1. University of California, San Diego, Scripps Institution of Oceanography, IGPP 0225, 9500 Gilman Drive, La Jolla, California 92037‐0225. Emails:
2. Formerly University of California, San Diego, Scripps Institution of Oceanography, La Jolla, California; presently Institut de Physique du Globe de Paris, Laboratoire Geosciences Marines, Case 89 Tour 24‐25‐4eme Etage, 4 Place Jussieu, Paris 75252, France.
3. Statoil Research and Development Center, NO‐7005, Trondheim, Norway.