Download River runoff, sea ice meltwater, and Pacific water distribution and

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. Ser. vol. 85, edited by O.M.
Johannessen,R.D. Muench, and J.E. Overland, pp. 5-20, AGU,
Washington,D.C., 1994.
Aagaard, K., L.K. Coachman,and E. Carmack, On the haloclineof the
Arctic Ocean,Deep Sea Res.,Part A, 28, 529-545, 1981.
Aagaard,K., J.H. Swift, and E.C. Carmack,Thermohalinecirculationin
the Arctic Mediterraneanseas,J. Geophys.Res.,90, 4833-4846, 1985.
Anderson, L., and J. Sarmiento, Redfield ratios of remineralization
determinedby nutrientdata analysis,Global Biogeochem.Cycles,8,
65-80, 1994.
Anderson,L.G., Chemical oceanographyof the Arctic and its shelf seas,
in Arctic Oceanography: Marginal Ice Zones and Continental
Shelves,CoastalEstuarineStud. vol. 49, editedby W.O. Smith Jr. and
J.M. Grebmeier,pp. 183-202,AGU, Washington,DC, 1995.
Anderson, L.G., and E.P. Jones, Transport of CO2 into Arctic and
Antarctic seas:Similarities and differences in driving processes,J.
Mar. Syst.,1, 81-95, 1991.
Anderson,L.G., E.P. Jones,R. Lindegren,B. Rudels,and P.I. Sehlstedt,
Nutrient regenerationin cold, high salinity bottomwater of the Arctic
shelves,Cont. ShelfRes., 8, 1345-1355, 1988.
Anderson, L.G., E.P. Jones, K.P. Koltermann, P. Schlosser, J.H. Swift,
and D.W. Wallace, The first oceanographicsectionacrossthe Nansen
Basinin the Arctic Ocean,Deep Sea Res.,Part A, 36, 475-482, 1989.
Anderson, L.G., G. BjOrk, O. Holby, E.P. Jones, G. Kattner, K.P.
Koltermann, B. Liljeblad, R. Lindegren, B. Rudels, and J. Swift,
Becker, P., and G. Bj/3rk, Residencetimes in the upperArctic Ocean,J.
Geophys.Res., 101, 28,377-28,396, 1996.
Bj/3rk, G., The vertical distributionof nutrientsand oxygen 18 in the
upperArctic Ocean,J. Geophys.Res.,95, 16,025-16,036,1990.
Blindheim,J., Cascadingof BarentsSeabottomwaterinto theNorwegian
Sea,Rapp. P. V. Reun. Cons.Int. Explor. Mer., 188, 49-58, 1989.
Blindheim,J., V. Borovkov, B. Hansen,S.-A. Maimberg, W.R. Turrell,
andS. Osterhus,
Upperlayercoolingandfreshening
in theNorwegian
Seain relationto atmosphericforcing,Deep Sea Res.,Part A, 47, 655680, 2000.
B6nisch, G., J. Blindheim, J.L. Bullister, P. Schlosser, and D.W.R.
Wallace, Long-term trends of temperature, salinity, density, and
transienttracersin the centralGreenlandSea, J. Geophys.Res., 102,
18,553-18,571, 1997.
Broecker, W.S., "NO," A conservative-mass tracer, Earth Planet. Sci.
Lett., 23, 100-107, 1974.
Broecker, W.S., T. Takahashi, and T. Takahashi, Sources and flow
patternsof deep-oceanwatersas deducedfrom potentialtemperature,
salinity, and initial phosphateconcentration,J. Geophys.Res., 90,
6925-6939, 1985.
Broecker,
W.S., S. Sutherland,
W. Smethie,T.H. Peng,andG. Ostlund,
Oceanicradiocarbon:Separationof the naturalandbombcomponents,
Global Biogeochem.Cycles,9, 263-288, 1995.
Broecker, W.S., S.L. Peacock, S. Walker, R. Weiss, E. Fahrbach, M.
Schroeder,U. Mikolajewicz, C. Heinze, R. Key, T.-H. Peng, and S.
Rubin, How much deep water is formed in the SouthernOcean?,J.
Geophys.Res., 103, 15,833-15,843,1998.
