空气净化材料

1、FormationoftheSouthPacificShallowSalinityMinimum:ASouthernOceanPathwaytotheTropicalPacificIntheeasternSouthPacificOcean,atadepthofabout200m,asalinityminimumisfound.Thisminimumisassociatedwithaparticularwatermass,theShallowSalinityMinimumWater(SSMW).SSMWoutcropsinafreshtongue(SAsubminA)centeredatabou

2、t45S.2sAsup-1a.-3人).Thetransformedwaterturnsnorthwardwiththegyrecirculationandcontributestothehydrographicstructureofthegyrefarthernorth.BecausetheSouthPacificprovidesmostofthesourcewatersthatupwellalongtheequatorialPacific,variabilityinSouthPacifichydrographymayinfluenceequatorialPacifichydrography

3、.Becauseone-halfofthetransformationisfoundtobecontrolledthroughEkmantransport,variabilityinwindforcingatthesouthernrimofthesubtropicalgyremaybeasourceforvariabilityoftheequatorialPacific.(ProQuestInformationandLearning:.denotesformulaeomitted.)IntroductionThetropical/extratropicalexchangeofwatercanb

4、eviewedasameridional-verticalorsubtropicalcell(STC)drivenbysubductionandupwelling,whichareconnectedviaEkmantransportandinteriorflow.Intheextratropicswaterissubductedfromthemixedlayerandflowsequatorwardininteriorwind-drivenpathwaysandwesternboundarycurrents.Neartheequatorthiswaterupwellsbackintothemi

5、xedlayerandistransportedpolewardtothesubductionsitesthroughthemeridionalcomponentoftheEkmantransportseeSchottetal.(2004)foranSTCreview.STCshavebeenidentifiedinalloceansusingobservationaldata(e.g.,JohnsonandMcPhaden1999;Schottetal.2002;Zhangetal.2003)andmodels(e.g.,McCrearyandLu1994;Rothsteinetal.199

6、8).OneinterestinstudyingtheSTCsistheirpotentialinvolvementinlow-frequencyclimatevariabilityoftheocean-atmospheresystem,becausebothupwellingandsubductionarecontrolledthroughair-seaexchangeofmomentum,heat,andfreshwater.InparticularthevariabilityofthePacificOceanSTCisofimportance,becauseitmayberelatedt

7、oElNi?o-SouthernOscillation(ENSO)withitsfar-reachingsocioeconomicimpacts.Pacificvariabilityhasbeendecomposedintointerannualandinterdecadalvariability(Zhangetal.1997).InterannualvariabilityisusuallyassociatedwiththeENSOphenomenonandmaybeexplainedthroughatmosphere-oceaninterplayneartheequator.Decadalv

8、ariability,however,couldbetheresultofvariabilitythatisgeneratedintheextratropicaloceanandsubsequentlyadvectedtotheequator.Twomechanismshavebeenproposedtogeneratevariabilitybasedontemperature(T)andtransport(v).Kleemanetal.(1999)proposedamodelsolelybasedonfluctuationsinSTCtransport,thatis,withoutfluct

9、uationsinthetemperaturefield(vT).Fluctuationsintransportgeneratesea-surfacetemperaturefluctuationsthatcouldfeedbackontheatmosphericcirculation.Observationalevidencefortransportfluctuationsinthenear-equatorialflowhasbeenpresentedby,forexample,McPhadenandZhang(2002)andMeinenetal.(2001).GuandPhilander(

10、1997)proposedamechanismbasedontheadvectionoftemperatureanomalieswithanaverageflowfield(vT).Subductedtemperatureanomaliesintheextratropicsappearwithatimelagattheequator.TheirupwellingcausestemperatureanomaliestoappearintheTropicsthatfeedbackontotheatmosphericmeridionalcirculation.Theappearanceofawarm

