Studies of the Canadian Arctic Archipelago water transport and its relationship to basin‐

24 Pages·2016·5.93 MB·English

PUBLICATIONS
Journal of Geophysical Research: Oceans
RESEARCH ARTICLE Studies of the Canadian Arctic Archipelago water transport and
10.1002/2016JC011634 its relationship to basin-local forcings: Results from AO-FVCOM
KeyPoints: YuZhang1,2,ChangshengChen1,2,RobertC.Beardsley3,GuopingGao2,ZhigangLai4,BethCurry5,
(cid:2)Ahigh-resolutionArcticOcean CraigM.Lee5,HuichanLin1,JianhuaQi1,andQichunXu1
FVCOMiscapableofsimulatingthe
CAAoutflow 1SchoolforMarineScienceandTechnology,UniversityofMassachusetts-Dartmouth,NewBedford,Massachusetts,USA,
(cid:2)Thebasin-scalesealevelpressureisa
majordriveroftheCAAoutflow 2InternationalCenterforMarineStudies,ShanghaiOceanUniversity,Shanghai,China,3DepartmentofPhysical
(cid:2)ThevolumefluxthroughDavisStrait Oceanography,WoodsHoleOceanographicInstitution,WoodsHole,Massachusetts,USA,4SchoolofMarineSciences,Sun
isnegativelycorrelatedwiththeflux Yat-SenUniversity,Guangzhou,China,5AppliedPhysicsLaboratory,UniversityofWashington,Seattle,Washington,USA
throughFramStrait
Correspondenceto: Abstract Ahigh-resolution(upto2km),unstructured-grid,fullycoupledArcticseaice-oceanFinite-Vol-
Y.Zhang, umeCommunityOceanModel(AO-FVCOM)wasemployedtosimulatetheflowandtransportthroughthe
[email protected]
CanadianArcticArchipelago(CAA)overtheperiod1978–2013.Themodel-simulatedCAAoutflowfluxwas
inreasonableagreementwiththefluxestimatedbasedonmeasurementsacrossDavisStrait,NaresStrait,
Citation:
LancasterSound,andJonesSounds.Themodelwascapableofreproducingtheobservedinterannualvari-
Zhang,Y.,C.Chen,R.C.Beardsley,
G.Gao,Z.Lai,B.Curry,C.M.Lee,H.Lin, abilityinDavisStraitandLancasterSound.ThesimulatedCAAoutflowtransportwashighlycorrelatedwith
J.Qi,andQ.Xu(2016),Studiesofthe thealong-straitandcross-straitseasurfaceheight(SSH)difference.Comparedwiththewindforcing,the
CanadianArcticArchipelagowater
sealevelpressure(SLP)playedadominantroleinestablishingtheSSHdifferenceandthecorrelationofthe
transportanditsrelationshiptobasin-
localforcings:ResultsfromAO-FVCOM, CAAoutflowwiththecross-straitSSHdifferencecanbeexplainedbyasimplegeostrophicbalance.The
J.Geophys.Res.Oceans,121, changeinthesimulatedCAAoutflowtransportthroughDavisStraitshowedanegativecorrelationwiththe
doi:10.1002/2016JC011634.
netfluxthroughFramStrait.Thiscorrelationwasrelatedtothevariationofthespatialdistributionand
intensityoftheslopecurrentovertheBeaufortSeaandGreenlandshelves.Thedifferentbasin-scalesurface
Received8JAN2016
forcingscanincreasethemodeluncertaintyintheCAAoutflowfluxupto15%.Thedailyadjustmentofthe
Accepted27MAY2016
modelelevationtothesatellite-derivedSSHintheNorthAtlanticregionoutsideFramStraitcouldproduce
Acceptedarticleonline2JUN2016
alargerNorthAtlanticinflowthroughwestSvalbardandweakentheoutflowfromtheArcticOcean
througheastGreenland.
1.Introduction
TheArcticOceanisapolarbasinwithitsmajorwatersourcesconsistingofrelativelycoldandfreshPacific
water inflowing through Bering Strait [Coachman and Aagaard, 1988], relatively warm and salty Atlantic
waterenteringthrough Fram Strait andthe Barents Sea [Fahrbach et al., 2001]and river runoff (Figure 1).
These water inflows coexist with outflows through two pathways: the Canadian Arctic Archipelago (CAA)
StraitsandtheeasternshelfofGreenlandconnectedtoFramStrait[AagaardandCarmack,1989].TheCAA
ischaracterizedbynumerousislands,straits,andnarrowandshallowchannelsorwaterpassages.TheCAA
waterfromNaresStrait,LancasterSound,andJonesSoundflowsintoBaffinBayandthenenterstheNorth
AtlanticOceanthroughDavisStrait[Tangetal.,2004;Cunyetal.,2005;Curryetal.,2011,2014],withasmall
portion flowing into the Labrador Sea through Hudson Strait [Straneo and Saucier, 2008]. Fram Strait can
carry the warm and salty Atlantic inflow named the West Spitsbergen Current (WSC) along the western
coastofSpitsbergenandthecoldandfreshArcticoutflownamedtheEastGreenlandCurrent(EGC)[Schlich-
tholzandHoussais,1999;Woodgateetal.,1999].
