空气净化材料Word文档格式.docx
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IntheeasternSouthPacificOcean,atadepthofabout200m,asalinityminimumisfound.Thisminimumisassociatedwithaparticularwatermass,the"
ShallowSalinityMinimumWater"
(SSMW).SSMWoutcropsinafreshtongue(S^submin^)centeredatabout45°
S.TheS^submin^appearstoemanatefromtheeasternboundary,againstthemeanflow.ThewatermasstransformationthatcreatesSSMWandS^submin^isinvestigatedhere.TheS^submin^andSSMWaretransformedfromsaltierandwarmerwatersoriginatingfromthewesternSouthPacific.Thefresheningandcoolingoccurwhenthewaterisadvectedeastwardatthepolewardsideofthesubtropicalgyre.Sourcesoffresheningandcoolingareair-seaexchangeandadvectionofwaterfromsouthofthesubtropicalgyre.Afreshwaterandheatbudgetforthemixedlayerrevealsthatbothsourcesequallycontributetothewatermasstransformationinthemixedlayer.Thefreshenedandcooledmixedlayerwaterissubductedintothegyreinterioralongthesouthernrimofthesubtropicalgyre.Subductionintothezonalflowrestrictsthetransformationofinteriorpropertiestodiffusiononly.Asimpleadvection/diffusionbalancerevealsdiffusioncoefficientsoforder2000m^sup2^s^sup-1^.ThetongueshapeoftheS^submin^isexplainedfromadynamicalviewpointbecausenorelationtoapositiveprecipitation-evaporationbalancewasfound.FreshestS^submin^valuesarefoundtocoincidewithslowesteastwardmixedlayerflowthataccumulatesthelargestamountsoffreshwaterinthemixedlayerandcreatesthefreshtongueattheseasurface.AlthoughtheSSMWisthedensestandfreshestmodeofwatersubductedalongtheSouthAmericancoast,thefresheningandcoolingintheSouthPacificaffectawholerangeofdensities(25.0-26.8kgm^sup-3^).Thetransformedwaterturnsnorthwardwiththegyrecirculationandcontributestothehydrographicstructureofthegyrefarthernorth.BecausetheSouthPacificprovidesmostofthesourcewatersthatupwellalongtheequatorialPacific,variabilityinSouthPacifichydrographymayinfluenceequatorialPacifichydrography.Becauseone-halfofthetransformationisfoundtobecontrolledthroughEkmantransport,variabilityinwindforcingatthesouthernrimofthesubtropicalgyremaybeasourceforvariabilityoftheequatorialPacific.
(ProQuestInformationandLearning:
...denotesformulaeomitted.)
1.Introduction
Thetropical/extratropicalexchangeofwatercanbeviewedasameridional-verticalorsubtropicalcell(STC)drivenbysubductionandupwelling,whichareconnectedviaEkmantransportandinteriorflow.Intheextratropicswaterissubductedfromthemixedlayerandflowsequatorwardininteriorwind-drivenpathwaysandwesternboundarycurrents.NeartheequatorthiswaterupwellsbackintothemixedlayerandistransportedpolewardtothesubductionsitesthroughthemeridionalcomponentoftheEkmantransport[seeSchottetal.(2004)foranSTCreview].STCshavebeenidentifiedinalloceansusingobservationaldata(e.g.,JohnsonandMcPhaden1999;
Schottetal.2002;
Zhangetal.2003)andmodels(e.g.,McCrearyandLu1994;
Rothsteinetal.1998).
OneinterestinstudyingtheSTCsistheirpotentialinvolvementinlow-frequencyclimatevariabilityoftheocean-atmospheresystem,becausebothupwellingandsubductionarecontrolledthroughair-seaexchangeofmomentum,heat,andfreshwater.InparticularthevariabilityofthePacificOceanSTCisofimportance,becauseitmayberelatedtoElNiñ
o-SouthernOscillation(ENSO)withitsfar-reachingsocioeconomicimpacts.Pacificvariabilityhasbeendecomposedintointerannualandinterdecadalvariability(Zhangetal.1997).InterannualvariabilityisusuallyassociatedwiththeENSOphenomenonandmaybeexplainedthroughatmosphere-oceaninterplayneartheequator.Decadalvariability,however,couldbetheresultofvariabilitythatisgeneratedintheextratropicaloceanandsubsequentlyadvectedtotheequator.Twomechanismshavebeenproposedtogeneratevariabilitybasedontemperature(T)andtransport(v).Kleemanetal.(1999)proposedamodelsolelybasedonfluctuationsinSTCtransport,thatis,withoutfluctuationsinthetemperaturefield(v'
T).Fluctuationsintransportgeneratesea-surfacetemperaturefluctuationsthatcouldfeedbackontheatmosphericcirculation.Observationalevidencefortransportfluctuationsinthenear-equatorialflowhasbeenpresentedby,forexample,McPhadenandZhang(2002)andMeinenetal.(2001).GuandPhilander(1997)proposedamechanismbasedontheadvectionoftemperatureanomalieswithanaverageflowfield(vT'
).Subductedtemperatureanomaliesintheextratropicsappearwithatimelagattheequator.TheirupwellingcausestemperatureanomaliestoappearintheTropicsthatfeedbackontotheatmosphericmeridionalcirculation.Theappearanceofawarmanomalyalongtheequatorstrengthenstheextratropicalwindand,throughanincreaseofevaporation,introducesacoldanomalyintheextratropics.Thesubductionofsuchacoldanomalythenappearswithatimelagalongtheequatorwhereitsupwellinginitiatesacoldanomalythere.However,thepersistenceoftemperatureanomaliessubductedintothesubtropicalgyreiscurrentlyunderdebate.Hindcastrunsofcoupledocean-atmospheremodelssuggestthatequatorialPacificisothermaldepthvariabilitymaybegeneratedbythelocalwindstress(andEkmanpumping)variabilityattheequatorratherthanfromanomaliesofextratropicalorigin(Schneideretal.1999).InparticularfortheNorthPacificlittlecouplingbetweenTropicsandextratropicswasfound.However,YeagerandLarge(2004)identifiedsea-surfacetemperaturevariabilityalongtheequatorgeneratedthroughisopycnaladvectionofnotonlytemperaturebuttemperature/salinity(T/S)anomalies.Theyanalyzedoutputofanoceanmodelforcedwith40yearsofrealisticsurfacefluxes.Theirworkemphasizestheroleofbothheatandfreshwateranomaliesindecadalvariability.
