1. Department of Physics, University of Toronto, Toronto, Ontario, M5S I A 7, Canada
2. Canadian Climate Centre, Environment Canada, University of Victoria, Victoria, British Columbia V8W 2Y2, Canada.
3. Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA.
4. Center for Meteorology and Physical Oceanography, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.