1. Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
2. U.S. Geological Survey, 6000 J Street, Placer Hall, Bldg 56, Sacramento, California 95619-6129, United States
3. Section of Oceanography, University of Bremen, Otto Hahn Allee 1, D-28359 Bremen, Germany