This paper was funded in part by a grant/cooperative agreement from the National Oceanic and Atmospheric Administration (NOAA), Contract NA17RJ1231, with the University of California, San Diego, and Contract NA050AR4311004 with the trustees of Columbia University. The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its subagencies. The anonymous comments of referees, and helpful discussions with A. G. Barnston, L. Clarke, I. T. Jolliffe, G. V. Kass, B. Rajagopalan, L. A. Smith, R. L. Smith, U. Lall, and D. S. Wilks are gratefully acknowledged.
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