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 and 500 hPa height from winter monthly means (for the period 1948-2002), obtained from the reanalyses of the National Centers for Environmental Prediction - National Center for Atmospheric Research (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 zonal mean zonal wind on the COWL-index derived from 500 hPa height data by Wu and Straus [2003].