The inflow and outflow transports through the Arctic varied seasonally and interannually in responses to
thebasin-scaleforcingwithinfluencesofglobalclimatechange[e.g.,Jahnetal.,2010;McGeehanandMas-
lowski,2012;Wekerleetal.,2013].Thequalitativeandaccurateestimationofthesevariabilities,inturn,iscrit-
ical to understanding the changes in the Arctic and the impacts on the North Atlantic Ocean. In the past
decades,manymeasurementshavebeenmadetoestimatethetransportsthroughBeringStrait[Coachman
and Aagaard, 1988; Roach et al., 1995; Woodgate et al., 2005, 2006,2010] and Fram Strait [Fahrbach et al.,
2001;Schaueretal.,2004,2008;Rudelsetal.,2008],butonlyafewweremadeintheBarentsSeaOpening
VC2016.AmericanGeophysicalUnion.
AllRightsReserved. [Ingvaldsen et al., 2004; Skagseth et al., 2008] and the CAA. The measurements in the CAA were mainly
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 1
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Figure1.Bathymetryandschematicofnear-surfaceanddeepcirculationsintheArcticOcean.Redarrows:thedeepcurrents.Bluearrows:
thenear-surfacecurrents.Redlines:thesectionswherethevolumefluxwasestimated.Box:theareaofDavisStraitthatisdisplayed
enlargedinthelower-leftcorner.Theredtrianglesindicatethelocationsofcurrent/hydrographicmeasurementsitesinDavisStrait.
focused on Nares Strait [Sadler, 1976; Mu€nchow et al., 2006, 2007; Mu€nchow and Melling, 2008], Lancaster
Sound [Prinsenberg and Hamilton, 2005; Melling et al., 2008; Prinsenberg et al., 2009; Peterson et al., 2012],
andJonesSound[Mellingetal.,2008],aswellasDavisStrait-adownstreamstraitcapturingwatertransports
fromthethreemainwaterpathways[Cunyetal.,2005;Curryetal.,2011,2014].
OurcurrentunderstandingonthevariabilityoftheCAAoutflowismainlybasedonbothobservationalfindings
andbasin-scaleArcticOceanmodels.Previousobservationssuggestedthattheseasurfaceheight(SSH)differ-
encebetweentheArcticshelfandBaffinBayisthekeymechanismtocontroltheCAAoutflowtransport[e.g.,
Mu€nchowandMelling,2008;Petersonetal.,2012].Severalmodelingeffortsweremadetoexaminephysicalproc-
essescontrollingtheSSHvariationintheCAA.Duetodifferencesindiscretealgorithm,gridresolution,bathym-
etryandcoastlineapproximation,externaldrivingforcingandlateralboundaryconditions,etc.,however,the
findings obtained from these models were diverse, often not consistent, and the mechanisms produced by
themdifferedsubstantiallyintermsofparticulars[Jahnetal.,2010;HoussaisandHerbaut,2011;McGeehanand
Maslowski,2012;Wekerleetal.,2013;Luetal.,2014].Whatarekeyexternalforcingcomponents,windsorsea
levelpressure(SLP)orboth,attributingtotheonsetandvariabilityoftheSSHdifferenceandthustheCAAout-
flow?Toourknowledge,thisquestionhasnotbeenwellunderstoodyetintermofaquantitativeassessment.
ItiswellknownthatFramStraitandtheCAAarethetworegionsfortheArcticOceanwateroutflows.The
outflowalongtheeastGreenlandcoastofFramStraitconsistsoftwomajorcurrentpathways:oneisfrom
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thecycloniccirculationintheEurasianBasinandalongtheLomonosovRidgeandtheotheristhecyclonic
slopecurrentsfromtheBeaufortSea/CanadianBasin(Figure1).Assumingthattheinflowandoutfloware
balanced,whentheinflowsaregiven,thevariationoftheoutflowthroughtheCAAcouldaffectthenetflux
throughFramStrait.Thissuggeststhatastate-of-the-artArcticOceanmodelshouldhaveasufficientresolu-
tiontobecapableofresolvingbothbasinandlocal-scalephysicalprocesses,particularlywithbetterrepre-
senting the complex geometry of the CAA including narrow straits and water passages. This type of
multiscale resolving model could provide us a tool to examine the role of the CAA in the Arctic Ocean
system.
The upper ocean circulation in the Arctic is dominated by the wind-drifting-driven and ice-drifting-driven
anticycloniccirculation[Proshutinskyetal.,2001;Steeleetal.,2001;Hollowayetal.,2007;Chenetal.,2016].
Due to its severe natural conditions and limitations in past research efforts, the Arctic still remains in an
insufficient monitoring status for both meteorological and oceanic conditions [Wekerle et al., 2013]. The
externalforcingusedtoforceamodelisbasedfullyontheregionalorglobalmeteorologicalmodeloutputs
with the lack of validation or calibration through observations. What level could a model-produced CAA
outflow transport be affected when different meteorological forcings are used? Is it critical to resolve the
local wind variability in narrow straits of the CAA when the CAA outflow is simulated? These questions
should be addressed, since a fair model-data comparison is required to take the model uncertainty into
account.
