第十章海洋及其在气候系统中的作用

全球气候系统GlobalClimateSystem第十章海洋及其在气候系统中的作用10.1海洋基本特征

10.2大洋环流10.3海-气相互作用10.4海洋对气候的影响10.5温盐环流第十章海洋及其在气候系统中的作用Thewatermoleculehasanumberofunusualproperties.Oneoftheseisthatithasaveryhighheatcapacity.HeatcapacityistheamountofheatthatmustbeputintoonegramofamaterialinordertoraiseitstemperaturebyonedegreeCelsius.Similarly,wemightthinkofitastheamountofheatthatescapesfromonegramofamaterialwhenitcoolsbyonedegreeCelsius.Asitsnameimplies,theheatcapacityisameasureoftheabilityofamaterialtostoreheatenergywhenitstemperatureisraisedandtoreleasethatsameamountofheatenergywhenitcools.Becauseofitshighheatcapacity,thewaterintheworldoceansisanenormousreservoirforthermalenergy.Itservestotemperworldclimate,absorbingheatfromtheSuninthetropicsandredistributingittomorenortherlyregionsviathegreatoceancurrents.Forthisreason,theGulfStreamisabletobringitswarmththousandsofkilometersfromthetropicstotheEuropeanshoresbeforecoolingtothetemperaturesofArcticwaters.

Contrariwise,regionsinwarmlatitudesthatarevisitedbycoldoceancurrentsenjoyanaturalformofairconditioning.ThedelightfulclimateoftheseacoastofCaliforniaisduetothiseffect.Thecircum-Antarcticcurrenttravelsfromwesttoeast,completelyencirclingthecontinentofAntarcticawithacoldcurrentandformingapartialbarriertothetransferofheatfromlowerlatitudestothefrigidAntarcticlandmass.海洋:

巨大的热机OceanheattransportestimatespositivenorthwardsinPetaWatts(1015W),10.1海洋基本特征10.2大洋环流10.3海-气相互作用10.4海洋对气候的影响10.5温盐环流第十章海洋及其在气候系统中的作用Therearethreemainprocessesthatmaketheoceanscirculate:tidalforces,windstress,anddensitydifferences.

Notethatthemajorhorizontaloceancirculationsystemsmirrorcloselythesemi-permanenthighandlowpressuresystems10.1海洋基本特征10.2大洋环流10.3海-气相互作用10.4海洋对气候的影响10.5温盐环流第十章海洋及其在气候系统中的作用10.1海洋基本特征10.2大洋环流10.3海-气相互作用10.4海洋对气候的影响10.5温盐环流第十章海洋及其在气候系统中的作用PacificENSOPDOPDO

ThePacificDecadalOscillation

PDOhasbeendescribedasalong-livedElNino-likepatternofPacificclimatevariabilitybecausethetwoclimateoscillationshavesimilarspatialclimatefingerprints,butverydifferenttemporalbehavior.

TwomaincharacteristicsdistinguishPDOfromENSO:first,20thcenturyPDO"events"persistedfor20-to-30years,whiletypicalENSOeventspersistedfor6to18months;second,theclimaticfingerprintsofthePDOaremostvisibleintheNorthPacific/NorthAmericansector,whilesecondarysignaturesexistinthetropics-theoppositeistrueforENSO.

SeveralindependentstudiesfindevidenceforjusttwofullPDOcyclesinthepastcentury:"cool"PDOregimesprevailedfrom1890-1924andagainfrom1947-1976,while"warm"PDOregimesdominatedfrom1925-1946andfrom1977through(atleast)themid-1990's.October-to-MarchaveragedPDOindicesbaseduponNorthPacificSSTandSLPpatterns.PositivevaluesindicatewarmphasesofPDO,whilenegativevaluesindicatecoolphasesofPDO.

ABCAtlanticNorthAtlantic(THC-NAO)TropicalAtlantic:TAD海气耦合模式模拟的北大西洋温盐环流强度指数与SLP的相关分布(周天军等2000)IndianOceanDipole,monsoon10.1海洋基本特征10.2大洋环流10.3海-气相互作用10.4海洋对气候的影响10.5温盐环流第十章海洋及其在气候系统中的作用Thedensityofseawateriscontrolledbyitstemperature(thermo)anditssalinity(haline),andthecirculationdrivenbydensitydifferencesisthuscalledthethermohalinecirculation.AntarcticBottomWaterAntarcticIntermediateWaterAntarcticSurfaceWaterSaltyShelfWaterRejectedSaltContinentalShelfCircumpolarDeepWaterUpwellingSimulatedD/OandHeinrichevents.a,Forcing,b,Atlanticoverturning,c,Atlanticsalinity(S)at608N,d,airtemperatureinthenorthernNorthAtlanticsector(60±708N),ande,temperatureoverAntarctica(temperaturevaluesaregivenasthedifferencefromthepresent-dayclimate,DT).

AndreyGanopolski&StefanRahmstorf,2001,NatureWillthethermohalinecirculationcollapse?Thecriticalpartofthethermohalinecirculation(THC)isthesinkingintheNorthAtlanticOcean.Tosomeextent,theTHCappearstobeself-sustaining.Ifsomeeventoccurstobreakthisself-sustainingchainofprocesses,thenthereisthepotentialforthecirculationtobreakdownrapidly(i.e.,overseveraldecades)andtoremaininareduced-circulationstateforseveralcenturies.

Somefairlysimplemodelsoftheworld'soceansdosimulatearapidbreakdownoftheTHC,whenthedensityofthewaterintheNorthAtlanticOceanisloweredbyaddingfreshwater(rain)and/orbywarming.IncreasedrainfallandwarmingovertheNorthAtlanticarebothexpectedasaresultofincreasedgreenhousegasconcentrations,andsoitcanbearguedthatglobalwarmingmaycausearapidcollapseofthethermohalinecirculation.

Hurrelletal.2001Whatwouldhappentoourclimateifthethermohalinecirculationcollapsed?Duetotheinteractionsbetweenmanycomponentsoftheclimatesystem,itisnotasimplemattertoestimatehowdifferentourclimatewouldbewithoutthecurrentthermohalinecirculation.CertainlythebiggestimpactwouldbeonthetemperatureovertheNorthAtlanticandEurope.Simulationswithacomplexclimatemodel,whosether