheir lowest PO4* concentrationsnear the LHW salinity range, as expected waterswith PO4* valuessimilar to thoseof the Atlantic water. Pacificwater is not presentin centraland easternEurasianBasin waterssampledduringthisstudy. from the low NO values of those waters [Jones and Anderson, 1986]. However, shelf stationsin this data set often deviate from between thecentralLomonosov RidgeandMorrisJesupPlateau the general trend. Stationsclose to the BarentsSea shelf just north of Svalbard (ARCTIC91 stations 4, 60, and 61; open trianglesOutsidethe main cluster of data in Figure 5a) exhibit much lower PO4* values. This may be the resultof mixing with Makarov Basin stations and Eurasian Basin stations located extend the secondmixing line of the above stationsto fresher waters(S -30.5-31.5) with higherPO4*values(-1.4/zmol kg'•), hereafter referred to as Mixed East Siberian and Chukchi Shelf water(MECS). CanadaBasinstations neartheChukchiAbyssal seaicemeltwater asindicated by the•5•80trendforthesesamples Plain andMorris JesupPlateaustationsin the westernAmundsen (open triangles above the river-Atlantic water mixing line in Basingroupalongthreemixinglines:(1) Atlanticwatermixing Figure 4a). There is a slight elevation of PO4* in the shelf withLHW, (2) LHW mixingwithPacificwaterorUHW, and(3) stationssampledin 1987 (ARK IV/3 stations269, 274, 276, and Pacificwateror UHW mixingwithMECS. 278; open circlesFigure 5a). This may reflect the influenceof The surfacewatersat the Morris JesupPlateau(ARCTIC91 nutrient regenerationat the sedimentwater interface [Anderson stations40, 42, and 43; solidtrianglesthat tracea high arch and Jones, 1991] becausethese sampleshave a higher salinity patternin Figure5b) containPacificwateror UHW. Slightly EKWURZEL ET AL.- FRESHWATER DISTRIBUTION away from the Morris JesupPlateau (ARCTIC91 stations37-39, 41, 44, and 45; solid trianglestracing a smaller arch patternin Figure5b), the surfacewatersfollow the LHW-UHW mixingline for salinities between 32.8 and 34.2 and follow the LHW-MECS waters with salinities between the 1994 data these waters are limited 32.8 and 34.2. to the Pacific In side of the MendeleyevRidge. In contrast,the morebuoyantcomponents of Pacific inflow OCEAN 9081 {a)•.• 0,9 mixing line for fresherwaters(S < 32.8). SeveralMendeleyev Ridge stations (AOS94 stations12-18) are the solid circles plotting onto the smaller arch patterns in Figure 5c. The similarity between these Mendeleyev Ridge stations and the stationsjust offshoreof the Morris JesupPlateau supportsthe followingconnectionor circulationpattern:buoyantfreshwaters over the MendeleyevRidge reachto just offshoreof the Morris JesupPlateau. Nutrient-rich Pacific water, most likely added by winter inflow, has salinitiesand temperaturessufficient for renewal of the halocline IN THE ARCTIC waters mix with river runoff and shelf waters to renew the freshestpart of the halocline and are presentover a larger portion of the Arctic Ocean (Canada Basin, central Makarov Basin, central Lomonosov Ridge, and western O ..•................... •................ :............ :......... + GR 87 0'6 f O,5 0.4 30 31 32 • 0NB 87 ,• GR 91 DAB 91 • NB 9! al AB 94 V NB94 & • 35 30 31 Salini• 32 33 34 Salin•y (c)1.t 1 0,9 •o.s 0.7 i 0,6 0,5 Amundsen Basin). 0.4 30 31 32 33 34 35 30 31 32 Salinity 4.3. NO/PO Aagaard et al. [1981] emphasizethat the seasonalcycle of summer melt and winter sea ice formation on the shelf seas is an importantprocessfor creatinga thick Arctic Oceanhalocline. If we can identify where shelf regions currently renew halocline waters,we may be ableto usethis informationto modelhow the halocline will respondto changesin winter sea ice formation ratesunderdifferentphasesof the Arctic Oscillation. Wilson and Wallace [1990] found that Arctic shelf waters haveregionallydistinctiveNO/PO ratios. They usedthe Redfield ratiosof Broecker[1974]in theirdefinitionof NO (9NO3'+ 02) andPO (135PO43' + 02) instead of therevised Redfieldratioby Broecke,' et al. [1985](175PO43+ 02). TheNO/POratiousedin 33 34 Salinity Figure 6. NO/PO(]3•) versussalinityplotsfor stations from (a) Nansen Basin, (b) Gakkel Ridge and Amundsen Basin, (c) Canadian Basin, and (d) end members within the data range for Atlantic water, Pacific water, LHW, and UHW and MECS. Boxes in each graph are the ranges of Wilson and Wallace [1990]. BS is Barents Sea, LS is Laptev Sea, CS is Chukchi Sea, ESS is East Siberian Sea, and legend abbreviationsare explainedin Figure4. AOS94 station 4 in Figure6c fallsbelowestablished NO/PO(•35 ) ratios. It representsa pool of high-salinityChukchishelfwater with the highestphosphateand silicate as well as the lowestoxygenvaluesrecordedin this dataset. this studyis the ratio by Wilsonand Wallace [1990], hereinafter referredto as (NO/PO(•35)), to be consistent with their values definedfor the shelf waters(Figure 6). Kara Sea datawere not availablefor their study,andthereforeratioswere definedonly for the BarentsSea shelf, the Laptev Sea shelf, the East Siberian Sea shelf, and the Chukchi Sea shelf. Coolingand fresheningof Atlantic water (boththe FSBW and ratios. This is consistent with the cappingof LHW by Laptev Sea shelf waters proposedby Rudelset al. [1996] and the directionof the Transpolar Drift [Thorndike and Colony,1982; Pfirman et al., 1997]. The Makarov Basin and western BSBW) on its transit to the eastern Eurasian Basin have been AmundsenBasin stationscontaina mixturebetweenhigher- proposedas the first stepstoward forming LHW [Jonesand salinityBarents SeaandLaptevSeaNO/PO(]35 ) ratiosandthe lower-salinity watersat theboundary between theNO/PO(]35 ) Anderson, 1986; Steele et al., 1995; Rudels et al., 1996]. The stationssampledfor this study supportthe above hypothesis ratios of East Siberian Sea and Chukchi Sea shelf