FramStraitisaninflow-outflowstraitcontainingboththeNorthAtlanticinflowalongthewestcoastofSval-
bardandtheArcticoutflowalongtheeastGreenlandshelf.Whetherornotamodelcanwellproducethe
NorthAtlanticinflowdirectlyaffectstherealityandaccuracyofthenetoutflowfluxthroughFramStraitand
hencethecirculationandiceintheArcticbasin.WhenaregionalmodelisappliedtotheArcticOcean,one
isrequiredtosetuptheboundaryconditionsinbothAtlanticandPacificOcean.Eitherone-wayortwo-way
nestingwithaglobalmodelisacommonapproachusedintheArcticOceanmodelsimulation.Aslongas
Fram Strait is considered, the success of this approach depends on whether or not a global model could
provide an accurate simulation of the North Atlantic water flux on the nesting boundary. The satellite-
derived SSH is widely used in the global ocean models to improve the low-frequency spatial variation of
thegradientoftheseasurfaceelevationandthusthebarotropiccomponentoftheoceancirculation[Mar-
shalletal.,1997;Madecetal.,1998;PacanowskyandGriffies,1999;SmithandGent,2002;Blecketal.,2002;
Chassignet et al., 2003, 2006; Chen et al., 2016]. The Archiving, Validation, and Interpretation of Satellite
Oceanographic data (AVISO) daily satellite-derived SSH product covers the ocean region bounded by
(cid:3)808S–808N. How will a regional Arctic Ocean model perform with the data assimilation of the satellite-
derivedSSHintheArcticOcean?HowwillthenetfluxthroughFramStraitchangewhentheSSHassimila-
tionistakenintoaccount?ConductingthemodelexperimentswithandwithouttheSSHassimilationcould
help us not only address these questions but also explore the dynamics controlling the net flux through
FramStrait.
Anewhigh-resolution,global-basinnested,ice-seacoupledArcticOceanmodelwasdevelopedbasedon
theunstructured-grid,Finite-VolumeCommunityOceanModel(hereafterreferredtoasAO-FVCOM)[Chen
etal.,2009;Gaoetal.,2011;Chenetal.,2016].Themodelwasdesignedtomeetthefollowingstate-of-the-
artArcticOceanmodelrequirements:(a)gridflexibilitytoresolvethecomplexcoastalgeometryandsteep
continentalslopes;(b)massconservationinanumericalcomputationalsensetoaccuratelysimulatemass,
heat,andsalttransport;(c)properparameterizationofverticalandlateralmixingtocapturewaterstratifica-
tion;(d)advanceddataassimilationmethodstointegrateobservationswithsimulationresults;and(e)mod-
ularstructurestofacilitateprocess-orientedandhindcast/forecastapplications[Chenetal.,2013].Usingthe
AO-FVCOM,wehavesimulatedtheseaiceandcirculationintheArcticfortheperiod1978–2013.TheAO-
FVCOMsimulationresultsprovideuswithanopportunitytovalidatethismodelthroughcomparisonswith
observationsintheCAAandexaminethephysicalprocessescontrollingArcticoutflowthroughtheCAA.
Inthispaper,weattempttoaddressthequestionsdescribedabove.Thesimulatedwatertransportisfirst
comparedwithobservationstakenintheCAAandDavisStraittoensuretheabilityoftheAO-FVCOMtorea-
sonably capture the seasonal and interannual variability of the CAA outflow. Then a series of process-
orientedmodel-experimentsareconductedtoexaminethelocalandbasin-scalephysicalprocessesassoci-
atedwithexternalforcing.
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 3
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The rest of the paper is organized as fol-
lows. In section 2, the model setup and
observational data are briefly described.
In section 3, the comparison of observed
and simulated water transports through
the CAA and Davis Strait is presented. In
section 4, the process-oriented experi-
ment results are discussed, with a focus
on the response of the CAA outflow to
thelocalandbasin-scalevariability.Insec-
tion5,sensitivitiesofthemodelperform-
ance to external forcing, grid refinement
and SSH assimilation are examined. Con-
clusionsaresummarizedfollowingdiscus-
sionsinsection6.
2.AO-FVCOMandObservational
Data
2.1.AO-FVCOM
The AO-FVCOM is an Arctic regional
coupled ice-ocean model nested within
the global FVCOM modeling system
[Chen et al., 2016]. The FVCOM is a prog-
nostic, unstructured-grid, Finite-Volume,
free-surface,3-D primitiveequationCom-
munity Ocean Model [Chen et al., 2003,
Figure2.TheunstructuredtriangulargirdoftheAO-FVCOMnestedwith
Global-FVCOM.BlackdashlinesindicatethenestingboundariesofAO- 2006,2007,2013].TheAO-FVCOMiscon-
FVCOMandGlobal-FVCOM.Bluelineindicatesthe62.58Nline,thenorthern figured using a spherical coordinate ver-
boundaryofSSTandSSHassimilation.Forthe36yearsimulation,theAO-
sion of FVCOM with a horizontal
FVCOMwasrunbymergingittotheGlobal-FVCOMwithahorizontalresolu-
tionupto2kmintheCanadianArcticArchipelago. resolutionvaryingfrom2to40km,witha
meanresolutionof(cid:3)5kmintheCAAand
(cid:3)12kminthecentralArcticOcean(Figure2).Ahybridterrain-followingcoordinateisusedinthevertical,
withatotalof45layers.Thes-coordinateisusedintheregiondeeperthanandequal225m,with10uni-
formlayers(thicknessof5m)nearthesurfaceandfiveuniformlayers(thicknessof5m)nearthebottom,
respectively.Ther-coordinateisspecifiedintheshallowcontinentalandcoastalregionsoflessthan225m.
Theses-coordinateandr-coordinatehaveatransitionatthe225-isobathatwhichthethicknessofalllayers
is 5 m. The AO-FVCOM can run either through nesting with Global-FVCOM or by merging into Global-
FVCOM as a single global-scale model. Global-FVCOM has a horizontal resolution of up to 2 km and the
sameverticalresolutionasAO-FVCOM.TheicemodelcoupledinAO-FVCOMandGlobal-FVCOMisUG-CICE:
anunstructured-grid,finite-volumeversionoftheLosAlamosCommunityIceCodedevelopedbyGaoetal.
