Observational Zonal Mean Flow Anomalies: Vacillation or Poleward Propagation?

North-south displacements and meridional vacillations of the eddy-driven jet are widely accepted as the dominant cause of variability of the observational zonal-mean zonal wind anomalies(denoted u]’).In this study,a new idea regarding the primary variability of the observational u]’ in the middle latitude troposphere is presented.It is hypothesized that there are two different classes of primary variability of the observational u]’:the poleward propagation of the u]’(abbreviated as PP) and meridional vacillations.To validate this hypothesis,one-point correlation maps of u]’ at 200-hPa during the boreal cold season(November-April) of every year from 1957-2002 are used as a criterion.Twelve PP years,in which the PP events are dominant in the variability of u]’,and 15 no_PP years,in which the PP events are recessive and the meridional vacillations are dominant in the variability of u]’,are examined.The results show that the variabilities of u]’ are different in the chosen PP and no_PP years.In the PP years,the PP events dominate the variability of u]’;however,the meridional vacillations are prevalent in the no_PP years.