It is a pleasure to thank A. W. Robertson for helpful discussions and the three reviewers for constructive comments; one of these, A. J. Majda, kindly provided the reference to Franzke et al. (2005). This research was supported by NSF Grant ATM-0081321 (for MG and DK), as well as NSF Grant OCE-02-221066 and DOE Grant 98ER6215 (for MG and SK).
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