11、anomalyalongtheequatorstrengthenstheextratropicalwindand,throughanincreaseofevaporation,introducesacoldanomalyintheextratropics.Thesubductionofsuchacoldanomalythenappearswithatimelagalongtheequatorwhereitsupwellinginitiatesacoldanomalythere.However,thepersistenceoftemperatureanomaliessubductedintoth

12、esubtropicalgyreiscurrentlyunderdebate.Hindcastrunsofcoupledocean-atmospheremodelssuggestthatequatorialPacificisothermaldepthvariabilitymaybegeneratedbythelocalwindstress(andEkmanpumping)variabilityattheequatorratherthanfromanomaliesofextratropicalorigin(Schneideretal.1999).InparticularfortheNorthPa

13、cificlittlecouplingbetweenTropicsandextratropicswasfound.However,YeagerandLarge(2004)identifiedsea-surfacetemperaturevariabilityalongtheequatorgeneratedthroughisopycnaladvectionofnotonlytemperaturebuttemperature/salinity(T/S)anomalies.Theyanalyzedoutputofanoceanmodelforcedwith40yearsofrealisticsurfa

14、cefluxes.Theirworkemphasizestheroleofbothheatandfreshwateranomaliesindecadalvariability.McCrearyandLu(1994)showedthatthermoclinewatermovingequatorwardoriginatesfromtheeastandpolewardsideofthesubtropicalgyres.ThissuggestsfortheSouthPacific(SP)thattheSouthernOcean(SO)canplayaroleinventilatingthePacifi

15、cequatorialthermocline(Toggweileretal.1991;Johnson2001).AprominentexchangepathfromtheSOtowardtheequatorinallSouthernHemisphereoceansisthefreshtongueofAntarcticIntermediateWater(AAIW).AAIWhasitsorigininthesubductionofpolarsurfacewaters(see,e.g.,Molinelli1978)andspreadsatthebaseofthesubtropicalgyres,a

16、tdepthsbetween600and1000m.However,withacoredensityanomalyofabout27.2kgmAsup-3A,AAIWliesbelowthewaterthatupwellsalongthePacificequator.UpwellingalongthePacificequatoris,rather,fedinthedensityrangeofwatersadvectedintheEquatorialUndercurrent(EUC)andthesubsurfacecountercurrents(SCC)(Roweetal.2000),thesu

17、bsurfacepathwaysfromwesttoeast(anddeepandshallow)alongtheequator.TheequatorialPacificisfedbyabout60%-3人(JohnsonandMcPhaden1999;Rodgersetal.2003).AsecondfreshtongueisfoundaboveAAIWintheinteriorofthesoutheastSP(atabout26kgmAsup-3A；seeFig.1)namedShallowSalinityMinimumWater(SSMW;Reid1973;TsuchiyaandTall

18、ey1996)orEasternSouthPacificIntermediateWater(EmeryandMeincke1986;Schneideretal.2003).SSMWisfoundinbothhemispheres.Itsformationhasbeenexplainedthroughthesinkingofsubantarcticsurfacewatersbelowhigher-salinitywaters(e.g.,Reid1973).Consequentlyoneexpectsacorrespondingsignatureintheseasurfacesalinitywhe

19、reSSMWissubducted.NorthPacificsurfacewatersfreshennorthofthesubtropicalgyreandalongtheeasternboundary(Fig.1)sothattheconceptualmodelforSSMWformationholdshere.IntheSP,however,alocalmeridionalfreshtongueislocatedatabout45.Consequently,asimplenorthwardtransferoffreshsurfacewaterwithsubsequentsubduction

20、cannotexplaintheformationofSSMWintheSPentirely.SSMWoutcropsinafreshsurfacetongue(SAsubminA)(seeFig.1).TheshapeofSAsubminApromptedanumberofinvestigatorstoinferawestwardflowatthesouthernrimoftheSPsubtropicalgyre(seeDeacon1977forareview).Suchaflow,incombinationwithcoastalfresh-waterinputandapositivepre