[2011].
The36year(1978–2013)simulationwasconductedbyGlobal-FVCOMmergingwithAO-FVCOMwithresolu-
tionupto2km.Themodelwasinitializedwiththe50yearspin-upoutputundera‘‘climatologic’’meteoro-
logicalforcingandriverdischargeconditions[Gaoetal.,2011]anddrivenby(a)astronomicaltidalforcing
witheightconstituents(M ,S ,N ,K ,K ,P ,O ,andQ ),(b)surfacewindstress,(c)netheatfluxatthesur-
2 2 2 2 1 1 1 1
faceplusshortwaveirradianceinthewatercolumn,(d)surfaceairpressuregradients,(e)precipitation(P)
minusevaporation(E),and(f)riverdischarges[Gaoetal.,2011;Chenetal.,2016].
The1978–2013simulationbeganon1January1978.Theatmosphericforcingwastakenfromthe6hourly
version-2datasetfortheCommonOcean-iceReferenceExperiments(CORE-v2)overtheperiod1978–2009
and then the National Center for Environmental Prediction and the National Center for Atmospheric
Research (NCEP/NCAR) data set over the period 2010–2013. A total of 766 rivers were included in the
Global-FVCOM. The river discharges collected from the U.S. Geological Survey and the Water Survey of
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 4
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10.1002/2016JC011634
Figure3.(left)Originaland(right)refinedunstructuredtriangulargridsconfiguredintheBaffinBayregionusedinthesensitivityexperi-
ments.Thehorizontalresolutionswere35and8kminthisregion,respectively,forthecaseswithoriginalandrefinedAO-FVCOMgrids.
Canadawerespecifiedusingdailyreal-timerecords.Fortheriverswithoutreal-timedischargerecords,the
climatologicallyaverageddailyrecordswereused.Toadjusttheinitialclimatologictemperatureandsalinity
to the real-time observation, the satellite-derived global daily sea surface temperature (SST) (ftp://data.
nodc.noaa.gov/pub/data.nodc/ghrsst/) andseasurfaceheight(SSH)(http://www.aviso.altimetry.fr/en/data/
products/sea-surface-height-products/global/msla-h.html) south of 62.58N was assimilated into Global-
FVCOMbyanudgingmethod.Givenaprioristatisticalassumptionaboutthemodelnoiseanderrorsinthe
observationaldata,thisassimilationmethodwastomergemodel-predictedvaluesdirectlytoobservations
[Chenetal.,2013].TheSSTassimilationwasconductedthroughthesurfacemixedlayerwithitsthickness
(whichcouldbetensofmeters)beingdeterminedusingthePWPmixedlayermodel[Priceetal.,1986].We
collectedallavailableT/Sobservationaldata(e.g.,NODC,JAMSTEC,andArgo)andassimilatedthisintothe
Global-FVCOMonthemonthlyaveragedscaletohelpensuresimulatedstratificationwouldbeconsistent
withobservations.ThemodelusedamodifiedMellorandYamadalevel2.5(MY-2.5)andSmagorinskytur-
bulentclosureschemesforverticalandhorizontalmixing,respectively[MellorandYamada,1982;Galperin
etal.,1988;Smagorinsky,1963].Thetimestepusedforintegrationwas300s.
ToexaminethesensitivityofthesimulatedCAAoutflow toexternalforcing, wereranthesimulationwith
differentatmosphericforcingfromtheNCEP/NCARreanalysisandtheEuropeanCentreforMedium-Range
WeatherForecasts(ECMWF)datasetsaswellasinadditionthehigh-resolution((cid:3)6km)hourlypolarwind
fieldproducedbytheFifth-GenerationPennState/NCARMesoscaleModel(MM5)fortheperiod2004–2010.
ToevaluatetheinfluenceofmodelresolutionontheaccuracyofthesimulatedCAAoutflow,wealsoreran
themodelbyrefiningthegridfrom35to8kminBaffinBay(Figure3).
2.2.ObservationalData
The simulated CAA outflow transport was compared with the observations of currents across Davis Strait
(Figure 1). The Bedford Institute of Oceanography (BIO) deployed six moorings across Davis Strait in Sep-
tember1987andtherecordscoveredtheperioduntilAugust1990[Cunyetal.,2005].Theseparationdis-
tancebetweenmooringswasabout30–60km.Ateachmooring,threeAanderaaRecordingCurrentMeters
(RCM5) were mounted at depths of around 150, 300, and 500 m. A new set of moorings were deployed
againinSeptember2004,with14moorings:4ontheshelfofBaffinIsland,4ontheshelfofwesternGreen-
land, and 6 in the interior of thestrait [Curry et al., 2011]. Thenumber of moorings variedwith the years.
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 5
Journal of Geophysical Research: Oceans
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21 Duringtheyear2009–2010,itbecame12
moorings:2ontheshelfofBaffinIsland,4
C
on the shelf of the western Greenland,
and 6 in the interior of the strait [Curry
18 A
et al., 2014]. The separation distance of
a
these moorings varied from (cid:3)0.1 to
b
15 SecN-aNres Strait ifi2n6leterksrmio(Aro.DvCAerPco)thuwesetsircheemDlfootopupn(cid:3)lte1er6d–Ci6nu5rtrkhementdinePptrhtohe-
Sec-L B between56and390mandSea-BirdElec-
m) tronics(SBE)37MicroCATsweremounted
k12 20 cm/s
20 10 cm/s in the depth between 20 and 500 m
1 D
×e ( 5 cm/s depending on the location [Curry et al.,
nc 2 cm/s 2014]. At each mooring in the interior of
sta 9 thestrait,1-3AanderaaRecordingCurrent
Di
Meters(RCM8)weremountedinthedeep
B G
af r region between 200 and 500 m. A
6 fin e detailed summary of the locations,
I e depths, record lengths, and types of
sla n l instruments deployed between Septem-
n a
d ber 2004 and September 2010 was pub-
n
3
d lished in the appendix of Curry et al.
