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1、1Multipleregimesandlow-frequencyoscillationsintheSouthernHemisphere'szonal-mean
owSeongjoonKoo,AndrewW.Robertson,andMichaelGhilDepartmentofAtmosphericSciencesandInstituteofGeophysicsandPlanetaryPhysics,UniversityofCalifornia,LosAngeles,LosAngeles,CaliforniaShorttitle:
2、MULTIPLEREGIMESANDLOW-FREQUENCYOSCILLATIONS2Abstract.Multipleregimesintheatmosphericzonal-mean
owoftheSouthernHemisphereareexaminedusing53yearsofdailyobservationalanalysisdata.Estimatesoftheprobabilitydensityfunctioninthereducedphasespacespannedbythetwoleadingempirica
3、lorthogonalfunctions(EOFs)arefoundtoexhibitsigni cantdeviationsfrombivariateGaussianity.Twomajorregimesareidenti ed,whichdescribetheextremephasesofatmosphericzonal-
owvacillation:inthehigh-latituderegimethejetisdisplacedpolewardfromitsclimatologicalmeanposition,whilei
4、nthelow-latituderegimeitisdisplacedequatorward.Thetwozonal-
owregimesaremaintainedbytheforcingduetotransienteddiesagainstsurfacefriction.Signi cantlow-frequencyoscillatorycomponents,withpeaksnear135and70days,arefoundintheEOFmodeassociatedwithzonal-
owvacillation.These
5、oscillationsaredescribedusingboththemulti-tapermethodandsingularspectrumanalysis.Closeexaminationoftherelationshipbetweentheregimesandtheoscillationssuggeststhatthehigh-andlow-latituderegimesareassociatedwiththeextremesofeitherlow-frequencyoscillation.Weillustratethep
6、otentialforimprovedlong-rangeforecastsofthetwoextremephasesofthevacillationusingthe70-daylow-frequencyoscillationasapredictor.31.IntroductionLow-frequencyvariability(LFV)oftroposphericmid-latitudecirculationintheSouthernHemisphere(SH)containsalargezonallysymmetriccomp
7、onent[Kidson,1988,1991;Karoly,1990].Thisisduetothelimitedroleoftopographyandland-seacontrastintheSHcirculation[Newton,1972].TheleadingLFVmodeofthezonal-meanzonal
owintheSHthustakestheformofexcursionsofthezonaljetfromitsclimatologicalmeanposition,withequivalentbarotr
8、opicdipolaranomaliescenterednear40Sand60S[Kidson,1988].HartmannandLo[1998]foundthismeridionalshiftofthemid-latitudezonaljett
