Multiple planetary flow regimes and the eddy forcing in Northern Hemisphere wintertime variability

Abstract

A cluster algorithm is applied to the zonal mean zonal wind ([u]) from winter monthly means of the reanalyses of NCEP/NCAR. Four reproducible and robust clusters are found: two regimes represent the opposite phases the Northern Hemisphere Annular mode (NAM), and two regimes represent the opposite phases of a pattern which can be identified with the Cold Ocean-Warm Land (COWL) pattern. This identification is accomplished by regressing the [u] on the COWL-index derived from 500 hPa height data by Wu and Straus [2002]. Transformed Eulerian (or Eliassen-Palm) eddy forcing analysis is used to examine the maintenance of these clusters. The results show that barotropic forcing by the eddy momentum fluxes appears to maintain the wind anomalies in the upper troposphere and balances the Coriolis acceleration associated with the anomalous transformed mean meridional circulation, while the Coriolis acceleration maintains the wind anomalies against friction in the lower troposphere. This mechanism holds for both NAM-like and COWL-like regimes.