Ward's hierarchical cluster algorithm has been applied to the NCEP reanalyzed monthly mean Northern Hemisphere (NH) wintertime geopotential height at 500 hPa (Z500). A combination of criteria involving distance, pattern correlation, well-defined branches of the family tree, significance testing, and comparison of hierarchical clustering results to other clustering methods has lead to a reasonable choice of four or five clusters. The four clusters obtained, using a subset of empirical orthogonal functions (EOFs) of Z500, reproduce the Northern Hemisphere Annular Mode (NAM), also called the Arctic Oscillation (AO) pattern, and the Cold-Ocean-Warm-Land (COWL) pattern in both polarities. During the occurrence of NAM-like regimes, the full-field centers of action over the Pacific and the Atlantic have positive or negative anomalies simultaneously. For the COWL-like regimes, the center of action over the Pacific is out of phase with that over the Atlantic. The coexistence of NAM-like and COWL-like regimes explains the apparent lack of correlation between the Z500 fluctuations near the Azores and the Northern Pacific.