We thank Michael Tippett, Tony Barnston, and J. Shukla for their insightful discussions about this research, which led to improvements and clarifications. We also thank three anonymous reviewers for their constructive comments and for pointing out important references. This research was supported by the National Science Foundation (Grants ATM0332910, ATM0830062, ATM0830068), the National Aeronautics and Space Administration (Grants NNG04GG46G, NNX09AN50G), and the National Oceanic and Atmospheric Administration (Grants NA04OAR4310034, NA09OAR4310058, NA05OAR4311004). The views expressed herein are those of the authors and do not necessarily reflect the views of these agencies.
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