Observational Zonal Mean Flow Anomalies: Vacillation or Poleward Propagation?

Abstract

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 vari-ability 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.

Key Words:

• zonal-mean zonal wind
• zonal index
• pole-ward propagation