21、cipitation-evaporationbalance,wasthoughttogeneratethesurfacesalinitypatternwithlowestsalinitiesintheeast(Davilaetal.2002;Schneideretal.2003).However,aclearsignalofwestwardflowbetweentheeastward-flowingsubtropicalgyreandtheeastward-flowingextensionoftheACCwasneverdetected.NeshybaandFonseca(1980)suspe

22、ctedtransienteddiescontributedtothewestwardtransportbutdatacoveragewastoosparsetoprovethisidea.InthispaperwestudywatermasstransformationinthesouthernpartoftheSPsubtropicalgyretoexplaintheformationofSAsubmnandSSMW.Afterintroducingthedata,transformationmechanismsformixedlayerandthermoclinewatersaredis

23、cussed.Azonalmixedlayerbudgetisusedtoquantifytheimportanceofair-seaexchangeandadvection.Thentheroleofdiffusioninthewatermasstransformationisdiscussed.TheformationofSAsubminAisexplainedasanadvectivefeature.Last,variabilityofthewatermasstransformationanditsrelationtodecadalvariabilityofthehydrographic

24、structureoftheSPandEquatorialPacificarediscussed.DataTheoceansurfaceandinteriordataproductsusedinthisstudyaremainlythe1990-99averagetemperature,salinity,andvelocityfieldsfromthebeta-7versionofthesimpleoceandataassimilation(SODA)analysis(Cartonetal.2000a,b).SODAinvolvesanoptimalinterpolationassimilat

25、ionofdatainanumericalmodel(theModularOceanModelMOM-2,GFDL,Princeton,NewJersey).Seasurfacetemperature,altimeterseasurfaceheight,andtemperatureandsalinityprofiledataareassimilated.Themodeldomaincoversalloceansbetween60S/Nwithahorizontalresolutionof1xiinlatitude/longitudeinthesubtropics,0.45xiintheTrop

26、ics.Themodelisoptimizedforphysicsoftheupperocean;14ofits20verticallevelsplacedintheupper500m.WewilltreattheSODAanalysisfieldsasupper-oceanclimatologies.Surfacefluxmomentum,evaporation,andprecipitationfieldsaretakenfromtheNationalCentersforEnvironmentalPrediction(NCEP)-NationalCenterforAtmosphericRes

27、earch(NCAR)reanalysis(Kistleretal.2001),whichwasalsousedtogenerateorforcetheSODAanalysis(Cartonetal.2000b).ComparisonstudiesbetweenNCEP-NCARreanalysisdataanddirectobservations(e.g.,Smithetal.2001)foundtheNCEPdatatooverestimatefluxes(latentandsensibleheat)byabout20WmAsup-2a.Thisintroducesanerrorforth

28、enetheatfluxaswellasforthefreshwaterflux(precipitation-evaporationbalance)vialatentheat.Wedecidedtouseboththeoriginaldata(NCEP)andaversionreducedby20WmAsup-2A(NCEP-20W)forthecalculations.Forcomparison,theSouthamptonOceanographicCenter(SOC)air-seafluxfields(Joseyetal.1998;GristandJosey2003)aswellasth

29、edaSilvaetal.(1994a,b)climatologyareused.Botharebasedonobservationaldataandbothclimatologiesareconstrainedviaalinearinverseanalysisusinghydrographicoceanheattransports.Upper-layerhydrographyanddynamicsThehydrographicstructureoftheSPsubtropicalgyreiscomplexincomparisonwiththeotheroceans.Temperature/s

30、alinity(T-S)diagramsofzonalsectionsinthesubtropicsat20Sand30S(Fig.2)inthethreeoceansrevealthattheinterioroftheSPcannotbecharacterizedbyasingleCentralWaterlineasinthesouthernIndianandAtlanticOceans,butshowsmorescatter,mostpronouncedatabout26kgmAsup-3A.ThesouthernIndianandAtlanticOceansarequitesimilar