waters. NO/PO(•35 ) ratiosin ChukchiAbyssalPlainsurfacewaterspoint Ridge within the BarentsSea NO/PO(•35 ) range(Figure6). The towarda ChukchiSeasource,whilethosein Mendeleyev surfacewaterspoint towardan East SiberianSea source. Hence exceptions to this trendare the lowerNO/PO(•35 ) ratiosof the markan increased progression of shelfinputsfrom Chukchi Abyssal Plain sampleswith salinities >34.2. Salmon the stations becausewaterswith salinitiesgreaterthan thoseof LHW all fall the BarentsSea, Kara Sea, Laptev Sea, East SiberianSea, and trendsfor otherCanadaBasin stations. The ChukchiAbyssal ChukchiSeaintothefreshestpartof theArctichalocline. Plain stations(star symbolson Figure 6c) and Morris Jesup 4.4. End-Member Characteristics Plateaustations(solid triangleson Figure 6b) exhibit mixing between Atlantic water-LHW and UHW with salinities around The mixing relationships outlinedaboveimply an intricate 32.8-33.1,whichare overlainby watersfalling on the mixingline interaction betweenthefreshwater components of the surfaceand and McRoy [1994] and McLaughlin et al. [1996] found similar with Chukchi Sea shelf waters. The freshest Nansen Basin, Gakkel Ridge, and central Amundsen Basin halocline waters are within the Barents Sea haloclinewaters. Eachgeochemical tracerhighlightsspecific freshwater components in a differentway. For example,high PO4* levels emphasizethe Pacific influence, but it would be NO/PO(•35 ) range.TheLomonosov Ridgestations rangebetween difficult to assessthe river runoff factor from PO4* alone. theBarents Seaandthelowerlimitof theLaptevSeaNO/PO(•35 ) Combining salinity,•5180,andPO4*allowsfor a quantitative 9082 EKWURZEL ET AL.: FRESHWATER DISTRIBUTION determinationto thesefreshwaterfractionsin each samplewhere all three measurements exist. IN THE ARCTIC OCEAN Table 1. ParametersUsed in Three-Componentand Four-Component MassBalance Ostlundand Hut [1984] first proposed combining15•80and Salinity salinity measurementsto separatethe river runoff from the sea ice meltwater contribution to the Atlantic derived water. methodwas successfullyemployedon the Mackenzieshelfin the Beaufort Sea [Macdonald and Carmack, 1991; Macdonald et al., 1995; Melling and Moore, 1995] and in the Eurasian Basin [Schlosseret al., 1994; Bauch et al., 1995; Frank, 1996]. We extend the oxygen isotope data into the easternEurasian Basin and Canadian Basin and recalculate the ARK 8•*O,%0 PO4*,#molkg4 This IV/3 and ARCTIC91 datapresented by Bauch et al. [1995], addingPO4* Atlantic water River runoff Sea ice 35 + 0.05 0 4 _+1 Pacific water 32.7 + 1 0.3 _+0.1 -18 + 2 surface + 0.7 _+0.05 0.1 _+0.1 0.4 + 0.2 (2.6 +_0.1) a -1.1 _+0.2 2.4 _+0.3 aFor each station this value is set as the surface8•80 measurement plusthe equilibriumfractionation factor[Macdonaldet al., 1995; Eicken, 1998]duringseaiceformation lOstlurid andHut,1984]. as a fourthcomponentto isolatethe Pacificfraction. The four-component massbalanceequationsare asfollows: fa+L+fr+fi=i, (2) faSa+ feSp-{-frXr-{-fiSi= Sin, (3) L818Oa -{.-L818Op nl-fr8180r -1.-f/818Oi = 81BOrn, (4) faPO4*a + fvPO4*,+ frPO4*r+ f,PO4*,= PO4*,, (5) TheAtlantic8180value(0.3_+0.1%o) originally determined by Ostlund andHut [1984]is supported by Meteor8 BarentsSea shelfand ARK IX/1 Fram Strait sections[Frank, !996], and by Atlantic water measurementsfrom ARK IV/3 [Schlosseret al., 1994], ARCTIC91 [Bauch et al., 1995], and AOS94 stations. Theriverend-member 15•80 concentration (-18%o)is basedon the river15180 measurements weighted by theirrespective discharge values(Figure 3) The question remains if the above weighted mean is wherefa,fr, f•' f, arethefractions of Atlanticwater,riverrunoff, when we do not currentlyhave 15180 data from sea ice meltwater,and Pacific water contributingto a measured representative watersample(subscript m) and Sx, 15•8Ox, andPO4*x are the smallerrivers and when we are accountingfor contributionfrom corresponding salinity, 15•80, and PO4* water mass high-latitudeprecipitation.To addressthis concern,we consulted concentrations. The fractionsf• are also used to calculate the the GlobalNetwork for Isotopesin Precipitation(GNIP) database volume of water attributableto each freshwatercomponent. At of the International Atomic Energy Agency (IAEA, 1998). All each stationthe fractionswere linearly interpolatedto a depthof http://www.iaea.org/programs/ri/gnip/gnipmain.htm, 300 m (or to the deepest depth if shallower than 300 m) and GNIP stations at latitudes >65øN are listed in Table 2. As with 15180 in extrapolated to the surface. This interpolated profile was theriverdata,thegeneraltrendis towardmoredepleted integratedby trapezoidalapproximationto yield a total heightin precipitationthe farthereastthe stationis locatedon a 360ø scale. The averagefor theseprecipitation datais -19.9%o15180, andfor meters(termedthe inventoryheight)asa proxyfor volume. Not all the waters in the Arctic Ocean halocline have a stations with monthly rainfall measurementsthe weighted is -18.1%o 15•80.If we werefocusing on determining a Pacific-derivedcomponent. The four-componentmassbalance average calculationswere run on all samplesto determinewhich waters more precise river runoff fraction for a shelf sea, we could use endmemberspecificto thatshelfarea.However,for the containa Pacificcomponent.If the four-component solutionof a the15•80 sampleresultsin a negative f, value,thenPacificwaterwasnot central Arctic Ocean, where all the river sources are mixed present. In this casewe usedthe salinityand15•80dataand together,one end memberis chosen,and sensitivitytestsfor the solved the three-component mass balance equations (i.e., effectof the uncertaintyin eachend memberserveas a guideto equations (1), (2), and(3) withoutthefv.fvS,,andf,15•O, our interpretations. Followingthe methodfirst