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Study Finds Small Scale Ocean Currents Cause Clustering of Floating Material

May 10, 2018

Researchers analyzed how satellite-tracked ocean surface drifters moved in the Gulf of Mexico to learn how other floating materials (oil, plastics, marine organisms) move. Half of the drifters dispersed over a region much larger than their initial spread, and the other half converged into clusters that were much smaller than their initial spread. Small fronts and vortices aggregated the drifters into clusters while larger currents slowly distributed the clusters over a wide region. These results help explain why floating materials often travel over large distances while maintaining their concentrations. The results also suggest that, under certain conditions, buoyant pollutants could collect along fronts or in eddies and possibly make cleanup easier. The researchers published their findings in Proceedings of the National Academy of Sciences of the United States of America: Ocean convergence and the dispersion of flotsam.

Classic dispersion models typically assume that patches of floating objects that are initially close together will spread apart horizontally over an increasingly large area without changing the surface area of these patches. While likely valid for large-scale flows (> 100 km horizontally), recent theoretical studies predict strong surface convergences and downwelling are associated with small-scale flows (<10 km). Investigating this new paradigm, this study examined the dynamics of large- and small-scale features and their influence on ocean transport.

As part of a multi-year and multi-experiment study to better understand and predict how the ocean moves floating material, CARTHE conducted a LAgrangian Submesoscale ExpeRiment or LASER. They deployed 1,000 ocean drifters near the Deepwater Horizon site where fresh, cold water from the Mississippi River converges with salty, warm Gulf water. The researchers in this study analyzed a subset of these drifters whose initial distribution was 25 km in diameter. Within one week, some drifters converged into a 60 m wide region before slowly dispersing. Other drifters spread over a 100 km wide area, while intermittently being concentrated into a small fraction of this region.

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