Abstract
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.
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