Cannack, E.C., Large-scalephysical oceanographyof polar oceans,in
Polar Oceanography,part A, PhysicalScience,editedby W.O. Smith
Jr., pp. 171-222,Academic,SanDiego,California,1990.
Carmack, E.C., R.W. Macdonald, R.G. Perkin, F.A. McLaughlin, and
R.J. Pearson,Evidencefor warming of Atlantic water in the southern
Canadian
Basin
of the Arctic
Ocean:
Results
from
the Larsen-93
expedition,Geophys.Res.Lett.,22, 1061-1064, 1995.
Carmack, E.C., K. Aagaard, J.H. Swift, R.W. Macdonald, F.A.
Mclaughlin,E.P. Jones,R.G. Perkin,J.N. Smith, K.M. Ellis, and L.R.
Killius, Changesin temperatureand tracer distributionswithin the
Arctic Ocean:Resultsfrom the 1994 Arctic Ocean section,Deep Sea
Res. Part B, 44, 1487-1502, 1997.
Cavalieri, D.J., and S. Martin, The contribution of Alaskan, Siberian, and
Canadiancoastalpolynyasto the cold halocline layer of the Arctic
Ocean,J. Geophys.
Res.,99, 18,343-18,362,
1994.
Clarke, W.B., W.B. Jenkins,and Z. Top, Determinationof tritium by
massspectrometric
measurement
of 3He,Int. Appl.Radiat.Isot.,27,
515-527, 1976.
Coachman,L.K., and C.A. Barnes,The contributionof Bering Sea water
to the Arctic Ocean, Arctic, 14, 147-161, 1961.
Coachman, L.K., and C.A. Barnes, Surface water in the Eurasian Basin of
the Arctic Ocean, Arctic, 15, 251-277, 1962.
Coachman, L.K., K. Aagaard, and R.B. Tripp, Bering Strait: The
Regional Physical Oceanography,172 pp., Univ. of Wash. Press,
Seattle, 1975.
Codispoti,L.A., and F.A. Richards,Micronutrientdistributionsin the
East Siberianand Laptev seasduringsummer1963,Arctic, 21, 67-83,
1968.
Cooper,L.W., T.E. Whirledge,J.M. Grebmeier,andT. Weingartner,The
nutrient,salinity, and stableoxygen isotopecompositionof Bering
and Chukchi Seas waters in and near the Bering Strait, J. Geophys.
Res., 102, 12,563-12,573, 1997.
EKWURZEL
ET AL.: FRESHWATER
DISTRIBUTION
Craig, H., Standard for reporting concentrationsof deuterium and
oxygen-18in naturalwaters,Science,133, 1833-1844,1961.
Craig, H., and B. Hom, Relationshipsof deuterium, oxygen 18, and
chlorinityin the formationof sea ice, Eos Trans. AGU, 49, 216-217,
1968.
Dickson,B., All changein the Arctic,Nature, 397, 389-390, 1999.
Dickson, R.R.J., J. Meincke, S.-A. Maimberg, and A.J. Lee, The "great
salinity anomaly"in the northernNorth Atlantic 1968-1982, Prog.
Oceanogr.,20, 103-151, 1988.
Dickson, R., J. Lazier, J. Meincke, P. Rhines, and J. Swift, Long-term
coordinatedchangesin the convectiveactivity of the North Atlantic,
Prog. Oceanogr.,38, 241-295, 1996.
Eicken, H., Deriving modesand ratesof ice growth in the Weddell Sea
from microstructural,
salinityand stable-isotope
data,in Antarcticsea
ice: Physical Processes,Interactions and Variability, Antarctic Res.
Ser., vol. 74, editedby M. O. Jeffries,pp. 89-122, AGU, Washington,
D.C., 1998.
Ekwurzel, B., Circulation and mean residence times in the Arctic Ocean
derived from tritium, helium, and oxygen-18 tracers,Ph.D. thesis,
Columbia Univ., New York, 1998.
Epstein, S., and T. Mayeda, Variation of oxygen-18 contentof waters
from natural sources,Geochem.Cosmochim.Acta, 4, 213-224, 1953.
