[1] Univ Bristol, Sch Geog Sci, Bristol BS8 1SS, Avon, England
[2] Univ Utrecht, Inst Marine & Atmospher Res, Utrecht, Netherlands
[3] Univ Twente, NL-7500 AE Enschede, Netherlands
[4] CALTECH, Jet Prop Lab, Pasadena, CA USA
[5] Univ Calif Irvine, Dept Earth Sci, Irvine, CA USA