Department of Mathematical Sciences, University of Wisconsin-Milwaukee, P. O. Box 413, Milwaukee, WI 53201, USA

Dept. of Atmospheric and Oceanic Sciences,& Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, CA 90095–1565, USA

Dept. of Oceanography, Florida State University, Tallahassee, FL, 32306, USA

Dept. of Atmospheric and Oceanic Sciences,& Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, CA 90095–1565, USA

Département Terre-Atmosphère-Océan, & Laboratoire de Météorologie Dynamique (CNRS and IPSL), Ecole Normale Supérieure, F-75231 Paris Cedex 05, France

Dept. of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

DAMTP, University of Cambridge, Cambridge, UK

Dept. of Atmospheric and Oceanic Sciences,& Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, CA 90095–1565, USA