[2014]. We also compared the model
results with observations taken in Nares
0 Strait [Sadler, 1976; Mu€nchow et al., 2006,
0 2 4 6 8 10 12 14 2007; Mu€nchow and Melling, 2008], Lan-
Distance (×102 km)
caster Sound [Peterson et al., 2012; Prin-
senbergandHamilton,2005;Mellingetal.,
Figure4.Thedistributionofthemeancurrentvectorsaveragedintheupper
400mandovera36yearperiodof1978–2013inBaffinBaywithconnection 2008; Prinsenberg et al., 2009; Peterson
toNaresStrait,LancasterSound,andJonesSound.Thecurrentswererescaled et al., 2012], and Jones Sound [Melling
usingtheroot-squarevectorscaleandresampledwitha35kmresolutionto
et al., 2008]. To examine the dynamical
makevectorsvisible.Blackboxes:theareawherethemeansimulatedSSH
wascalculatedintheupstreamanddownstreamofNaresStraitandLancaster relationshipoftheCAAoutflowtransport
Sound.Blacklines:thesectionswheretheSSHdifferencewascalculatedin withtheArcticBasinvariability,thesimu-
NaresStraitandLancasterSound.Thebacklineconnectedbetweenaandb
isthestreamlineusedfortheanalyticalsolution. lated transports were compared with the
observations taken in Bering Strait
[Coachman and Aagaard, 1988; Roach
etal.,1995;Woodgateetal.,2005,2006,2010],BarentsSeaOpening[Ingvaldsenetal.,2004;Skagsethetal.,
2008],andFramStrait[Fahrbachetal.,2001;Schaueretal.,2004,2008;Rudelsetal.,2008].
3.ValidationsoftheSimulatedCAAWaterTransport
3.1.MeanTransport
The CAA outflows from Nares Strait, Lancaster Sound, and Jones Sound enter the North Atlantic Ocean
throughDavisStrait.ThegeometriesofmajorwaterpassagesoftheCAAwerereasonablyresolvedinAO-
FVCOMandthesimulated36yearaveragedresultscapturedtheCAAcirculationpatternsuggestedbypre-
viousobservations.TheAO-FVCOMshowedthecoastal-intensifiedoutflowsthroughNaresStrait,Lancaster
Sound and Johnes Sound (Figure 4). These flows brought the relarively cold and fresher water into Davis
Straittoformastrongsouthwardcoastalcurrentnamedthe‘‘BaffinIslandCurrent(BIC)’’alongthewestern
shelfofDavisStrait[Tanget al.,2004;Cuny etal.,2005]. Themodelsuggested aflowconnectivityamong
thesethreeCAAoutflowpassages,whichdirectlycontributedtotheformationofBIC.DavisStraitwaschar-
acterized by the northward inflow along the eastern slope and shelf, which could be traced back to the
inflowofmixedWestGreenlandCoastal andSlopeCurrentsliketheobservationsdescribedinCurryetal.
[2014].
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 6
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Table1.ComparisonoftheVolumeFluxBetweenAO-FVCOMandObservationalEstimatesa
FVCOM(Sv)
Location Vol.Flux Std. Obs.(Sv) Source
DavisStrait 21.83 0.80 Simulation(1978–2013)
21.51 0.46 22.661.0 Cunyetal.[2005](Sep1987toAug1990)
21.94 0.61 22.360.7 Curryetal.[2011](Oct2004toSep2005)
22.14 0.76 21.660.2 Curryetal.[2014](Oct2004toSep2010)
NaresStrait 20.81 0.33 Simulation(1978–2013)
20.6760.1 Sadler[1976](AprtoJun1972)
20.73 0.48 20.860.3 Mu€nchowetal.[2006](Aug2003)
20.73 0.48 20.9160.1 Mu€nchowetal.[2007](Aug2003)
20.85 0.32 20.5760.09 Mu€nchowandMelling[2008](Aug2003toAug2006)
LancasterSound 20.71 0.17 Simulation(1978–2013)
20.70 0.30 20.7560.25 PrinsenbergandHamilton[2005](Aug1998toSep2001)
Mellingetal.[2008](Aug1998toAug2004)
20.68 0.32 20.760.4 Prinsenbergetal.[2009](Aug1998toAug2006)
20.65 0.32 20.760.3 Petersonetal.[2012](Aug1998toAug2006)
20.65 0.32 20.53 Petersonetal.[2012](Aug1998toAug2011)
20.62 0.33 20.4660.34 PrinsenbergandHamilton[2005](Aug1998toSep2001)
JonesSound 20.31 0.05 Simulation(1978–2013)
20.31 0.03 20.3 Mellingetal.[2008](1998–2002)
BeringStrait 0.88 0.08 Simulation(1978–2013)
0.8 CoachmanandAagaard[1988](1976–1977)
0.81 0.27 0.8360.25 Roachetal.[1995](Sep1990toSep1994)
0.86 0.08 0.8 Woodgateetal.[2005](1990–2004)
0.86 0.08 0.7–1.0 Woodgateetal.[2006](1991–2004)
0.85 0.08 0.6–1.0 Woodgateetal.[2010](1991–2007)
BarentsSeaOpening 2.07 0.36 Simulation(1978–2013)
2.29 1.39 1.5 Ingvaldsenetal.[2004](Aug1997toAug2001)
2.21 0.34 1.8 Skagsethetal.[2008](1997–2006)
FramStrait 21.10 0.70 Simulation(1978–2013)
20.46 1.03 24.262.3 Fahrbachetal.[2001](Sep1997toSep1999)
20.51 1.12 2262-2462 Schaueretal.[2004](Sep1997toAug2000)
20.58 0.50 2262.7 Schaueretal.[2008](1997–200s6)
20.83 0.74 21.7 Rudelsetal.[2008](1980–2005)
aThemodel-estimatedtransportwasbasedonthe36yearaveragedcurrents.Note:thepositivesign,inflow;thenegativesign,
outflow.