outlinedby Ostlundand Hut variables). valueswerecalculatedfor eachstationas To preparefor the massbalancecalculation,a representative [1984],the seaice 15180 tracer concentration must be determined for each water mass before it entersthe Arctic Ocean (i.e., Fram Strait, Bering Strait, river mouth, and sea ice at the surface). Using a single-tracer concentrationfor each end member averagesover the natural variability of the system. For example,eachriver doesnot have the exactsame15•Ovalue(Figure3). Sensitivitytestsfor the centralArctic Oceandata reveal that the uncertaintyin the range of each end-memberconcentrationdoes not significantlyaffect the first order resultsobtainedby the mean end-membervalues (see section4.5). The Atlantic water salinity (Table1) is well constrainedand is basedon the Meteor 8 sectionbetweenNorway and Svalbardon the Barents Sea shelf and the Polarstern ARK IX/1 Fram Strait sectionat 79øN [Frank, 1996]. Initial seaice salinitycan have a wide range (5 < S < 12), but multiyear ice has a salinity with a mean value of 4 _+1 [Untersteiner, 1968; Pfirman et al., 1990; Melling and Moore, 1995; Melnikov, 1997]. The Pacific salinity end-memberconcentrationhas been set to 32.7 _+1 [Roachet al., 1995]. the surface water 15180measurement plus the maximum equilibriumfractionationfactorof 2.6 _+0.1%o[Macdonaldet al., 1995; Eicken, 1998]. The Bering Strait data by Cooper et al. [1997]areusedfor the Pacific15180end-member value(-1.1 _+ 0.2%0). The Atlantic PO4* end-member concentrationis based on PO4* data reportedby Broecker et al. [1995] and the Atlantic water data at ARK IV/3 and ARCTIC91 stations close to Fram Strait. The PO4* concentrationfor river runoff was calculated from phosphateandoxygendatafrom the Ob andYeniseyRivers [Makkaveevand Stunzhas,1995] andMackenzieRiver phosphate data[Macdonaldet al., 1987]. The MackenzieRiver phosphatesalinityrelationshipimpliesa net lossof phosphatein the estuary possiblydueto particlescavenging. Extensiveseaice phosphatemeasurements were performedfor all seasonsduring the drifts of the Arctic and AntarcticResearch Institute "North Pole" ice stations NP-22 and NP-23. Melnikov [1997] reports a seasonal decrease (June to November) of phosphateconcentrationsas the ice algae utilize the nutrients (a) Greenland 0o Sea ice Canada co½½½½ meltwater ;iiii inventory height (meters) - 150 ø Russia (b) Greenland 0o Sea ice Canada' •,-co• •• • •• -10 growth -5 inventory II ,, -o height (meters) ..... - 150 ø Russia Plate 2. Net sea ice meltwaterwater columninventoryexpressedas height in metersfor the integrated water columnbetweenthe surfaceand 300 m (or bottommeasurement if shallowerthan 300 m) depthat each station.(a) Positivenumbersare net seaice meltwateradditionand (b) negativenumbersrepresent net freshwaterremovedto form seaice but are depictedhere as positivebar heights. Also shownare the inventoriesFrank [1996] calculatedfor the 1993 ARK IX/4 and 1995 ARK XI/1 stationsusing a threecomponentmassbalancewith slightlydifferentend-memberconcentrations. 9084 EKWURZEL ET AL.: FRESHWATER DISTRIBUTION IN THE ARCTIC OCEAN Table2.Average MonthlyPrecipitation and/5•aO DataforallStations Greater Than65øNLatitude in theGNIP DatabaseMaintainedby the IAEA Station Country Latitude Longitude Average Altitude, m Precipitation per Month, Average •80, %0 mm 71.3øN 68.2øN 156.8øW 133.3øW 7 68 11.1 -17.8 16.2 -27.6 76.1øN 69.1øN 74.7øN 119.2øW 105.1øW 95.0øW 15 27 67 9.9 11.5 10.9 -27.0 -25.8 -27.8 Canada Canada Canada Denmark Canada 80.0øN 68.5øN 72.4øN 76.5øN 82.3øN 85.6øW 81.2øW 78.0øW 68.8øW 62.2øW 10 8 54 77 62 Scoresbysund Greenland Denmark 70.5øN 22.0øW Danmarkshavn Greenland Nord Greenland Denmark Denmark 76.5 øN 81.6øN 18.4øW 16.7øW 12 35 7 6 Barrow Alaska Inuvik N.W.T. MouldBayN.W.T. Cambridge BayN.W.T. Resolute Bay Eureka N.W.T. Hall Beach N.W.T. Pond Inlet N.W.T. Thule Greenland Alert N.W.T. United States of America Canada Canada Canada Canada Ny Alesund IsfjordRadio Norway Norway 78.2øN 78.1øN 11.9øE 13.6øE Racksund Abisko Kiruna Gallivare Sweden Sweden Sweden Sweden 66.0 øN 68.2øN 67.9øN 67.1 øN 17.4øE 18.5øE 20.2øE 20.3øE 432 392 505 359 66.3øN 22.30E 252 68.6øN 80.4øN 69.5øN 69.2øN 33.0øE 58.0øE 61.4øE 86.1øE 46 20 53 Lapptrask Murmansk Krenkel Polar GMO Amderma Dudinka Sweden RussianFederation RussianFederation RussianFederation Russian Federation 7.0 -30.5 14.8 -25.8 16.1 -28.9 15.8 14.0 -24.2 -30.8 37.9 18.3 14.4 -13.8 - 18.5 -25.1 34.5 40.7 - 11.8 -9.6 -13.0 -13.3 32.7 -13.9 -12.9 -13.9 40.2 -12.9 15.8 36.3 -21.4 -15.7 48.1 -15.9 total inventoryheightof usuallymuchlessthan0.5 m. However, sea ice measurementsrecord a decreasingtrend in phosphate certainend-memberuncertaintiescreatea larger error that must available in the sea ice. Statistics for all the NP-22 and NP-23 in content fromyoungseaice(-0.3 _+0.07/•molkg'•) to first-year be keptin mindwheninterpretingthe meanresultspresented seaice (-0.2 _+0.15/•molkg'l) to multiyearseaice(-0.13 _+0.08 the figures. The river runoff inventoryheight error is most /•molkg-l) [Melnikov,1997]. Phosphate measurements in seaice sensitiveto the river and Atlantic /5•aO end members, which fromtheBeaufort Searangefrom-0.05 to - 0.6/•molkg'l, with produce -0.8 to -1.4 m mean absolute inventory height a few individualsamplesevenhigherwithineachice core(R. W. Macdonald, Institute of Ocean Sciences, personal communication, 1998). These phosphate measurements, combinedwith the calculatedoxygensaturationconcentration at salinity4 and temperature-0.2 øC, are usedfor the seaice endmemberPO4*concentration (Table 1). Mixing of water masses,biological production,and decay productsof organicmaterialin the sediments createa widerange of PO4* valuesfor AOS94 ChukchiSeastationscloseto Bering Strait (Figure 5). The maximum