1 Geosciences Department and Laboratoire de Météorologie Dynamique (CNRS and Institut Pierre-Simon Laplace), École Normale Supérieure and PSL University, 75231 Paris Cedex 05, France.
2 Department of Atmospheric and Oceanic Sciences, University of California at Los Angeles, Los Angeles, CA 90095-1567.
3 Departments of Mathematics and of Finance, Imperial College London, London SW7 2BX, United Kingdom.