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1、15NOVEMBER2001CHAOANDCHEN3497TheOriginofMonsoonsWINSTONC.CHAOLaboratoryforAtmospheres,NASAGoddardSpaceFlightCenter,Greenbelt,MarylandBAODECHENGESTCenter,UniversityofMaryland,BaltimoreCounty,Baltimore,Maryland(Manuscriptreceived12September2000,in®nalform11May2001)ABSTRACTThemonsoonisinterprete
2、dasanintertropicalconvergencezone(ITCZ)substantiallyaway(morethan108)fromtheequatorandtheexistenceoftheITCZdoesnothavetorelyonland±seacontrast.Land±seacontrastcanprovideafavorablelongitudinallocationfortheITCZbutthisrolecanbereplacedbyseasurfacetemperaturecontrastinthelongitudinaldirection.Th
3、us,theinterpretationofthemonsoonpresentedhereindiffersfromthelong-heldfundamentalbeliefthatitsbasiccauseisland±seathermalcontrastonthecontinentalscaleinthesensethattheexistenceoflandmassisnotconsideredanecessaryconditionformonsoons.Throughgeneralcirculationmodelexperiments,supporthasbeenfound
4、forthisinterpretation.TheAsianandAustraliansummermonsooncirculationsarelargelyintactinanexperimentinwhichAsia,maritimecontinent,andAustraliaarereplacedbyoceanwithseasurfacetemperature(SST)takenfromthatofthesurroundingoceans.Thus,intheseareasland±seacontrastisnotanecessaryconditionformonsoon.T
5、hisalsohappenstotheCentralAmericansummermonsoon.ThesamethingcanalsobesaidabouttheAfricanandSouthAmericansummermonsoons,ifthesecontinentsarereplacedbyoceanofsuf®cientlyhighSST.ItisalsoshownthatintheAsianmonsoonthechangeresultingfromsuchreplacementisduemoretotheremovaloftopographythantotheremov
6、alofland±seacontrast.IntheAsianandAustralianwintermonsoonsland±seacontrastalsoplaysonlyaminorrole.TheoriginoftheITCZsandtheirlatitudinallocationshavebeenpreviouslyinterpretedbyChao.Thecirculationassociatedwithanoff-equatorITCZ,previouslyinterpretedbyChaoandChenthroughamodi®edGillsolutionandbr
7、ie¯ydescribedinthispaper,explainsthemonsooncirculation.ThelongitudinallocationoftheITCZisdeterminedbythedistributionofsurfaceconditions.ITCZsfavorlocationsofhighSSTasinthewesternPaci®candIndianoceans,ortropicallandmass,duetoland±seacontrast,a