PO4* concentrationat the Pacific inflow salinity range for the AOS94 stationsis -1.8-2 differences, respectively, anda maximumabsoluteuncertainty of -1.6 m. Similarly, sea ice meltwater inventoriesare most sensitiveto Atlantic and river /5•aOend members,creatinga maximum absolute error of- 1.8 m. The Pacific water inventoryis sensitiveto the Atlantic PO4* end member, producinga mean differenceof-0.9 m and a maximumdifferenceof 2.5 m in absoluteinventoryheight,andis extremelysensitiveto the PacificPO4* endmember,with mean and maximum differencesin absoluteinventory height of--2.4 and 12 m, respectively.The stationsclosestto BeringStraitare mostsensitiveto the PacificPO4* endmemberbecausetheyhave /•mol kg'l. The PacificPO4* end memberis slightlyhigher highfractionsof Pacificwater. becauseit is basedon Bering Seaphosphatedatacloserto Bering Despitethefactthatthesensitivity testssuggest thefreshwater inventoriesare sensitiveto the PO4* end-memberuncertaintythe Strait[Codispotiand Richards,1968;BjOrk,1990]. errors do not significantlychangethe spatial distributionof 4.5. End-Member Sensitivity Tests Pacific water and thereforeare useful for mappingwater masses in the Arctic Ocean. This is due to the presencein the Arctic of Sensitivitytestswere performedto assessthe impact of the two water masses(Atlantic and Pacific) that carry signalscloseto uncertainties in each end-member concentration on the Atlantic, river runoff, sea ice meltwater, and Pacific fractions. All samples collectedwithin the upper300 m of the watercolumnfrom ARK IV/3, ARCTIC91, AOS94, and ARK XII/1 (a 1996 expedition not reportedin this contribution)werecalculatedusingthe mean theminimum(0.7 /•molkg'•) andmaximum(2.4/•molkg'•) of the observedrange in the global oceanPO4* [Broeckeret al., 1998]. 4.6. Sea Ice end-member concentrations listed in Table 1. These results were Predominantfreshwatercirculationpatternscan be inferred then compared to calculations where one end-member concentration was changedat a time to the maximumerrorvalue from (1) solutionsof the massbalanceequationspresentedalong (seeEkwurzel[1998] for details). Uncertaintiescausean errorin a seriesof sectionsfrom 0 to 300 m depthbetweenthe Barents EKWURZEL ET AL.' FRESHWATER DISTRIBUTION IN THE 1994 .3.33 .3.3.3.3.3 ARCTIC OCEAN 1991 ', .3 ', .3 9085 1987 66666666666 •. •.•'•.•.•. •-- (D •-- 0 0 0 o -5o -lOO -15o -2oo -25o -3oo o -5o -lOO -15o -2oo -25o -3oo 0 • -50 - -100 - -150 - -200 - -250 - -300 - 0 000000 ..; .i: ß • ß I Chukchi 0 Lomonosov tssalMendeleyev Ridge Ridge Makarov Basin ' . . I Lomonosov Ridge o Q• •c,m, • • •o• c=.c_ B•dge Gakkel •m ••sen c3 -4500 0 00000 00000 500 1000 1500 2000 2000 2500 sin 3000 Distance [km] Figure 7. River runoff,sea-icemeltwater,and Pacificwaterfractiondistributionin the upper300 m water columnalongseveralsectionsspacedover severalyearsand threeexpeditionsbetweenthe Chukchi shelf north of Bering Strait and the Barentsshelf north of Svalbard. The year of each expeditionis locatedat the top of the sectionsand the stationnumber codesare P, Polarstern (ARK IV/3); O, Oden (ARCTIC91); and L, Louis S. St-Laurent (AOS94). Figure 1 depicts the location of each station. Numbersaboveeachstationin each sectionare the stationinventoryexpressedas heightin metersfor the integratedwater columnbetweenthe surfaceand 300 m (or bottommeasurementif shallowerthan 300 m) depthat eachstation. For the seaice meltwatersections, positivenumbersreflectfreshwateradditionby seaice meltingandnegativenumbersrepresentfreshwaterremovalduringseaice formation. Shelfnorthof Svalbardandthe ChukchiSea(Figure7) and(2) maps of total freshwater fraction inventories for each station Following theinterpretation of Ostlund andHut [1984],negative sea ice fractions and inventories represent the portion of freshwaterusedto form seaice, whereaspositivevaluesindicate becausethere were significant property shifts in the Arctic freshwatergeneratedby seaice melting. SouthernNansenBasin surfaceandhaloclineduringthe early 1990s. Indeed,theseseries surface and halocline waters sampled in 1987 and 1991 are of sections serveto highlightsomeof thesechanges, particularly formed from inflowing Atlantic water and sea ice meltwater, over the Lomonosov Ridgewherethereis geographic overlap. consistent with haloclinewaterformationprocesses described by (Plates1 and2). The sections do notrepresent a synopticview 9086 EKWURZEL ET AL.: FRESHWATER DISTRIBUTION Steele et al. [1995] and Rudels et al. [1996]. The near-surface waters observedin this region contain as much as 2% of sea ice meltwater,<1% of river runoff, and no Pacific water (Figure 7). A narrow region (-200 km) containing sea ice meltwater immediately north of Svalbard widens to -500 km farther east betweenSvalbardand Franz JosephLand (Plate 2). This may reflect Atlantic the different influence of the Fram Strait Branch of IN THE ARCTIC OCEAN passedover the Lomonosov Ridge, and was responsiblefor higherriver runoff fractionsin the westernand centralAmundsen Basin (in 1991) than were found in the easternAmundsenBasin (in 1993 and 1995). For 1993, Morison et al. [1998] calculateda surfacecurrentof 0.02-0.03m s'l flowingalongtheMendeleyev Ridge toward Greenland,addingsupportto the East SiberianSea source area. water and Barents Sea shelf water on the surface mixed layer and halocline formation [Pfirman et al., 1994; Schauer et al., 1997]. Net sea ice melting extendsto the shelf slopenear SevernayaZemlya, where 1993 (ARK IX/4) sea ice meltwater 4.8. Pacific Water The Pacific fraction, discussed in this context, is not a calculationwith a salinity point of zero but rather is a fraction inventories remain in excess of 1 m [Frank, 1996]. The 1993 represented by the Pacific water salinity. The highest ARK IX/4 datain the NansenBasinat