[1] Univ E Anglia, Sch Environm Sci, Norwich NR4 7TJ, Norfolk, England
[2] British Antarctic Survey, Cambridge CB3 0ET, England
[3] Natl Oceanog Ctr, Southampton SO14 3ZH, Hants, England
[4] Oregon State Univ, Coll Ocean & Atmospher Sci, Corvallis, OR 97331 USA