1. British Antarctic Survey, Cambridge CB3 0ET, UK.
2. Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, UK.
3. Univ Lyon, ENS de Lyon, Univ Claude Bernard, CNRS, Laboratoire de Physique, F-69342 Lyon, France.
4. Grantham Institute and Department of Earth Science and Engineering, Imperial College London, London, SW7 2AZ, UK.