the LaptevSeashelfbreak concentrationof Pacific water, away from the ChukchiSea shelf, showmostly positiveseaice meltwaterinventoriesjust offshore is foundbetween75 and 100 m depthover the ChukchiAbyssal of the shelf break. Plain (AOS94 stations8, 10, and 11) with fractions of >50% of In 1987 and 1991 a sharpfront existsin the NansenBasin,where Pacific water fraction in the core of the 33.1 salinity water we observe a shift in surface and halocline source waters from (Figure 7). A measurablePacific water fraction in the 33.1 net sea ice melting to net sea ice formation, a river runoff salinity water does not extend all the way to the Lomonosov increase, and the absence of Pacific water (Plates 1 and 2). Ridge in the AOS94 data; only the more buoyant (S < 33.1) ARCTIC91 station 58, ARK IV/3 station 358, and ARCTIC91 surfaceportion of the Pacific inflow extendsto the Lomonosov station 10 trace this front. Ridge. One Chukchi Sea shelf slopestation,AOS94 station6, has a lower Pacific water fraction near 33.1 salinity than do the 4.7. River Runoff adjacent stations, suggestingthat the AOS94 section does not The most strikingfeature of the massbalancesolutionsis the cross the shelf at the dominant location for UHW renewal. The high river runoff fractions (>14%) in the upper 50 m over the high Pacific water fraction core at the Chukchi Abyssal Plain Mendeleyev Ridge (AOS94 stations 14, 16, 17, 18, and 20 in stationsis associated with a NO/PO(135 ) signatureof MECS. Figure 7). The NO/PO(135 ) ratio for the MendeleyevRidge Thereforesomeof the Pacificinflow probablyspreadstowardthe surfacewaterspointsto an East SiberianSea sourcearea(Figure East SiberianSea, renewsthe haloclinenear 33.1 salinity,flows 6). Dissolved barium concentrationssuggestMackenzie River off the shelf, and spreadstoward the Chukchi AbyssalPlain. water may alsobe presentover the MendeleyevRidge [Guay and Herald Canyon is the closestsubmarinecanyonto the Chukchi Falkner, 1997]. Hencein 1994 the dominantlocationfor mixing Abyssal Plain stations (Figure 1) and may guide the denser of river runoff that has left the Siberianshelf is the Mendeleyev Pacific water to the ChukchiAbyssalPlain stations. Weingartner Ridge. et al. [1998] analyzed1991-1992 currentmeter and temperature On the Chukchi shelf slopewe observedhigherupper water and salinity mooring data and concluded that December and columnriver runoff percentagesthan in the adjacentshelf waters January, when open water areas had high heat loss, was the or in waters seaward of the shelf break (Figure 7). AOS94 period of most dense water formation and fresh Bering Strait station 6 located over the Chukchi Sea shelf slope has higher inflow wasdivertedthroughHopeValley towardHeraldCanyon. river runoff fractions (>10% in the upper 50 m) than does The surfacesamplesfor the Pacificwaterpercentage in Figure Chukchi Sea shelf station 2 (>2%) and Chukchi Abyssal Plain 7 can be comparedwith the Pacific water calculationsof Joneset stations8, 10, and 11 (>6% in the upper 50 m). This suggests al. [1998] for the upper 30 m. The Pacific water percentagefor strongtopographicsteeringof river runoff flow after it has left surfacesamplesdepictedin Figure7 are slightlylower thanthose the shelf break. of Jones et al. [1998] becausethe nitrate-phosphatemethod Three componentmassbalancecalculationsfor the 1995 Kara calculates the Atlantic water and Pacific water fractions but does Sea and 1992, 1993, and 1995 Laptev Sea demonstratethat the not separateout the river runoff or sea ice meltwater fractions. high river runoff fractions on the shelf decreasedramatically Thesedifferences are smallcompared to thegeneralsimilarity seaward of the shelf break [Bauch, 1995; Frank, 1996]. In betweenthesetwo differentapproaches thatreconstruct a similar contrast,the upper 50 m of the Makarov Basin alongthe AOS94 spatialdistributionfor the Pacific water mass. section contained more than 10% river runoff. The LouisS. St-Laurentcrossedthe Lomonosov Ridgein 1994closeto the stationsOdenoccupiedin 1991providingan opportunity to examinetemporalchangesin waterproperties at These combined river runofffractionresultssuggest thatduringtheearly 1990sa significantportionof Pechora,Ob, Yenisey,Kotuy, and Lena river waters did not flow off the shelf closest to their river deltas. the samelocation. Near the LomonosovRidge, stationsfrom Instead, they remained mostly on the shelf, circulating bothcruisesare almostidenticalwith respectto river runoffand cyclonicallyinto the East Siberian Sea, where most of the river seaice fractions,but the AOS94 stationsexhibit a significant runoff moved off the shelf along the MendeleyevRidge and decrease in Pacific water fraction and increase in Atlantic water Chukchi Plateau. fraction comparedto the ARCTIC91 stations(Figure 8). Furthermore,the river runoff inventorydistribution(Plate 1) ARCTIC91 station 23 contains more than 18% Pacific water near in combination with the NO/PO(135 ) results(Figure6) andthe thesurfacecompared to AOS94station30, whichcontains only barium data [Guay and Falkner, 1997; Guay et al., 1999] 3-4% at the surface. This is equivalentto a decrease in Pacific suggests thefollowingcentralArcticOceancirculation pattern:at water inventory from 9.2 m in 1991 to 1.6 m in 1994. the Mendeleyev Ridge and Chukchi Plateauthe Eurasianriver Essentially,the 13 m Pacific water inventoryisoplethshifted water mixed with North American river water, circulated across from directly over the Lomonosov Ridge in 1991 to the the CanadaBasin(andperhapsinto the CanadianArchipelago), MendeleyevRidge in 1994 (Plate 1). EKWURZEL (a) o ,_, 50 ET AL.' DISTRIBUTION 9087 •, 5o 4.9. PossibleCausesfor Observed Arctic Ocean Changes Questionsremain as to the causesfor theseobservedchanges in the Arctic Ocean