1. Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado, and Department of Mathematics and Statistics, San Diego State University, San Diego, and Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California
2. Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado
3. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California
4. Department of Mathematics and Statistics, San Diego State University, San Diego, California