Fairbanks,R.G., The origin of continentalshelf and slopewater in the
NewYorkBightandGulfof Maine:Evidence
fromH2•80/H2•60
ratio
measurements,
J. Geophys.Res.,87, 5796-5808, 1982.
Frank, M., Spurenstoffuntersuchungen
zur zirkulation im Eurasischen
Becken des Nordpolarmeeres,Ph.D. thesis, Ruprecht Karls Univ.
Heidelberg,Heidelberg,Germany,1996.
IN THE ARCTIC
OCEAN
9091
Macdonald,R.W., andE.C. Carmack,The role of large-scale
under-ice
topography
in separating
estuary
andoceanonanArcticshelf,Atmos.
Ocean, 29, 37-53, 1991.
Macdonald,
R.W., C.S. Wong,andP.E. Erickson,The distribution
of
nutrientsin the southeasternBeaufort Sea: Implications for water
circulation
andprimaryproduction,
J. Geophys.
Res.,92, 2939-2952,
1987.
Macdonald,R.W., E.C. Carmack,F.A. McLaughlin, K. Iseki, D.M.
Macdonald,and M.C. O'Brien, Compositionand modificationof
water massesin the Mackenzie shelf estuary,J. Geophys.Res., 94,
18,057-18,070, 1989.
Macdonald,R.W., D.W. Paton,E.C. Carmack,and A. Omstedt,The
freshwater
budgetandunder-icespreading
of the MackenzieRiver
waterin theCanadian
BeaufortSeabasedon salinityand•80/•60
measurements
in waterandice,J. Geophys.
Res.,100, 895-919,1995.
Macdonald,R.W., E.C. Carmack,F.A. McLaughlin,K.K. Falkner,and
J.H.Swift,Connections
amongice,runoffandatmospheric
forcingin
theBeaufort
Gyre,Geophys.
Res.Lett.,26, 2223-2226,1999.
Makkaveev,P.N., andP.A. Stunzhas,
Hydrochemical
characteristics
of
theKaraSea,Oceanology,
Engl.Transl.,34, 600-605,1995.
Manley,T.O.,andK. Hunkins,
Meso-scale
eddies
of theArcticOcean,J.
Geophys.
Res.,90, 4911-4930,1985.
Maslanik,J.A.,M.C. Serreze,andR.G. Barry,Recentdecreases
in Arctic
summerice coverandlinkagesto atmospheric
circulation
anomalies,
Geophys.
Res.Lett.,23, 1677-1680,1996.
McLaughlin, F.A., E.C. Carmack, and R.W. Macdonald,Physical and
geochemical
propertiesacrossthe Atlantic/Pacificwatermassfront in
the southernCanadianBasin,J. Geophys.Res.,101, 1183-1197, 1996.
Gonfiantini,R., The •5-notation
andthe mass-spectrometric
measurement Melling, H., Hydrographicchangesin the CanadaBasin of the Arctic
techniques,
in StableIsotopeHydrology:Deuteriumand Oxygen-18in
Ocean,1979-1996,J. Geophys.Res.,103, 7637-7645, 1998.
the WaterCycle,editedby J.R. Gat and R. Gonfiantini,pp. 35-84, Int.
Melling, H., and R.M. Moore, Modification of halocline sourcewaters
At. EnergyAgency,Vienna, 1981.
during freezing on the Beaufort Sea shelf: evidencefrom oxygen
Gordeev, V.V., J.M. Martin, I.S. Sidorov, and M.V. Sidorova, A
isotopesanddissolvednutrients,Cont.ShelfRes.,15, 89-113, 1995.
reassessment
of the Eurasianriver input of water, sediment,major
Melnikov, I.A., The Arctic Sea Ice Ecosystem,204 pp., Gordon and
elements, and nutrients to the Arctic Ocean, Am. J. Sci., 296, 664-691,
1996.
Grebmeier, J.M., L.W. Cooper, and M.J. DeNiro, Oxygen isotopic
compositionof bottom seawater and tunicate cellulose used as
indicatorsof water massesin the northernBering and ChukchiSeas,
Limnol. Oceanogr.,35, 1182-1195, 1990.
Guay, C.K., and K.K. Falkner,Bariumas a tracerof Arctic haloclineand
river waters,Deep SeaRes.,Part B, 44 (8), 1543-1569, 1997.