basin, but mounting atmospheric and oceanographicevidencepoints to potentialforcing mechanisms. Atmosphericpressurefield shiftsknown as the Arctic Oscillation A ARCTIC91-21 c31O0•/•,RCTIC91-23 A, ARCTIC91-26 n AOS94-29 n AOS94-29 ß AOS94-30 ß AOS94-30 and the NAO 150 150 0 OCEAN contribution has quantified these shifts, which now can be incorporated into modelsthattestproposedforcingmechanisms. AARCTIC91-26 10 IN THE ARCTIC (b) o AARCTIC91-21 •/•RCTIC91-23 100 FRESHWATER -30 20 -20 (d) (c) o achieved an extreme state for winter climatic -10 Sea ice meltwater fraction [%] River runofffraction [%] forcing over the Arctic and Nordic Seasin the 1990s [Dicksonet al., 1996; Walsh et al., 1996; Thompsonand Wallace, 1998; Dickson, 1999]. The shift in atmosphericforcing is associated with a decreasein the extent of the Beaufort gyre, a shift to a cyclonic wind-forced circulation, and a decreasein the extent of v• '•/• Lomonosov % •, 50 - c310• k Basin n Makaro •• • • Ridge t• i•(• / Amundsen Arctic sea ice [Johannessen et al., 1995; Serreze et al., 1995; Maslanik et al., 1996; Proshutinsky and Johnson, 1997; Levi, 1 2000]. •/•,RCTIC91-23 210' n AOS94-29 •• 10 20 • • •. 30 inflow that renews the halocline near 33.1 salinity. Coachman and Barnes [1961] definedthisportionof the haloclineasWinter BeringSeaWater. Cooperet al. [1997] define it as a winter ventilationsignature. Treshnikov[1985] presentsa silicatesectionfor a locationsimilar to that of the AOS94 section. The Treshnikov 1989 that towardlessanticyclonicwind forcing). Amundsen Basin stations for a cruise in 1996 (ARK XII/1) just east of ARCTIC91 stations record a decreasein the river Historical data can be used to compare temporal changesin Pacific Sea after o. Figure 8. Comparisonof 1991 and 1994 stationsnear the LomonosovRidge for (a) river runoff fraction, (b) sea ice meltwater fraction (positive = meltwater; negative = ice formation), (c) Pacific water fraction, and (d) location for thesestations. of the denser in the Beaufort coincides with a shift in the Arctic Oscillation index (i.e., a shift Pacificwater fraction [%] the distribution in sea ice melt Basin ß AOS94-30 15g '-0 Macdonald et al. [1999] document a dramatic 4-6 m increase A, ARCTIC91-26 section starts at runoff inventories from 10 to 12 m in 1991 to 6 to 8 m in 1996 [Ekwurzel, 1998]. These temporal and spatially separateddata setslend supportto Steele and Boyd's[1998] hypothesisfor the retreatof the cold haloclinelayer they observedin 1995 from the Eurasian Basin as a shift in Siberian shelf river runoff outflow from the AmundsenBasin to the Makarov Basin. Thus dynamic atmosphericforcing may be reorganizing the mean freshwater circulationpatternsenoughto diminishthe freshwatercap in the AmundsenBasinduringthe 1990s. BjSrk [1990] useda one-dimensionalcirculationmodel for the upper 300 m of the Arctic Ocean and found that varying the Bering Strait inflow had a significant impact on the halocline nutrient profiles. Winter hydrographicsurveysin the Beaufort Sea and southern Canada Basin, between 1979 and 1996 are a 71øN, 175øW, near Wrangel Island (near AOS94 station 2 at 72ø08'N, 168ø50'W) and ends at 89øN, 90øE, i.e., north of ARCTIC91 station 18 (88ø11'N, 99øE). UHW has traditionally been associatedwith a nutrient maximum. Therefore variability in the extent of UHW can be determinedby comparingsilicate sections to the Treshnikov silicate section (Figure 9). The remarkabletime serieswhere Melling [1998] observedcooling (by 0.12øC at the 34.5 isohaline) of the lower halocline, a thickeningof the Atlantic layer, and the warming and freshening of the water below the Atlantic water temperaturemaximum in the 1990s. Melling [1998] calls upon the weakening of the Siberian High to have caused a decrease in upper halocline farthestnorthwardextentof the 20/,tmolkg't silicatemaximum renewal from the Pacific side. on the AOS94 sectionis to the southernedge of the Mendeleyev AOS94 stationsandmay lie alonga path wherethe Pacific water Furthermore,Carmack et al. [1995, 1997], and Swift et al. [1997] propose that the warmer Atlantic water, found in the Makarov Basin in 1993 and 1994, may arise from warmer Atlantic inflow sourcewaters in the Norwegian Sea. The most positive phase of the NAO in the late 1980s and early 1990s resultedin an increasein temperatureof Atlantic water flowing north through Fram Strait to the highest recorded values [Dickson, 1999]. Zhang et al. [1998] presentmodel resultsfor the 1989-1996 time period of a 0.2 Sv increaseof Atlantic water inflow through Fram Strait and a 0.5 Sv increasethrough the Barents Sea branch, which is balanced by a 0.7 Sv increase in outflow of Arctic water through Fram Strait. This leads to an circulates to Fram Strait. increase of heat and salt in the Arctic Ocean and a decrease in ice Ridge, whereasthe 20 /,tmolL't silicate contouron the Treshnikov section extends across the Makarov Basin to the LomonosovRidge. Silicateconcentrations up to 40/,tmolkg'• weremeasured at T3 Ice station 703 in the Canada Basin near the center of the Alpha-MendeleyevRidge in 1969, over the Alpha Ridge from CESAR Ice stationin 1983, and at the LomonosovRidge near the North Pole in 1979 from LOREX Ice station[Kinneyet al., 1970; Moore et al., 1983; Jones and Anderson, 1986]. The T3 and CESAR stations are located closer to the Canadian shelf than the Other Canadian Basin data record the shifts in the Atlantic water and Pacific water front in the early 1990s. McLaughlinet al. [1996] andMorison et al. [1998] report data collectedin 1993 that record the frontal positionbetween Atlantic and Pacific water masses at the Alpha-Mendeleyev Ridge, and Carmacket al. [1997] also documentthis feature using data from 1994. The mass balance approachof this volume. They proposethat the increasedpenetrationof Atlantic water into the Arctic Ocean is linked to a weakening of the Beaufortgyre and an increasein cyclonicsystemsthat track into the Arctic from the North Atlantic. UHW distributionduring the early 1990s has changed from that seen during surveys in the 1960s, 1970s,and 1980s.This changemay be part of a shift 9088 EKWURZEL (a) ET AL.' FRESHWATER DISTRIBUTION IN THE ARCTIC Svalbard Chukchi Sea 1994 , oooo , , , , c:x::x:•,, , , , 'T•T•T 'T , , , , , a•a• a• , 1987 1991 ', ',', ', o OCEAN , , , , , , 6666666 oo , -5O .