Key points are not available for this paper at this time.
Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, No. 2 (2012):15-19, doi:10.5670/oceanog.2012.37.
Laurent et al. (Fri,) studied this question.