1. Ocean and Ice Branch, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
2. School of Civil Engineering and Geosciences, University of Newcastle, Newcastle Upon Tyne, NE1 7RU, UK.
3. Department of Geosciences and EMS Environment Institute, Pennsylvania State University, University Park, PA 16802, USA.
4. Earth and Space Research, Seattle, WA 98102–3620, USA.