responsetochangesinatmosphericgreenhousegasconcentrationsIntheAGCMsimulationbyHostetleretal2006theprescribedbiasadjustedSSTsproduceapositivemassbalanceoverthenorthernhemisphereicesheetsandleadto ID: 439220
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QuaternaryScienceReviews25(2006)1150 1151HowcoldwerethetropicsandsubtropicsattheLastGlacialMaximum?Thespatialpatternandmagnitudeoftropicalandsubtropicalseasurfacetemperatures(SSTs)attheLastGlacialMaximum(LGM)havebeenamajorchallengeformanyyears.TherangeoftheLGMtropicalandsubtropicalcoolingremainsamajorsourceofuncertaintiesforourunderstandingoftheLGMclimate.TheglobalSST responsetochangesinatmosphericgreenhousegasconcentrations.IntheAGCMsimulationbyHostetleretal.(2006)theprescribedbias-adjustedSSTsproduceapositivemassbalanceoverthenorthernhemisphereicesheets,andleadtogreatercoolinginthenorthernhemisphereandlesscoolinginthesouthernhemisphere,relativetothecasewithprescribedCLIMAPSSTs.Thisinterhemisphericcontrastinthesimulatedresponse(greatercoolinginthenorthernhemisphereandlesscoolinginthesouthernhemisphere)mayresultinasouthwardshiftoftheIntertropicalConvergenceZone(ITCZ)overthePacicandtheAtlanticinafullycoupledocean-atmospheresystem.ThereisnocoupledfeedbackintheAGCMsimulationswithSSTsspeciedastheoceansurfaceboundaryconditions.Wouldthebias-adjustedSSTpatternbemaintainedwiththeAGCM-simulatedresponseiftheatmosphere oceansystemwerefullycoupled?Howwilltheresultsbeaffectedbythecoupledfeedbackinvolvingrealisticoceandynamicsandair seainteractions?Thebias-adjustedCLIMAPSSTsinHostetleretal.agreewellwithmanyrecentgeochemicalSSTestimates,exceptthatfromcoreV21 30intheeasterntropicalPacic.TheLGMSSTestimatedbytheMg/CamethodatcoreV21 30(southoftheequatorintheeasterntropicalPacic)showsaweakercooling(1.2Koutavasetal.,2002).Meanwhile,thesameMg/CamethodindicatesthatLGMSSTsfromthewesternPacicwarmpoolandfromnorthoftheequatorintheeasterntropicalPacicweremuchcolder(Leaetal.,2000Stottetal.,2002).Theseresultssuggestthatrelativetopresent,theLGMnorth southSSTcontrastacrosstheequatorintheeasterntropicalPacicandeast westSSTcontrastinthesoutherntropicalPacicwerereduced,indicatinganElNinpatterninthesoutherntropicalPacicandasouthwardshiftofthePacicITCZduringtheLGM(Koutavasetal.,Stottetal.,2002).Thequestionishowtoreconcilethebias-adjustedCLIMAPSSTsderivedfromfaunaltransferfunctionswiththereducednorth southSSTcontrastintheeasterntropicalPacicandthereducedeast westSSTcontrastinthesoutherntropicalPacicestimatedbytheMg/Camethod.AmoreaccurateestimateoftheLGMtropicalPacicSSTcontrastiscrucialtoourunderstandingoftheITCZpositionandtheoceancirculationinthetropicalPacicattheLGM.ReferencesHostetler,S.,Pisias,N.,Mix,A.,2006.SensitivityofLastGlacialMaximumclimatetouncertaintiesintropicalandsubtropicaloceantemperatures.QuaternaryScienceReviews,thisissue,doi:10.1016/j.quascirev.2005.12.010Koutavas,A.,Lynch-Stieglitz,J.,MarchittoJr.,T.M.,Sachs,J.P.,2002.Elnino likepatterniniceagetropicalPacicseasurfacetemperature.Science297,226 230.Lea,D.W.,Pak,D.K.,Spero,H.J.,2000.ClimateimpactofLateQuaternaryequatorialPacicseasurfacetemperaturevariations.Science289,1719 1724.Pinot,S.,Ramsteim,G.,Harrison,S.P.,Prentice,I.C.,Guiot,J.,Stute,M.,Joussaume,S.,1999.TropicalclimatesattheLastGlacialMaximum:comparisonofpaleoclimatemodelingintercom-parisonproject(PMIP)simulationsandpaleodata.ClimateDynamics15,857 874.Rind,D.,Peteet,D.,1985.TerrestialconditionsattheLastGlacialMaximumandCLIMAPseasurfacetemperatureestimates:aretheyconsistent?.QuaternaryResearch24,1 22.Stott,L.,Poulsen,C.,etal.,2002.SuperENSOandglobalclimateoscillationsatmillennialtimescales.Science297,222 226.Yin,J.H.,Battisti,D.S.,2001.TheimportanceoftropicalseasurfacetemperaturepatternsinsimulationsofLastGlacialMaximumclimate.JournalofClimate14,565 581.RongZhangGFDL/AOSProgram,ForrestalCampus,Route1,Prince-tonUniversity,Princeton,NJ08542,USAE-mailaddress:Rong.Zhang@noaa.gov.Context/QuaternaryScienceReviews25(2006)11501151