1. Institute of Theoretical Geophysics and Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, United Kingdom;
2. School of Mathematics, University of New South Wales, Kensington NSW 2052, Australia
3. Faculty of Science, University of Bristol, Bristol BS2 6BB, United Kingdom
4. BP Institute, Department of Earth Sciences, and Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0EZ, United Kingdom;