In Davis Strait, the observed mean volume transports and uncertainties, which were estimated based on
combineddirectcurrentandhydrographicmeasurementsmadeoverperiodsofSeptember1987toAugust
1990, October 2004 to September 2005, and October 2004 to September 2010, were 22.661.0 Sv [Cuny
etal., 2005],22.360.7Sv[Curry etal.,2011], and21.660.2Sv[Curryetal., 2014],respectively(Table1).
Sincethedifferencesofthesethreetransportvalueshadthesameorderofmagnitudeasthemeasurement
uncertainty,wecouldnotestimatetheinterannualvariationofthetransportbasedonthesethreemeasure-
ment period data. Correspondingly, the simulated volume transports calculated over the same periods as
observationswere21.5,21.9,and22.1Sv(Table1),whichwere21.1and20.4Svsmallerthanobserved
valuesovertheperiodsofSeptember1987toAugust1990andOctober2004toSeptember2005,respec-
tively,and0.5SvlargerthantheobservedvalueovertheperiodofOctober2004toSeptember2010.The
simulatedvolumetransportshowedatendencytoincreaseoverthethreemeasurementperiods.Wealso
estimated the 36 year (1978–2013) averaged volume transport through Davis Strait, which was (cid:3)21.8 Sv
withastandarddeviationof60.8Sv(Table1).Thisvaluewasveryclosetothemeanvalueof22.1Svwith
theuncertaintyrangeof0.2–1.0SvestimatedfromthemeasurementsofCunyetal.[2005]andCurryetal.
[2014].
Reasonable agreement between simulated and observed transports in Davis Strait was consistent with the
model-datacomparisonsmadeinNaresStrait,LancasterSoundandJonesSound.NaresStraitisoneofthe
majorwaterpassagesofArcticoutflowenteringtheCAAandhasawidthof(cid:3)35km.Theobservedvolume
transports and uncertainties were 20.6760.1 Sv [Sadler, 1976], 20.860.3 Sv [Mu€nchow et al., 2006],
20.9160.1Sv[Mu€nchowetal.,2007],and20.5760.09Sv[Mu€nchowandMelling,2008].Thesetransportval-
ues were estimated based on current measurements made across Robeson Channel over the period April–
June, 1972, the ship-board ADCP/hydrographic survey data across Kennedy Channel in early August 2003,
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 7
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four cross-strait ADCP/hydrographictransects takeninAugust2003 inRobeson Channel,northern Kennedy
Channel,southernKennedyChannel,andSmithSound,and3yearcurrentmeasurementsovertheperiodof
August2003toAugust2006,respectively.Themeantransportvalueaveragedoverthesefourmeasurement
periodswas20.74Svwiththemeasurementuncertaintyrangeof60.09–0.3Sv.
The AO-FVCOM simulations covered the two measurement periods of August 2003 and August 2003 to
August 2006. Correspondingly, the simulated volume transports for these two periods were 20.73 and
20.85 Sv, respectively (Table 1), which were 0.13 Sv smaller than the mean value of 20.86 Sv averaging
fromMu€nchowetal.[2006]andMu€nchowetal.[2007],and0.28Svlargerthantheobservedvaluereported
byMu€nchowandMelling[2008].
Itshouldbepointedoutthattheobservedtransportbasedon3yearcurrentmeasurementsovertheperiod
ofAugust2003toAugust2006[Mu€nchowandMelling,2008]wasestimatedbyexcludingtheupper30m.
We recomputed the simulated transport over the same period for the case by excluding the upper 30 m
layer,whichequaledto20.68Sv,0.17Svsmallerthanthetotaltransportthroughouttheentirewatercol-
umn. Compared with theobserved transport, thesimulatedtransport with excludingtheupper 30 m still
showedanoverestimationbyavalueof0.11Sv.Ifweconsideredthemeasurementuncertaintyof0.09Sv,
thesimulatedtransportwasveryclosetotheobservedtransport.Thisresultalsosuggestedthattheuncer-
taintyduetoeitherdifferentsamplingresolutionsintheverticalandhorizontalormodelaccuracydueto
inaccurateexternalandboundaryforcingcouldbeintherangeof0.02Sv.
The 36 year AO-FVCOM simulation showed that Nares Strait accounted for (cid:3)44% of the total transport
throughDavisStrait.Thesimulated36yearmeantransportthroughNaresStraitwas20.81Svwithastand-
arddeviationof60.33Sv(Table1),whichwasabout0.07Svlargerthanthemeanvalueestimatedbased
onthefourmeasurements.Thisdifferencewaswithinthemeasurementuncertaintyrange.
