An Update of Cluster Analysis of the Northern Hemisphere Wintertime Regimes

The method of self-organizing maps (SOMs) is used on the Northern Hemisphere wintertime 10-day low-pass-filtered 500-hPa geopotential height field and compared with the hierarchical cluster analysis known as “Ward's method”. Both cluster analyses are based on a 57-year data combined by ERA-40 reanalysis data from 1957 to 1978 and ERA-Interim reanalysis data from 1979 to 2014. The SOM method captures the three regimes identified using Ward's method, but with more robust and stable spatial patterns: the negative polarity of the North Atlantic Oscillation (NAO) pattern, the "Alaska blocking" pattern, and the negative polarity of the Pacific-North American (PNA) pattern. An additional regime with a positive PNA-like pattern recovered in SOM clustering does not appear robustly in clustering based on Ward's method. The update of cluster analysis in this study confirms that the PNA pattern and the NAO exhibit significant skewness.