1. Department of Earth Sciences, Dartmouth College, HB 6105 Fairchild Hall, Hanover, New Hampshire 03755, U.S.A
2. Department of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street West, Montreal, Quebec, H3A 2K6, Canada, and Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, U.S.A