Lancaster Sound, located in thewest of Baffin Bay, is about 100 km wide. Moored current measurements
wereinitiated in western LancasterSound in 1998 and continued until 2011.The observed meanvolume
transports and standard deviations, estimated based on measurements over the periods of 1988–2001,
1998–2004,and1998–2006,were20.7560.25Sv[PrinsenbergandHamilton,2005],20.760.4Sv[Melling
etal.,2008],and20.760.3Sv[Prinsenbergetal.,2009],respectively.Usingthesamemeasurementdataas
Prinsenberg et al. [2009], Peterson et al. [2012] recalculate the mean volume transport over 1998–2006 by
introducinganimprovedalgorithm,whichproducedatransportof20.53Sv.Itwasclearthatduetoinsuffi-
cient spatial sampling, the transport estimated by different methods could cause a difference of 0.17 Sv.
Usingthesameapproachandextendingthedatatocovertheperiodof2007–2011,Petersonetal.[2012]
reportedameanvolumetransportandstandarddeviationof20.4660.34Sv.
SimilartotheresultsshowninPrinsenbergandHamilton[2005],Mellingetal.[2008],andPrinsenbergetal.
[2009], the simulated mean transport over the periods with measurements shows minor differences from
the36yearmeanvalue.Theywere20.70Svovertheperiodsof1998–2001;20.68Svovertheperiodsof
1998–2004; and 20.65 Sv over the periods of 1998–2006, and 20.62 Sv over the periods of 1998–2011
(Table 1). The simulated transport was 0.12 and 0.16 Sv larger than the values reported by Peterson et al.
[2012].Theseerrorsweresmallerthan0.17Sv,thedifferenceoftransportsestimatedbyPrinsenbergetal.
[2009]andPeterson etal.[2012]withdifferentalgorithms.Thesimulated36 yearmeantransport through
Lancaster Sound was 20.71 Sv with a standard deviation of 60.17 Sv and showed that the outflow from
LancasterSoundaccountedfor(cid:3)39%ofthetotalCAAoutflowthroughDavisStrait.
JonesSoundislocatedtothenorthofLancasterSound.Thetwonarrowchannels,CardiganStraitandHell
Gate,arewaterpassagesintheCAAthatoutflowthroughJonesSound[Wekerleetal.,2013].Severalefforts
weremadetoestimatethetransportthroughthesetwochannelsandthemeantotalvolumetransportesti-
matedbasedoncurrentmeasurementsover1988–2002was20.3Sv:20.2SvthroughCardiganStraitand
20.1SvthroughHellGate[Mellingetal.,2008].Correspondingly,overthesameperiod,thesimulatedmean
total transport and a standard deviation through Cardigan Strait and Hell Gate 20.3160.03 Sv (Table 1),
withadifferenceof0.01Svcomparedwiththeobservations.The36yearsimulatedtotaltransportoverthe
period 1978–2013 through Jones Sound was 20.3160.05 Sv, implying that the yearly mean transport
remained relatively constant, with small interannual variability in the standard deviation range of 0.05 Sv.
The36yearAO-FVCOMsimulationshowedthattheoutflowthroughJonesSoundaccountedfor(cid:3)17%of
thetotaloutflowtransportthroughDavisStrait.
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 8
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-0.5
-1
v)
S
ort (-1.5
p
ns -2
a
me tr-2.5
u
ol
V -3
-3.5
04-05 05-06 06-07 07-08 08-09 09-10
0.5
Sv) 0
ort (
p
ns-0.5
a
e tr
m
olu -1
V
-1.5
98-99 99-00 00-01 01-02 02-03 03-04 04-05 05-06 06-07 07-08 08-09 09-10 10-11
Time (year)
Figure5.Comparisonsof(top)yearly(October–September)simulated(red)andobserved(blue)volumefluxesinDavisStraitoverthe
period2004–2010and(bottom)yearly(August–July)simulated(red)andobserved(blue)volumefluxesinLancasterSoundoverthe
period1998–2011.Thebluesolidverticalbarinthetopplotistheobserveduncertainty[Curryetal.,2014].Theredandbluedashedverti-
calbarsarethesimulatedandobservedstandarddeviations.
SeveralmodelingeffortsweremadetoestimatetheCAAoutflowtransportinpreviousstudies.McGeehan
andMaslowski[2012]usedtheNavalPostgraduateSchoolArcticModelingEffort(NAME)modeltosimulate
theArcticOceancirculationovertheperiod1979–2004.Withahorizontalresolutionof1/128((cid:3)9km),the
simulated26yearmeanvolumetransportplusstandarddeviationwas21.5560.29SvthroughDavisStrait;
20.7760.17SvthroughNaresStrait,and20.7660.12SvthroughLancasterSound.ThewidthsofCardi-
ganStraitandHellGatewere(cid:3)10km,whichcouldnotbewellresolvedinNAME,andthetransportresult
fromJonesSoundwasnotdescribedintheirpaper.Thesimulatedtransportswereingoodagreementwith
observationsinNaresStraitandLancasterSound,butabout0.55Svlowerthanthemeanobservationsin
DavisStrait.Wekerleetal.[2013]appliedtheunstructured-gridFiniteElementSeaice-OceanModel(FESOM)
tosimulatetheCAAoutflowtransportovertheperiod1968–2007.Withahorizontalmodelresolutionofup
to (cid:3)5 km, the simulated 40 year mean volume transport was 21.8160.31 Sv through Davis Strait,
20.9160.16 Sv through Nares Strait, 20.8660.16 Sv through Lancaster Sound, and 20.0460.01 Sv
throughJonesStrait.ThetransportsestimatedbytheFESOMandAO-FVCOMhadnosignificantdifference
inDavisStrait,NaresStrait,andLancasterSound,buttheFESOM-estimatedtransportthroughJonesSound
was(cid:3)0.26SvsmallerthanboththeobservedandAO-FVCOMvalues.Thisdifferenceaccountedfor87%of
thetotaltransportthroughthissound.
