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The stratospheric mean-meridional circulation (MMC) and eddymixing are compared among six meteorological reanalysis data sets: NCEP-NCAR,NCEP-CFSR, ERA-40, ERA-Interim, JRA-25, and JRA-55 for the period 1979–2012.The reanalysis data sets produced using advanced systems (i.e., NCEP-CFSR,ERA-Interim, and JRA-55) generally reveal a weaker MMC in the NorthernHemisphere (NH) compared with those produced using older systems (i.e.,NCEP/NCAR, ERA-40, and JRA-25). The mean mixing strength differs largelyamong the data products. In the NH lower stratosphere, the contribution ofplanetary-scale mixing is larger in the new data sets than in the old datasets, whereas that of small-scale mixing is weaker in the new data sets.Conventional data assimilation techniques introduce analysis incrementswithout maintaining physical balance, which may have caused an overly strongMMC and spurious small-scale eddies in the old data sets. At the NHmid-latitudes, only ERA-Interim reveals a weakening MMC trend in the deepbranch of the Brewer–Dobson circulation (BDC). The relative importance ofthe eddy mixing compared with the mean-meridional transport in thesubtropical lower stratosphere shows increasing trends in ERA-Interim andJRA-55; this together with the weakened MMC in the deep branch may imply anincreasing age-of-air (AoA) in the NH middle stratosphere in ERA-Interim.Overall, discrepancies between the different variables and trends therein asderived from the different reanalyses are still relatively large, suggestingthat more investments in these products are needed in order to obtain aconsolidated picture of observed changes in the BDC and the mechanisms thatdrive them

Inter-comparison of stratospheric mean-meridional circulation and eddy mixing among six reanalysis data sets