Guay, C.K., G.P. Klinkhammer, K.K. Falkner, R. Benner, P.G. Coble,
T.E. Whitledge, B. Black, F.J. Bussell, and T.A. Wagner, Highresolutionmeasurementsof dissolvedorganic carbon in the Arctic
Ocean by in situ fiber-optic spectrometry,Geophys.Res. Lett., 26,
1007-1010, 1999.
Hakkinen, S., An Arctic source for the Great Salinity Anomaly: A
simulationof the Arctic ice-oceansystemfor 1955-1975,J. Geophys.
Res., 98, 16,397-16,410, 1993.
Johannessen,
O.M., M. Miles, and E. Bjorgo, The Arctic'sshrinkingsea
ice, Nature, 376, 126-127, 1995.
Jones,E.P., and L.G. Anderson,On the origin of the chemicalproperties
of the Arctic Ocean halocline,J. Geophys.Res., 91, 10,759-10,767,
1986.
Breach, Newark, N.J., 1997.
Moore, R.M., M.G. Lowings,andF.C. Tan, Geochemicalprofilesin the
central Arctic Ocean: Their relation to freezing and shallow
circulation,J. Geophys.Res.,88, 2667-2674, 1983.
Morison, J., M. Steele, and R. Andersen, Hydrography of the upper
Arctic Ocean measuredfrom the nuclear submarineU.S.S. Pargo,
Deep SeaRes.Part A, 45, 15-38, 1998.
Olsson,K., Dissolvedcarbonin the ArcticOcean,Ph.D. thesis,G/3teborg
Univ., G0teborg,Sweden,1997.
Ostlund,H.G., andG. Hut,ArcticOceanwatermassbalancefromisotope
data,J. Geophys.Res.,89, 6373-6381, 1984.
Pavlov, V.K.,
L.A. Timokhov, G.A. Baskakov, M.Y.
Kulakov, V.K.
Kurazhov, P.V. Pavlov, S.V. Pivovarov, and V.V. Stanovoy,
Hydrometeorological
regime of the Kara, Laptev, and East-Siberian
Seas,APL-UW TM1-96, Appl. Phys. Lab. Univ. of Wash., Seattle,
1996.
Pfirman, S., M.A. Lange, I. Wollenberg,and P. Schlosser,Sea ice
characteristics
and the role of sedimentinclusionsin deep-sea
deposition:
Arctic-Antarctic
comparisons,
in GeologicalHistoryof the
Polar Oceans:Arctic VersusAntarctic,editedby U. Bleil and J.
Thiede,pp. 187-211, Kluwer Acad., Norwell, Mass., 1990.
Pacific watersin the upperArctic Ocean:Implicationsfor circulation, Pfirman, S.L., D. Bauch, and T. Gammelsr•d, The Northern Barents Sea:
Water mass distribution and modification, in The Polar Oceans and
Geophys.Res.Lett., 25, 765-768, 1998.
Their Role in Shaping the Global Environment,Geophys.Monogr.
Kinney, D.J., M.E. Arhelger, and D.C. Burrell, Chemicalcharacteristics
Ser., vol. 85, edited by O.M. Johannessen,R.D. Muench, and J.E.
of the water masses in the Amerasian Basin of the Arctic Ocean, J.
Overland,pp. 77-94, AGU, Washington,D.C., 1994.
Geophys.Res., 75, 4097-4104, 1970.
Kipphut, G.W., Glacial meltwater input to the Alaska CoastalCurrent: Pfirman, S.L., R. Colony, D. Ntirnberg, H. Eicken, and I. Rigor,
Reconstructing
the origin and trajectoryof drifting Arctic seaice, J.
Evidence from oxygen isotopemeasurements,
J. Geophys.Res., 95
(C4), 5177-5181, 1990.
Geophys.Res.,102, 12,575-12,586,1997.
A.Y., and M.A. Johnson,Two circulationregimesof the
Lehmann,M., and U. Siegenthaler,Equilibrium oxygen and hydrogen Proshutinsky,
wind-driven Arctic Ocean, J. Geophys.Res., 102, 12,493-12,514,
isotopefractionationbetweenice and water, J. Glaciol., 37, 23-26,
Jones, E.P., L.G. Anderson, and J.H. Swift, Distribution of Atlantic and
1991.
