1. School of Civil Engineering and the Environment and the Tyndall Centre for Climate Change Research, University of Southampton, Southampton SO17 1BJ, UK.
2. Laboratoire d’Etudes en Géophysique et Océanographie Spatiales LEGOS-CNES, Observatoire Midi-Pyrénées, 18 Av. E. Belin, 31401 Toulouse cedex 9, France.