-. -100 E • -150 Q -200 -250 -300 I 0 Chukchi MendeleyevRidge Ridge Makarov •.. • Basin • -4500 0 500 t 1000 1500 Distance [km] (169 ø W, 72 ø N) ' I Lomonosov Lomonosov 2000 • Gakkel Ridge c • 2000 t • •-• Ridge Nansen Basin 2500 3000 Distance [km] (90ø N) (b) o -5o• -lOO • -150Q ,-200 t t (175ø W, 71ø N) (90ø E, 89 ø N) Figure 9. (a) Silicate(#mol kg']) sections followingthe formatof Figure7. Silicatedataare from Anderson et al. [1989' 1994], and Swiftet al. [1997]. (b) Silicate(#mol L-]) sectionadaptedfrom Treshnikov [ 1985]. between the balance of Atlantic and Pacific inflow strengthin combinationwith the shiftsin the atmospheric circulation. 4.10. Renewal of Halocline Waters Rudelset al. [1996] suggestthat the haloclinewaters,once formed,follow the Atlanticwatercirculation.The 3H-3Heage profilerepresents thebalanceof watersrenewingthedensest part of the halocline combined with shear between the LHW (slow circulation)and the Atlantic water (relativelyrapid circulation). Cavalieri and Martin [1994] calculate the total contributionof dense watersfrom Arcticcoastalpolynyasto thecoldhaloclineto for thetracercontentto be reduced by a factorof 1/e. The3Hbe 0.7-1.2 Sv. Presumably,a smaller portion of this total 3Heagediffersfromthisbulkresidence timebecause thetracer renewsthedensest portionof thehaloclineeachyear. age representsthe mean transittime from the sourceregiondue production This is a much smaller rate when compared to the flow of to advection and mixing processes. Becker and BjOrk [1996] injected river water with age zero and a constant tritium Atlantic water estimated to be 1 Sv from the Fram Strait branch concentrationat each model level to producean age distribution [Rudels,1987] and2 Sv from the BarentsSeabranch[Blindheim, 19891. of the relative amount of each year's river runoff within each LHW directlyover the GakkelRidge andLomonosovRidge layer. The weightedmean age from the age distributionwithin There are severalways to define the "age"of water. Becker and BjOrk [1996] define bulk residencetime as the time it takes agesthanthewaters ontheflanksof eachmodellevelgivesa similarageto theARK IV/3 3H-3He (~ 150m) hashigher3H-3He resultsusedin their comparison. each ridge (or in the caseof the Gakkel Ridge, morecentral As first observed in 1987 data [Schlosser et al., 1990], the ARCTIC91 and AOS94 stations, with the exception of those Nansen Basin and Amundsen Basin stations). Halocline age contoursdome over the Gakkel Ridge and LomonosovRidge fromthesouthern NansenBasin,have3H-3Heagesthatincrease (Figure10). Immediatelybelowthe haloclinewaters,overthe Ridge,the trendof agecontours reverses: relatively graduallyfrom the surfaceto the lower halocline(~34.2 salinity) Lomonosov recentlyrenewedwaterextendsdeeperto the ridgecompared to and then increasedramaticallyto 34.5 salinity. This increasein watersin theMakarovBasin. age gradientis followed by a decreasein age gradientto the relativelyslowrenewalof adjacent The highest3H-3Heagesare foundover the GakkelRidge Atlantic water (Figure 10). EKWURZEL ET AL.' FRESHWATER DISTRIBUTION IN THE ARCTIC OCEAN 9089 Svalbard Chukchi Sea 1994 1991 1987 6666 r,j:) 0 CO 001 CO 4 -50 8 ,--. -lOO E ß= 12. 14 150 • -200 -250 -300 Lomonosov Chukchi Ridge c Mendeleyev Ridge Makarov Lomonosov > Ridge Basin •) Gakkel •: Ridge Nansen Basin -4500 500 1000 1500 2000 2000 2500 3000 Distance [km] Distance [km] Figure10. 3H-3He agesections followtheformatof Figure7. (ARCTIC91 stations11, 12, 14-16, 48, and 49) and the Makarov Basinadjacentto the LomonosovRidge(ARCTIC91 station26). The mean3H-3Heage measured for the 34.2 e 0.2 salinity surfaceis 9.6 e 4.6 years. Despite the weaknessof the central However, there is a narrow band of low tracer age water on the Arctic Ocean mean flow field, propertiescould be transported EurasianBasin flanks of the Gakkel Ridge and the Lomonosov rapidly acrossthe basin by eddies [Manley and Hunkins, 1985; Ridge. This patternimpliesa rapidcycloniccirculationaround Aagaard and Carmack, 1994]. A high-resolutionmodel forced the EurasianBasinperimeterin narrowboundarycurrentssimilar with 1992 winds calculated (1) increased annual mean velocities within 0-45 m at the shelf slopeperimeterthroughoutmostof the to thepatterndescribed by RudeIset al. [1994]. LHW agesdo not exhibita monotonic 3H-3Heageincrease from the Eurasian Basin to the Canada Basin. The age distribution implies either that this portion of the LHW is frequentlyventilatedfrom localshelfsources or isopycnalmixing Arctic Ocean, the Barents Sea shelf, Chukchi Plateau, and the Alpha-Mendeleyev Ridge and (2) high surface eddy kinetic energyin the CanadaBasin, especiallynear the ChukchiPlateau (W. Maslowski, personal communication, 1998). The3H-3Heage Chukchi Abyssal Plain LHW stations show mixing between Barents Sea and Chukchi Sea plus East Siberian Sea waters. Most LHW at Canadian Basin stationsin this study retain the resultssupportthe notion that boundarycurrentscarry the major properties along the Arctic Ocean perimeter (Atlantic water inflow and shelf renewal) and the eddiesrapidly transportthese propertiesinto the interior. Similar trendshavebeenobservedin BarentsSea NO/PO(135 ) signature. This suggeststhat shelf the