Study Finds Small Scale Ocean Currents Cause Clustering of Floating Material

CARTHE drifter trajectories in the Gulf of Mexico superimposed on AVISO surface currents. Red squares mark drifters positions on 9 March 2016 and the tails are 14 days long. (Credit: Edward Ryan and Tamay Ozgokmen from the University of Miami)

CARTHE drifter trajectories in the Gulf of Mexico superimposed on AVISO surface currents. Red squares mark drifters positions on 9 March 2016 and the tails are 14 days long. (Credit: Edward Ryan and Tamay Ozgokmen from the University of Miami)

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.

The team examined different drifter clusters (groups of three of more drifters with less than 1 km distance between them) to separate the dispersive and convergent components. Their analysis suggests the co-existence of small convergence structures (scales as small as a few meters) embedded within and advected by larger scale structures.

Surface convergence, downwelling, and associated vertical exchange appeared to concentrate at fronts and within cyclonic vortices into which the fronts converged. During the field experiment, scientists who were simultaneously conducting underwater observations and measuring vertical velocity near drifter clusters found water moving downward with some water moving back upward.

“To observe floating objects spread out over a region the size of a city and then concentrate into a region smaller than a football stadium was just amazing,” said author Eric D’Asaro.  “We knew there would be some concentration, but the magnitude seen was quite stunning.” D’Asaro said that the observed downward pull of water is similar to the spinning vortex that forms near a bathtub drain where a small area of water sinks while water from the surrounding larger area moves toward the vortex.

“This is probably how the vertical exchange in the ocean ultimately works,” said author Andrey Shcherbina. “Even though we think about ocean mixing as a large-scale process, once we start looking closer we begin to realize that it might actually happen episodically, on very small scales, at select hotspots that flash here and there.”

Together, the small-scale clusters followed by large-scale spreading set the overall extent and fine-scale texture of floating material and could create unique communities of organisms, amplify impacts of toxic material, and create opportunities for efficient material recovery. For example, the researchers hypothesized that a 10 μm thick oil slick covering a region 10 km wide could converge into a 100 m wide region with 10 cm thickness.

The findings have broader implications for how the ocean behaves as improved finer-grained models could better capture processes such as plankton blooms, carbon transport, and water circulation.

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N7HQ3WZR, doi:10.7266/N7KW5DH7, doi:10.7266/N7W0940J, doi:10.7266/N7H130FC, doi:10.7266/N7S75DRP, and doi:10.7266/N7610XQ6.

The study’s authors are Eric A. D’Asaro, Andrey Y. Shcherbina, Jody M. Klymak, Jeroen Molemaker, Guillaume Novelli, Cedric M. Guigand, Angelique C. Haza, Brian K. Haus, Edward H. Ryan, Gregg A. Jacobs, Helga S. Huntley, Nathan J.M. Laxague, Shuyi Chen, Falko Judt, James C. McWilliams, Roy Barkan, A.D. Kirwan, Jr., Andrew C. Poje, and Tamay M. Özgökmen.

************

This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment II (CARTHE II).

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

© Copyright 2010-2018 Gulf of Mexico Research Initiative (GoMRI) – All Rights Reserved. Redistribution is encouraged with acknowledgement to the Gulf of Mexico Research Initiative (GoMRI). Please credit images and/or videos as done in each article. Questions? Contact web-content editor Nilde “Maggie” Dannreuther, Northern Gulf Institute, Mississippi State University (maggied@ngi.msstate.edu).