L6tolle, R., J.M. Martin, A.J. Thomas, V.V. Gordeev, S. Gusarova, and
I.S. Sidorov, •80 abundanceand dissolvedsilicatein the Lena delta
andLaptevSea(Russia),Mar. Chem.,43, 47-64, 1993.
Levi, B.G., The decreasingArctic ice cover, Phys. Today, 53, 19-20,
2000.
Ludin, A., R. Weppernig, G. B0nisch, and P. Schlosser, Mass
spectrometricmeasurementof helium isotopesand tritium, L-DEO
Tech.Rep. 98-6, 42 pp., Lamont-DohertyEarth Obs., Palisades,New
York, 1998.
1997.
Rey, F., and H. Loeng,The influenceof ice andhydrographicconditions
on the developmentof phytoplanktonin the BarentsSea, in Marine
Biologyof Polar Regionsand Effectsof Stresson Marine Organisms,
edited by J.S. Gray and M.E. Christiansen,pp. 49-63, John Wiley,
New York, 1985.
Roach, A.T., K. Aagaard, C.H. Pease, S.A. Salo, T. Weingartner, V.
Pavlov, and M. Kulakov, Direct measurementof transportand water
propertiesthrough the Bering Strait, J. Geophys.Res., 100, 18,44318,457, 1995.
9092
EKWURZEL
ET AL.: FRESHWATER DISTRIBUTION
IN THE ARCTIC OCEAN
Roether,W., Water-CO2
exchange
set-upfor theroutinet8oxygen
assay Swift,J.H.,E.P.Jones,K. Aagaard,
E.C. Carmack,
M. Hingston,
R.W.
of naturalwaters,Int. J. Appl. Radiat. Isot., 321,379-387, 1970.
Rudels, B., On the Mass Balance of the Polar Ocean, With Special
Emphasison Fram Strait, Rep. 188, 53pp., Norsk Polarinst.,Oslo,
Norway, 1987.
Rudels, B., E.P. Jones, L.G.
Anderson, and G. Kattner, On the
intermediatedepth waters of the Arctic Ocean, in The Polar Oceans
and Their Role in Shapingthe Global Environment,Geophys.Mongr.
Ser., vol. 85, edited by O.M. Johannessen,
R.D. Muench, and J.E.
Overland,pp. 33-46, AGU, Washington,D.C., 1994.
Rudels, B., L.G. Anderson, and E.P. Jones, Formation and evolution of
the surfacemixed layer and haloclineof the Arctic Ocean,J. Geophys.
Res., 101, 8807-8821, 1996.
Salmon, D.K., and C.P. McRoy, Nutrient-basedtracers in the western
Arctic: A new Lower Halocline Water defined, in The Polar Oceans
and Their Role in Shapingthe Global Environment,Geophys.Mongr.
Ser., vol. 85, edited by O.M. Johannessen,R.D. Muench, and J.E.
Overland,pp. 47-61, AGU, Washington,D.C., 1994.
Schauer, U., R.D. Muench, B. Rudels, and L. Timokhov, Impact of
easternArctic shelf waterson the NansenBasinintermediatelayers,J.
Geophys.Res., 102, 3371-3382, 1997.
Schlosser, P., G. B6nisch, B. Kromer, K.O. Mtinnich, and K.P.
Koltermann, Ventilation rates of the waters in the Nansen Basin of the
Arctic Oceanderivedfrom a multitracerapproach,J. Geophys.Res.,
95, 3265-3272, 1990.
Schlosser, P., G. B6nisch, M. Rhein, and R. Bayer, Reduction of
deepwaterformationin the GreenlandSea duringthe 1980s:Evidence
from tracer data, Science, 251, 1054-1056, 1991.
Schlosser, P., D. Bauch, R. Fairbanks, and G. B6nisch, Arctic river-
runoff: Mean residencetime on the shelvesand in the halocline,Deep
Sea Res., Part A, 41, 1053-1068, 1994.
Schlosser, P., J.H. Swift, D. Lewis, and S.L. Pfirman, The role of the
Macdonald, F.A. McLaughlin, and R.G. Perkin, Waters of the
MakarovandCanadaBasins,DeepSeaRes.,Part H, 44, 1503-1529,
1997.
