I would like to express my appreciation to A. Mori for stimulating discussion about independent component analysis and the difference between independence and simple uncorrelatedness. Thanks are extended to three anonymous reviewers for giving many appropriate and constructive comments. I think the paper was greatly improved by these comments. NCEP–NCAR reanalysis data were provided by the CIRES Climate Diagnostics Center, Boulder, Colorado (available on their Web site at http://www.cdc.noaa.gov). This research is supported by a Grant-in-Aid for Scientific Research of the Japanese Ministry of Education, Science, Sports and Culture, and by the cooperative research project of the Center for Climate System Research, University of Tokyo. The GFD-DENNOU Library of the Japanese meteorological community was used to draft the figures.
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