1. Mathematical Institute, University of St Andrews, North Haugh, St Andrews KY169SS, United Kingdom
2. Institute of Water Problems, Russian Academy of Science, 3 Gubkina Street, 119333 Moscow, Russia
3. P. P. Shirshov Institute of Oceanology, Russian Academy of Science, 36 Nakhimovsky Prospect, 117997 Moscow, Russia
4. Laboratoire d’ Océanographie Physique et Spatiale, IUEM, UBO/UBL, Technopole Brest Iroise, 29280 Plouzane, France