31、intheirT/Sdistributionsoverthedensityrange26-27kgmAsup-3A,whichmatchesalsothewesternSPcharacteristic.Towardtheeast,however,theSPisconsiderablyfresherandcolderonisopycnalsthantheothertwooceans.Toinvestigatethisfeaturewerecallthemechanismsresponsibleforthetransformationofpropertiesofthemixedlayerandth

32、ewaythemodifiedpropertiesaretransferredintotheinterior(Fig.3).Themixedlayerisdominatedbymixingonvarioustimeandspacescales(see,e.g.,Woods1985).Wateradvectednearthesurfacefromotheroceanregions,suchasthefresherwaterfromtheACCintheSPorrainwater,israpidlyblendedintothemixedlayerandchangeitsproperties.The

33、interiorconnectionwiththemixedlayer,theoutcrop,migratesmeridionallythroughtheseasonalcycleofair-seaexchange,polewardinsummerandequatorwardinwinter.Thismovementisaccompaniedbyaseasonalstratificationunderneaththemixedlayer,theseasonalthermocline.Animportantcontrolsurfaceinthermocline-mixedlayersystemi

34、sthebaseofthedeepestmixedlayer,usuallyfoundinlatewinter.Mostdramaticchangesinthedepthofthemixedlayerbaseusuallyoccurbetweenlatewinterandearlyspring.Thedepthchangetrapswinterpropertiesintheseasonalthermoclineandanetfluxfrommixedlayerintothermoclineoccurs(Stommel1979;Woods1985;Cushman-Roisin1987).Howe

35、ver,exchangeoccursyear-roundandcanhavebothdirections:fluxoutofthemixedlayer(subduction)andintothemixedlayer(obduction;QiuandHuang1995).Thedominantventilationtimescalesofthemixedlayerareseasonal,excludingpossiblelonger-termchangesinthemixedlayertopographyandforcing.Theinterior/thermoclineventilationt

36、imescalesdependonthevolumeofneighboringisopycnalsandtheannualmeansubductionrateintotherespectiveisopycnals.Ventilationtimesofthepermanentthermoclinearelargerthanayear,uptotheorderofdecades.Large-scaleflowinthemixedlayeristhesumofgeostrophicandageostrophiccomponents(e.g.,Wijffelsetal.1994).Theageostr

37、ophiccomponent,mainlytheEkmancurrent,dominatesnearthesurfacebutvanisheswithdepthwherethegeostrophiccomponentgainimportance.TheverticalintegraloftheEkmancurrent,theEkmantransport,constitutestheupperbranchoftheSTCoverlargepartsofthemidlatitudesubtropicalgyres.Atlowlatitudesaninteriorgeostrophicreturnf

38、lowhasalsobeendocumented(Wyrtki1981).AdirectestimateoftheEkmancurrentsandassociatedtransportsispossiblefromacombinationofhydrographicandvelocitymeasurements(see,e.g.,Wijffelsetal.1994).However,ifoneisonlyinterestedintheverticalintegratedtransportcomponents,theycanbederivedfromthemeridionalandzonalwi

39、ndstress(e.g.,Gill1982).Incontrasttothemixedlayer,thethermoclineisdominatedbylateral/isopycnaltransportofproperties,ratherweakcross-isopycnalmixing,andgeostrophicflow.SurfaceforcingandmixedlayerdynamicsWatermasstransformationoccursatboundariesthroughexchangeprocessesorintheinteriorthroughmixing.Mixe

40、dlayerwaterisconstantlytransformedthroughair-seainteraction,lateraladvectioninthemixedlayer,andentrainmentatthemixedlayerbase.StrongestadvectioninthemixedlayeroccursnearthesurfacebyEkmanflow.OverlargepartsofthesubtropicalgyresthewindfieldispredominatelyzonalandconsequentlytheEkmantransportismeridion

