简介：TheNCEP/NCARreanalysisdatahavebeenemployedtodiagnosevariationsofthezonalmeanflowinborealsummer.TwoleadingEOFmodesarefoundtodominatethespatialandtemporalchangesofthesummertimezonalmeanwindsinthetroposphere.EOF1showsthedistributionofzonal-meanflowanomalieswithhighervarianceintheNorthPolarRegion,whereastheEOF2showsthedistributionofzonal-meanflowanomalieswithhighervarianceintropicalandextra-tropicalregions.TheEOF1andEOF2haverespectivelytheperiodicitiessimilartothoseofAOandENSO.SignificantlagcorrelationshavebeenfoundbetweenEOF1andENSO,andbetweenEOF2andAO,intheseasonsincludingspring,autumn,andwinter.However,nosignificantcorrelationshavebeenfoundbetweenEOF1insummerandENSOinanyotherseasons,andbetweenEOF2insummerandAOinotherseasons,nomatterhowbigthelagthatrepresentsnumberofseasonshasbeenset.Theseresultssuggestthattheprincipalmodesofsummertimezonalmeanflowcouldbestatisticallyseparatedfromeachother.Hence,EOF1andEOF2arephysicallyrelatedtotheAOandENSOrespectively.Atheorycalledquasi-geostrophicnon-accelerationtheoremhasbeenusedtopartlyexplainthepossiblemechanismsofthemaintenanceofthetwoprincipalmodes.ThecompositedifferencesofthedivergenceofEliassen-Palmflux(E-Pflux)betweenpositiveandnegativeyearsasobtainedfromthetimeseriesofEOF1andEOF2displaythedistributionsthatcontributetothezonalmeanwindanomaliesrepresentedbyEOF1andEOF2,respectively.TheplanetaryotherthanthesynopticwavesdominatethebehaviorsoftheE-Pfluxes,suggestingthecrucialroleoftheplanetarywavesinthemaintenanceofthezonalmeanflowanomalies.Theresidualcirculationaswellasthefriction,whichcancelthedivergenceoftheE-Pflux,alsoplayanimportantroleinsomeplaces.TheseresultsareveryhelpfulforourbetterunderstandinghowtheanomalouszonalmeanflowsmaintainandhowtheENSOandAOinfluencetheglo

简介：热带季节内振荡和北极涛动的持续异常会对全球大气环流产生影响,同样会对东亚地区的旱涝造成影响。本文针对云南及周边地区近年来频繁出现的极端干旱事件进行了分析。发现当热带大气季节内振荡（MJO）和北极涛动（AO）两者持续异常时,会对云南的降水产生影响。2009/2010秋、冬、春3季连旱就是由MJO和AO出现极端异常（MJO异常不活跃;AO异常偏弱）造成的。2011年云南主汛期极端伏旱的产生则是由于MJO持续不活跃和西太平洋高压脊异常偏北造成的。