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