The authors are grateful to Dr. Matthew Wheeler for providing his code of the wavenumber-frequency filter. We thank Dr. H. Hendon for his comments that led to an improved version of the manuscript. This research was supported by the Climate Dynamics Program, National Science Foundation under Grants ATM0073023. The second author is affiliated with International Pacific Research Center (IPRC), which is sponsored in part by the Frontier Research System for Global Change.
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