1. a School of Earth and Ocean Sciences , University of Victoria , P.O. Box 3055, Victoria, BC, V8W 3P6 E-mail:
2. b School of Earth and Ocean Sciences , University of Victoria , P.O. Box 3055, Victoria, BC, V8W 3P6
3. c Polar Science Center , Applied Physics Laboratory , Seattle, WA, U.S.A.
4. e Lawrence Livermore National Laboratory , Livermore, CA, U.S.A.
5. d National Center for Atmospheric Research , Boulder, CO, U.S.A.