1. Centre for Polar Observation and Modelling, Earth Sciences, University College London, London, United Kingdom
2. Centre for Polar Observation and Modelling, Earth Sciences, University College London, London, United Kingdom, and National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado
3. Department of Mathematics, King’s College London, London, United Kingdom, and Institute for Theoretical Physics, Georg-August-University Göttingen, Göttingen, Germany