1. Department of Mathematics, University College London, London WC1E 6BT, UK
2. School of Mathematics, University of East Anglia, Norwich NR4 7TJ, UK
3. Centre for Polar Observation and Modelling, Department of Meteorology, University of Reading, Reading RG6 6BB, UK
4. Department of Mathematics and Statistics, University of Otago, PO Box 56, Dunedin 9016, New Zealand