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Proc Natl Acad Sci U S A. 2018 Feb 6;115(6):1162-1167. doi: 10.1073/pnas.1718453115. Epub 2018 Jan 16.

Ocean convergence and the dispersion of flotsam.

Author information

1
School of Oceanography, College of the Environment, University of Washington, Seattle, WA 98105; dasaro@apl.washington.edu.
2
Applied Physics Laboratory, University of Washington, Seattle, WA 98105.
3
School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada, V8W 3P6.
4
Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada, V8W 3P6.
5
Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095.
6
Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL 33149.
7
Naval Research Laboratory, Stennis Space Center, MS 39529.
8
School of Marine Science and Policy, College of Earth, Ocean and Environment, University of Delaware, Newark, DE 19716.
9
Lamont-Doherty Earth Observatory, Earth Institute, Columbia University, Palisades, NY 10964.
10
Department of Atmospheric Sciences, College of the Environment, University of Washington, Seattle, WA 98195.
11
Mesoscale and Microscale Meteorology Laboratory, National Center for Atmospheric Research, Boulder, CO 80307.
12
Department of Mathematics, College of Staten Island, Staten Island, NY 10314.

Abstract

Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km2 converged into a 60 × 60 m region within a week, a factor of more than 105 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s-1 and 0.01 ms-1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material.

KEYWORDS:

dispersion; eddy; ocean; submesoscale; vertical velocity

Conflict of interest statement

Conflict of interest statement: E.A.D. and T.F. are coauthors on a 2014 paper. This was a brief announcement that did not involve any scientific collaboration.

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