McCrearyandLu(1994)showedthatthermoclinewatermovingequatorwardoriginatesfromtheeastandpolewardsideofthesubtropicalgyres.ThissuggestsfortheSouthPacific(SP)thattheSouthernOcean(SO)canplayaroleinventilatingthePacificequatorialthermocline(Toggweileretal.1991;
Johnson2001).AprominentexchangepathfromtheSOtowardtheequatorinallSouthernHemisphereoceansisthefreshtongueofAntarcticIntermediateWater(AAIW).AAIWhasitsorigininthesubductionofpolarsurfacewaters(see,e.g.,Molinelli1978)andspreadsatthebaseofthesubtropicalgyres,atdepthsbetween600and1000m.However,withacoredensityanomalyofabout27.2kgm^sup-3^,AAIWliesbelowthewaterthatupwellsalongthePacificequator.UpwellingalongthePacificequatoris,rather,fedinthedensityrangeofwatersadvectedintheEquatorialUndercurrent(EUC)andthesubsurfacecountercurrents(SCC)(Roweetal.2000),thesubsurfacepathwaysfromwesttoeast(anddeepandshallow)alongtheequator.TheequatorialPacificisfedbyabout60%fromtheSP.Thedensestwaterthatupwellsis26.8kgm^sup-3^(JohnsonandMcPhaden1999;
Rodgersetal.2003).
AsecondfreshtongueisfoundaboveAAIWintheinteriorofthesoutheastSP(atabout26kgm^sup-3^;
seeFig.1)named"
(SSMW;
Reid1973;
TsuchiyaandTalley1996)orEasternSouthPacificIntermediateWater(EmeryandMeincke1986;
Schneideretal.2003).SSMWisfoundinbothhemispheres.Itsformationhasbeenexplainedthroughthesinkingofsubantarcticsurfacewatersbelowhigher-salinitywaters(e.g.,Reid1973).ConsequentlyoneexpectsacorrespondingsignatureintheseasurfacesalinitywhereSSMWissubducted.NorthPacificsurfacewatersfreshennorthofthesubtropicalgyreandalongtheeasternboundary(Fig.1)sothattheconceptualmodelforSSMWformationholdshere.IntheSP,however,alocalmeridionalfreshtongueislocatedatabout45°
S.Consequently,asimplenorthwardtransferoffreshsurfacewaterwithsubsequentsubductioncannotexplaintheformationofSSMWintheSPentirely.
SSMWoutcropsinafreshsurfacetongue(S^submin^)(seeFig.1).TheshapeofS^submin^promptedanumberofinvestigatorstoinferawestwardflowatthesouthernrimoftheSPsubtropicalgyre(seeDeacon1977forareview).Suchaflow,incombinationwithcoastalfresh-waterinputandapositiveprecipitation-evaporationbalance,wasthoughttogeneratethesurfacesalinitypatternwithlowestsalinitiesintheeast(Davilaetal.2002;
Schneideretal.2003).However,aclearsignalofwestwardflowbetweentheeastward-flowingsubtropicalgyreandtheeastward-flowingextensionoftheACCwasneverdetected.NeshybaandFonseca(1980)suspectedtransienteddiescontributedtothewestwardtransportbutdatacoveragewastoosparsetoprovethisidea.
InthispaperwestudywatermasstransformationinthesouthernpartoftheSPsubtropicalgyretoexplaintheformationofS^submin^andSSMW.Afterintroducingthedata,transformationmechanismsformixedlayerandthermoclinewatersarediscussed.Azonalmixedlayerbudgetisusedtoquantifytheimportanceofair-seaexchangeandadvection.Thentheroleofdiffusioninthewatermasstransformationisdiscussed.TheformationofS^submin^isexplainedasanadvectivefeature.Last,variabilityofthewatermasstransformationanditsrelationtodecadalvariabilityofthehydrographicstructureoftheSPandEquatorialPacificarediscussed.
2.Data
Theoceansurfaceandinteriordataproductsusedinthisstudyaremainlythe1990-99averagetemperature,salinity,andvelocityfieldsfrom