Cluster Analysis of the Northern Hemisphere Wintertime 500-hPa Height Field: Spatial Patterns

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Abstract
Hierarchical cluster analysis based on the method of Ward is performed on the Northern Hemisphere wintertime 10-day low-pass-filtered 500-hPa height field, using the NMC operational analyses for the period 1946–85. Input data are gridded fields at 5-day intervals, a total of 702 maps, each with 445 grid points. The measure of similarity between maps is the squared height difference, averaged over all grid points; that is, the squared “distance” between the maps in multidimensional phase space. The closest two of the 702 maps are merged to form a cluster that, in subsequent calculations, replaces the maps from which it was formed. This procedure (modified slightly, to deal with the differing numbers of maps in the clusters) is repeated 701 times until all the maps have been merged to form a single cluster whose centroid corresponds to the climatological mean map. The two clusters involved in the final merger, the pair of smaller clusters that merged to form each of them, and so on, are represented... Abstract Hierarchical cluster analysis based on the method of Ward is performed on the Northern Hemisphere wintertime 10-day low-pass-filtered 500-hPa height field, using the NMC operational analyses for the period 1946–85. Input data are gridded fields at 5-day intervals, a total of 702 maps, each with 445 grid points. The measure of similarity between maps is the squared height difference, averaged over all grid points; that is, the squared “distance” between the maps in multidimensional phase space. The closest two of the 702 maps are merged to form a cluster that, in subsequent calculations, replaces the maps from which it was formed. This procedure (modified slightly, to deal with the differing numbers of maps in the clusters) is repeated 701 times until all the maps have been merged to form a single cluster whose centroid corresponds to the climatological mean map. The two clusters involved in the final merger, the pair of smaller clusters that merged to form each of them, and so on, are represented...

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