41、al(Fig.4a).InthecontextoftheSTC,thezeromeridionalEkmantransportline,locatedintheSPatabout30S(Fig.4a),istheseparatorbetweenhigher-andlower-latitudeoriginofthesurfacewatersparticipatingintheSTC.ThezeromeridionalEkmantransportlinecoincideswithasurfacedensityanomalyofabout25kgmAsup-3A(Fig.4b).Overonesea

42、sonalcycle(oneyear)therelativepositionbetweendensityandzeromeridionalEkmantransportisfairlyfixedintheeastwhileitismorevariableinthewest,changingintheTasmanSeabetween30Sinsummerand40Sinwinter(notshownhere).Thereverseistruefortheseasonalmovementofthesurfaceoutcropdensity(notshownhere):Inthewestitfollo

43、wsthemovementofthezeromeridionalEkmantransport(CkgmAsup-3人)whileintheeastvariabilityishigher(0.5kgMAsup3A).AlthoughthemeridionalEkmantransportdominatesoverlargepartsoftheSP,thezonalEkmantransportplaysaroleintheeasternboundaryupwellingregion.Itadvectsrelativelycoldandsalinewaterfromtheupwellingregion

44、andexplainsthehomogeneousandrelativehighsurfacedensityintheeasternsubtropics(Fig.4b).Oceanheatloss(Fig.4c)isintenseinthewesternboundarycurrentregionandintheTasmanSea.Here,warmtropicalwatersareadvectedpolewardintoregionsunderlyingacolderatmosphere.Highevaporationleadingtohighlatentandsensibleheatloss

45、isaconsequence.iand50Sfromabout20*o80WisasignificantfeaturerelatedtothenorthwardadvectionoffreshandcoldwaterthroughEkmantransportfromthesouthaswellastoapositivefreshwaterflux.Bothadvectionandthepositivefresh-waterfluxpromotetheformationofashallowbarrierlayerinsummer,blockingtheverticalheatexchange,a

46、ndsurfacewaterswarmmoreintensely.Highinterannualvariabilityoftheseasurfacetemperatureinsummerhasbeenobservedinthisregion(A.Montecinos2002,personalcommunication),whichispossiblyrelatedtothevariabilityinbarrier-layerintensity.Thebarrierlayeriserodedinautumn/winterthroughconvectiveoverturningdrivenbyhe

47、atloss.Apositivefreshwaterfluxcanbefoundsouthof30S(Fig.4d)andisconnectedwiththewesternequatorialPacificthroughtheSouthPacificconvergencezone(SPCZ).TheSPCZisaresultofconvergentflowaroundtheIndonesianlowandhighpressureovertheeasternsubtropicalSouthPacific.ThesurfacepatternofEkmantransportaswellasheata

48、ndfreshwaterfluxessuggeststhattheupperbranchoftheSTCcanbesplitintotwotransformationregions:Oneisforwaterdenserthanabout25kgmAsup-3Athatistransformedsouthof30S.Thisbranchwillbediscussedbelowinmoredetail.Thesecondbranchisforwaterlessdensethan25kgmAsup-3Athathasitssourceintheequatorialupwelling.Heremix

49、edlayerwateristransformedthroughthenegativefreshwaterfluxintheeastandthepositivefreshwaterfluxinthewest.Wewillnotdiscussthisbranchindetail.ToillustratehowthewaterparcelsbehaveintheEkmanlayer,trajectorieswerecalculatedforparticlesreleasedat5and53r(Fig.4e).Theaverageoftheuppertwolayervelocitiesfromthe

50、SODAanalysiswasused.TheflowpatternsareoverlargepartsasonewouldexpectfromtheEkmanlayertransport(Fig.4a).Thetrajectoriesareorientedmeridionallyandconvergeat30S.Tworegionsarenotreachedbyparticles:Inthesouthwest,westofNewZealand,particleshavetheiroriginfartherwest,southofAustralia.Thisisinagreementwithw

