空气净化材料.docx

空气净化材料.docx

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空气净化材料

FormationoftheSouthPacificShallowSalinityMinimum:

ASouthernOceanPathwaytotheTropicalPacific

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,becauseitmayberelatedtoElNiño-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"ShallowSalinityMinimumWater"（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