Cluster analysis of the Northern Hemisphere wintertime 500-hPa height field: spatial patterns

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 three most reproducible clusters, which together account for ~ 1/3 of the 702 maps in the dataset, can be reconstructed remarkably well from linear combinations of the two leading EOFs of the covariance matrix. They are related to features of the probability density function (PDF) in a two-dimensional phase space defined by the expansion coefficients of these EOFs. One is marked by a closed anticyclone over the southern tip of Greenland, one by a ridge over the Gulf of Alaska, and one by a ridge over the Rockies. In comparison to other clusters of comparable size, their centroids are conspicuously far from the climatological mean map. Positive 500-hPa height anomalies in excess of 200 m are observed in association with the first two clusters, over regions of large variance and strong positive skewness of the 500-hPa height field. -from Authors