3.2.SeasonalandInterannualVariability
Wecompared theAO-FVCOM-simulatedannuallyandmonthlyaveragedvolumetransportswithobserva-
tionsinDavisStraitovertheperiod2004–2010summarizedbyCurryetal.[2014]andinLancasterSound
over the period 1998–2011 described by Prinsenberg and Hamilton [2005], Prinsenberg et al. [2009], and
Petersonetal.[2012].InDavisStrait,theobservedannualmeanoutflowtransportwas22.0Svover2004–
2005, gradually decreased to 21.3 Sv over 2005–2008, increased to 21.8 Sv over 2008–2009 and then
droppedto21.5Svover2009–2010(Figure5,top)[seealsoCurryetal.,2014].Thesimulatedannualmean
outflow transport was 0.1 Sv smaller over 2004–2005, 0.5, 0.6, and 0.5larger over 2005–2008,2008–2009,
and2009–2010,respectively.Themeasurementuncertaintiesoverthesefourperiodswere0.5,0.6,0.4and
0.5 Sv, respectively. Considering these measurement uncertainties, the simulated annual mean outflow
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 9
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0.5
Observed 6-year mean (10/2004-09/2010)
0 Simulated 6-year mean (10/2004-09/2010)
v)-0.5
S
ort ( -1
nsp-1.5
me tra-2-.52
u
ol -3
V
-3.5
-4
1 2 3 4 5 6 7 8 9 10 11 12
Time (month)
3
Observed anomaly (minus 6-year mean: 10/2004-09/2010)
2.5
Simulated anomaly (minus 6-year mean: 10/2004-09/2010)
v) 2
ort (S 1.51
nsp 0.5
e tra-0.50
m
u -1
ol
V-1.5
-2
-2.5
Jan Jan Jan Jan Jan Jan
2005 2006 2007 2008 2009 2010
Figure6.Comparisonsofmonthlysimulatedandobserved(top)volumefluxesand(bottom)anomaliesinDavisStraitovertheperiod
2004–2010.Blue:observed,red:monthlyfluxandanomalyaveragedoverthe6yearsimulationperiodofOctober2004toSeptember
2010.Thebluesolidverticalbarinthetopplotistheobservationaluncertainty.Theredandbluedashedverticalbarsarethesimulated
andobservedstandarddeviations.
transport was in reasonable agreement with observations. The AO-FVCOM captured the interannual vari-
abilityoftheoutflowthroughDavisStraitoverthemeasurementperiods2004–2010.
TheinterannualvariabilityoftheoutflowtransportwasalsoobservedandcapturedbyAO-FVCOMinLan-
casterSound(Figure5,bottom).Bothobservationsandmodelsuggestedadecreaseover2002–2008and
anincreaseover2008–2009,thenadecreaseover2009–2011.Sincenomeasurementuncertaintywasgiven
intheobservedtransportinLancasterSound,itwasdifficulttoestimatemodelerrorsinthisregion.
ItappearedthattheobservedandsimulatedCAAoutflowtransportthroughDavisStraitexhibiteddifferent
seasonalvariationpatterns(Figure6,top).Over2004–2010,thetransportestimatedbytheobserveddata
showedthatthe6yearmonthlyaveragedobservedtransportvariedseasonallyinarangeof(cid:3)1.3Sv,with
theminimumvalueof21.0SvinNovemberandthemaximumvalueof22.3SvinJune.Thesimulatedsea-
sonalvariabilitywasinasimilarrangeof(cid:3)1.4Sv,butwiththeminimumvalueof21.4SvoccurringinOcto-
ber and the maximum value of 22.8 Sv occurring in January. Both observed and simulated monthly
transport anomalies were in a range of 61.6 Sv except December 2009, in which the simulated anomaly
wasabove2.0Sv(Figure6,bottom).
Thedifferencebetweenmaximumandminimumtransportswas1.3Svfortheobservationsand1.4Svforthe
model,whichwasclosetothemeasurementuncertaintyof(cid:3)1.1Sv.Fortheobservations,thetransportdiffer-
encebetweenOctoberandNovemberwasonly0.1Sv,butthemeasurementuncertaintyforthese2months
was0.8–1.0Sv.Iftakingthemeasurementuncertaintyintoaccount,thetimingdifferencebetweenobserved
and simulated minimum transports should not be significant. Similarly, the observed transport difference
betweenJanuaryandJunewas0.6Sv,whichwaswithinthemeasurementuncertaintyof0.8–1.1Sv.Forthe
samereason,onecouldnotconfirmthatthetransportwaslargerinJunethaninJanuary.Luetal.[2014]used
theNucleusforEuropeanModelingoftheOcean(NEMO)tosimulatethetransportthroughDavisStraitover
theperiod1998–2007,andtheirresultsalsoshowedamaximumtransportinwinterratherthansummer.Itis
prematuretoattributethedifferenceinthetimingofthemaximumtransportshowninmodelsandobserva-
tionstothemodeluncertaintyrelatingtoexternalforcing,ice,andwaterstratification.Thereisnodoubt,how-
ever,thatmoreattentionshouldbepaidonthisissueinfutureobservationsandmodeling.
ZHANGETAL. NUMERICALSTUDYONTHECAAVOLUMEFLUX 10