51、hatweseefromtheinteriorpropertiessuggestingwesternPacificwatertobesimilartothesouthernIndianOceanwater(Fig.2).Theotherexceptionalregionisbetween10and30SintheeasternSP.Themixedlayerhereispopulatedthroughparticlesthatoriginatefromtheeasternboundaryupwelling.FromtheEkmantransportdivergencethemeanvertic

52、alpumpingtermwasderived(Fig.4f).MoreorlessthewholegyreisdominatedbyEkmanpumping.ExceptionsaretheeasternboundaryupwellingregionandundertheSPCZnorthof20S.Ingeneral,EkmanpumpingisstrongernorthofthezeromeridionalEkmantransportline.ExchangesbetweenmixedlayerandthermoclineThemixedlayerisnotonlymodifiedthr

53、oughair-seaexchangeandadvection,butalsothroughthenetexportofwaterthroughsubductionatthebaseofthemixedlayer(Fig.3).EarlierSPstudies(deSzoeke1987;HuangandQiu1998;KarstensenandQuadfasel2002)haveshownthatinourregionofinterest,southof30S,thesubtropicalgyreisdominatedbysubduction.Consequentlyweconsiderint

54、hefollowingonlythenetexport.SubductionintheSPhasbeenanalyzedbydeSzoeke(1987)basedontheclassicalLuytenetal.(1983)model,whichaccountsforanEkman-pumping-drivensubductiononly(cf.Fig.4f).Laterstudiesconsideredtheseasonalmixedlayervariability(HuangandQiu1998;KarstensenandQuadfasel2002).However,fordensitya

55、nomaliessmallerthan26.6kgmAsup-3人transportsaresimilarinallpublicationsandoforder25Sv(Sv=10Asup6AmAsup3AsAsup-1A).-3人，twosubductionregionsareidentified.BothincorporateequatorialupwellingaswellaswaterfromsouthofthezeromeridionalEkmantransportline.IntheTasmanSea,inthewest,waterwithanearlyconstantsalini

56、tyof35.5andtemperaturesbetween15and25subducted.PartofthewaterhasbeencalledSouthPacificSubtropicalModeWater(RoemmichandCornuelle1992)althoughSouthPacificWesternSubtropicalModeWater(SPWSTMW)appearstobemoreappropriatetodistinguishitfromitseasterncounterpart.ThesubductedwatersdenserthanSPWSTMWresembleth

57、eT-SpropertiesoftheothertwoSouthernHemispheregyres(Fig.2).WithinthemodewaterdensityrangebutintheeasternSP,ventilationoccursoveramuchwiderrangeofsalinitiesfrom34to36.5andtemperaturesfrom10o25C,buttheytendtocompensateindensity.Notetheweaksurfacedensitygradientatthesurface(Fig.4b).Watersubductedinthisr

58、egionhasbeentermedSouthPacificEasternSubtropicalModeWater(SPESTMW;HanawaandTalley2001;WongandJohnson2003)and,asitswesterncounterpart,incorporatesequatorialandsouthernsourcewaters.Last,athirdsubductionregionislocatedatthesouthernrimoftheSPsubtropicalgyre.Here,mixedlayerwaterwithcharacteristicsinbetwe

59、enwesternandeasternwaterissubductedand,aswillbeshownbelow,modifiestheinteriorpropertiesthroughlateraldiffusion.Foreachregionwestern(S1),eastern(S2),andsouthern(S3)(Fig.6),theannualmeansubductionrate(SAsubannA)wasevaluatedfromananalysisoftheverticalvelocitiesatthebaseofthewintermixedlayer(H)(Marshall

60、etal.1993):SAsubannA=-wAsub屮-uAsub屮?H.isH).Twocontributionsmakeuptheannualmeansubduction:1)thecorrectedEkmanpumpingterm(-WAsubHA),whichincorporatesacorrectionforthemeridionalbarotropicforcingofthemixedlayerthroughthewind(Williams1989),and2)thecomponentofthehorizontalgeostrophicflowperpendiculartothe