Takahashi,T., W.S. Broecker,and S. Langer,Redfieldratio basedon
chemicaldatafrom isopycnalsurfaces,
J. Geophys.
Res.,90, 69076924, 1985.
Thompson•'
D.W.J.,
andJ.M.Wallace,
TheArctic
Oscillation
signature
in
the wintertimegeopotential
heightandtemperature
fields,Geophys.
Res. Lett., 25, 1297-1300, 1998.
Thorndike, A.S., and R. Colony, Sea ice motion in responseto
geostrophicwinds,J. Geophys.Res.,87, 5845-5852, 1982.
Treshnikov,A.F., Arctic Atlas, 204 pp., NauchnoIssled.Inst., Moscow,
1985.
Untersteiner,
N., Naturaldesalination
andequilibriumsalinityprofileof
perennialseaice, J. Geophys.Res.,73, 1251-1257, 1968.
Unterweger, M.P., B.M. Coursey, F.J. Schima, and W.B. Mann,
Preparationand calibrationof the 1978 National Bureauof Standards
tritiated-waterstandards,
Int. J. Appl.Radiat.Isot.,31, 611-614, 1980.
Walsh, J.E., W.L. Chapman,and T.L. Shy, Recentdecreaseof sea level
pressurein the centralArctic, J. Clim., 9, 480-486, 1996.
Weingartner,T.J., D. Cavalieri, K. Aagaard,and Y. Sasaki,Circulation,
densewater formation, and outflow on the northeastChukchi shelf, J.
Geophys.Res., 103, 7647-7661, 1998.
Wessel,P., and W.H.F. Smith, New versionof the GenericMapping
Tools released,Eos Trans. AGU, 76, 329, 1995.
Wilson, C., and D.W.R. Wallace, Using the nutrientratio NO/PO as a
tracer of continental shelf waters in the central Arctic Ocean, J.
Geophys.Res., 95, 22,193-22,208, 1990.
Zhang, J., D.A. Rothrock,and M. Steele,Warming of the Arctic Ocean
by a strengthened
Atlantic inflow: Model results,Geophys.Res.Lett.,
25, 1745-1748, 1998.
large-scaleArctic Oceancirculationin the transportof contaminants,
Deep Sea Res., Part B, 42, 1341-1367, 1995.
Serreze,M.C., J.A. Maslanik, J.R. Key, and R.F. Kokaly, Diagnosisof
B. Ekwurzel, Departmentof Hydrologyand Water Resources,The
the recordminimumin Arctic seaice areaduring1990 andassociated
Universityof Arizona, J. W. HarshbargerBldg., Tucson,AZ, 85721snowcoverextremes,Geophys.Res.Lett., 22, 2183-2186, 1995.
Smethie, W. M., Jr., P. Schlosser, G. B6nisch, and T. S. Hopkins, 0011, (e-mail: [email protected]).
Renewal
and circulation
of intermediate
waters in the Canadian Basin
observedon the SCICEX 96 cruise,J. Geophys.Res., 105, 1105-1121,
2000.
Steele,M, and T. Boyd, Retreatof the cold haloclinelayer in the Arctic
Ocean,J. Geophys.Res., 103, 10,419-10,435, 1998.
Steele, M., J.H. Morison, and T.B. Curtin, Halocline water formation in
the BarentsSea,J. Geophys.Res., 100, 881-894, 1995.
Swift, J.H., and K. Aagaard, Seasonal transitions and water mass
formation in the Iceland and GreenlandSeas,Deep Sea Res.part A,
28, 1107-1129, 1981.
R. G. Fairbanks, R. A. Mortlock, and P. Schlosser,Lamont-Doherty
EarthObservatory
of ColumbiaUniversity,Palisades,
NY 10964,(e-mail:
fairbank @ ldeo.columbia.edu,
mortlock @ ldeo.columbia.edu,
[email protected]).
J. H. Swift, ScrippsInstitution of Oceanography,University of
California,SanDiego,Mail Code0214, GilmanDr., La Jolla,CA 920930214, (e-mail:[email protected]).
(ReceivedJuly14, 1999;revisedSeptember
11, 2000;
acceptedNovember7, 2000.)