Canada Basin [Smethie et al., 2000]. renewal at this salinity is usually a minor addition in the CanadianBasin. Isopycnalmixingis quiterapidaccordingto the tracerages,with renewalfrom the shelfsourcesprobablybeinga 5. Conclusions is quite rapid on the basin scale. The NO/PO(135 ) datafrom second-order effect. UHW 3H-3Heageswere remarkablyuniformat 4.3 e 1.7 years. However,thereare slightdifferencesthat may be related to broadcirculation patterns.Slightlyyounger 3H-3He ages(-2-5 The salinity,nutrient,dissolvedoxygen,•5•80,tritium, and heliumdatacollectedin the early 1990sextendour insightinto the possiblecirculationpatternswithin the Arctic Ocean. AOS94 stationsdid not samplethe Beaufort Gyre but insteadwere years) over the Mendeleyev Ridge, comparedto the adjacent ChukchiAbyssalPlain stations(-9 years), are further evidence for thejunctureof the MendeleyevRidge and the East Siberian Sea as the main releasepoint from the shelf for freshwaterflow an UHW renewal location (East Siberian Sea close to the Chukchi Abyssal Plain). These tracer data also establish the in 1994. freshwaterfrom BarentsSea, Kara Sea, LaptevSea, and East Most of the Eurasian Basin waters in the 33.1 _+ 0.3 situatedcloseto theEastSiberianSeaandcapturedevidencefor MendeleyevRidgeasthe main areafor river runoff(a mixtureof salinityrangedo not exhibitthe nutrientpeakanddo not contain Siberian Sea) to flow off the shelf in 1994. This freshwater Pacific-derived UHW. The stations that do have determinable mixes with freshwater from the Canadian rivers and circulates Pacificwaterfractionsdemonstrate a slightincreasein 3H-3He throughtheCanadian BasinbackovertheLomonosov Ridge. It agefrom the LomonosovRidge (- 3-4 years)to the Morris Jesup formsa freshwatercap within the westernAmundsenBasinand north of Greenland. Plateau(-8 years). 9090 EKWURZEL ET AL.: FRESHWATER DISTRIBUTION PO4* is an effectivetracer,in combination with 8•80 and salinity,for calculatingthe Pacific water fractionof surfaceand halocline waters. The balance between Pacific and Atlantic water fractions changedbetween 1991 and 1994 over the Lomonosov Ridge, signaling the retreat of the Pacific water front from the Lomonosovto the Mendeleyev Ridge. It remainsto be seen if this shift is temporaryor long term. Dramatic changesbeganto occur in the convectiveregionsof the north Atlantic suchas the Greenland Sea in the 1980s [Schlosser et al., 199; B6nisch et al., 1997] and now dramatic shifts are occurring between water massesin the Arctic Ocean in the 1990s. Evidencesuggeststhat changesin atmosphericforcingregimescollectivelyknown as the Arctic Oscillationand NAO may be driving the reorganizationof freshwater masses in the Arctic Ocean. Acknowledgments. This researchrelied on the generoussupportof Louis S. St-Laurent. Numerous Water masses and circulation OCEAN in the Eurasian Basin: Results from the Oden91 expedition,J. Geophys.Res.,99, 3273-3283, 1994. Bauch,D., Thedistribution of 15180 in theArcticOcean:Implications for the freshwaterbalanceof the haloclineand the sourcesof deepand bottomwaters,Rep. 159, 144 pp., Alfred WegenerInst. for Polar and Mar. Res.,Bremerhaven,Germany,1995. Bauch, D., P. Schlosser, and R.G. Fairbanks, Freshwater balance and the sourcesof deep and bottom watersin the Arctic Oceaninferred from thedistribution of H2180,Prog.Oceanogr., 35, 53-80,1995. Bayer, R., P. Schlosser, G. B/3nisch, H. Rupp, F. Zaucker, and G. Zimmek,Performanceandblankcomponents of a massspectrometric systemfor routinemeasurement of heliumisotopesand tritiumby the 3Heingrowth method., Sitzungsber. 5 Heidelberger Akad.der WissenschaftenMathematisch-NaturwissenschaftlicheKlasse, Berlin, Germany1989. Becker,P., The effect of Arctic river hydrologicalcycleson Arctic Ocean circulation, Ph.D. thesis, Old Dominion Univ., Norfolk, Va., 1995. the chief scientists,officers, and crew of the FS Polarstern, IB Oden, and CCGS IN THE ARCTIC individuals from the Alfred Wegener Institute for Polar and Marine Research(Germany), Bedford Institute of Oceanography (Canada), Institute of Ocean Sciences (Canada), Scripps Institution of Oceanography(United States), and Universityof G/3teborg(Sweden)providedhigh-qualityhydrographicand nutrient data. Measurementof the unpublishedportion of the tritium, helium,and180dataanalyzedin thisstudybenefited fromtheassistance of N. Asif, G. BOnisch,D. Breger,D. Darro, S. Hellman,S. Khatiwala,L. Kohandoust, A. Ludin, G. Mandal, M. K. Mendelson, M. Peacock, R. Weppernig, D. Smith, and F. Zaucker. We thank Rob Macdonald for sharinghis sea ice phosphatemeasurements from the BeaufortSea, and HajoEickenfor hisinsights regarding 15180 fractionation in seaice. The sectionsand mapswere producedusing GMT [Wesseland Smith, 1995]. The invaluable assistanceof Vicki Childers much improved the GMT sectionand map figures. Critical reviews on early drafts by Stephanie Pfirman, JamesSimpson,and William SmethieJr. are muchappreciated. Anonymousreviewers provided valuable suggestionsfor improving the manuscript.The W. M. Keck Foundation provided the funds for the helium isotopemassspectrometers usedto measurethe helium samples analyzedin this study. Financialsupportwas providedby the Office of Naval Research (grants N0014-90-J-1362, N00014-94-1-0809, and N00014-93-1-0830) and the National ScienceFoundation(grantDPP 9022890). LDEO contribution6098. References Aagaard,K., and E.C. Carmack,The role of seaice and otherfreshwater in the Arctic circulation,J. Geophys.Res., 94, 14,485-14,498, 1989. Aagaard, K., and E.C. Carmack, The Arctic Ocean and climate: A perspective,in The Polar Oceans and Their Role in Shaping the Global Environment,Geophys.Mongr. 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(ReceivedJuly14, 1999;revisedSeptember 11, 2000; acceptedNovember7, 2000.)