Study Shows that Floating Bamboo Plates Capture Strength of Small-Scale Ocean Currents
– JANUARY 27, 2020
Researchers optically tracked 600 biodegradable bamboo plates floating in the Gulf of Mexico for 2.5 hours to better understand how small-scale currents (scales of minutes and meters) affect surface dispersion. Ten minutes after releasing the plates onto the water’s surface, the plates collected in narrow parallel convergence zones (known as windrows and sometimes referred to as streaks) as a result of Langmuir circulation (counter-rotating vortices aligned with wind direction). The spacing between the streaks was initially 15 meters apart, then widened to 40 meters after 20 minutes, and then slowly widened to 50 meters over 60 minutes. Comparison of statistics from nearby ocean drifters and the floating bamboo plates suggest that the plates provide a better indication of the strength of small-scale currents than do drifters. The results point to the important role that small-scale motions play in organizing floating material, such spilled oil, and is an important component in ocean transport predictions, which can inform spill response plans. The results also provide insight into how energy is redistributed in the ocean through currents at various scales.
The authors published their findings in the Journal of Physical Oceanography: Small-scale dispersion in the presence of Langmuir circulation.
Ocean currents at many scales cause floating material (i.e. plastics, wreckage debris, oil) to move. Historically, there has been more research about larger-scale circulations, which are detectable by satellite imagery, than smaller-scale circulations, which are difficult to study because of their fast and short-lived action.
“Smaller ocean motions are too large to observe in the lab, but too small to observe from space,” explained study author Henry Chang, “This study targets precisely those scales, where material is moved around within minutes to a few hours and across distances of 1 to 100 m.”
The team are members of the CARTHE research consortium who is studying how near-surface currents transported Deepwater Horizon oil. CARTHE has conducted four unprecedented experiments from the open Gulf of Mexico to shorelines (GLAD, SCOPE, LASER and SPLASH). The release and tracking of the 600 bamboo plates for this study took place during the January-February 2016 open-ocean LASER experiment that deployed 1,000 biodegradable ocean drifters. This experiment provided sufficient data to calculate statistics of transport properties influenced by Langmuir circulation.
Before the bamboo plate experiment, the team colored the plates with red, yellow, and magenta biodegradable paint to contrast with the ocean surface and distinguish between plates. The team tossed the bamboo plates into the ocean from a small boat. Within a few minutes, the plates soaked up enough water to become submerged 1–2 cm depth, preventing them from being subjected to direct wind forcing.
A high-resolution (8688 x 5792 pixel) color image was taken every 15 seconds by a Ship-Tethered Aerostat Remote Sensing System (STARRS), a helium-filled balloon, from an altitude of 150 m (Study Tests STARRS Imaging of Short-Lived Small-Scale Dispersion on Ocean’s Surface). Before analyzing the data, the team processed the raw images by 1) detecting plates, 2) converting pixel coordinates to physical coordinates, and 3) linking plates between images.
Conductivity, temperature, and depth surveys conducted in the area earlier that day revealed weak lateral density gradients, suggesting weak frontal and submesoscale activity. Therefore, Langmuir circulation should have been the dominant dynamic feature present in the small scales measured during the bamboo plate experiment.
“Langmuir circulation is a common phenomenon, causing buoyant material such air bubbles, seaweed, oil, or plastic waste to collect into streaks on the ocean surface,” explained Chang in summary. “In addition to this surface signature, Langmuir circulation also increases vertical mixing and the transfer of heat and gases between the ocean and atmosphere. Understanding and quantifying these effects is, therefore, important in the context of pollution issues in the ocean and of the global climate system.”
Data for this study are available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at: DOI 10.7266/N7M61H9P, DOI 10.7266/N7W0940J, DOI 10.7266/N7S75DRP, DOI 10.7266/n7-93j3-mn56, and DOI 10.7266/n7-8h5w-nn91.
The study’s authors are Henry Chang, Helga S. Huntley, A. D. Kirwan Jr., Daniel F. Carlson, Jean A. Mensa, Sanchit Mehta, Guillaume Novelli, Tamay M. Özgökmen, Baylor Fox-Kemper, Brodie Pearson, Jenna Pearson, Ramsey R. Harcourt, and Andrew C. Poje.
By Nilde Maggie Dannreuther. Contact firstname.lastname@example.org with questions or comments.
This research was made possible in part by grants from the Gulf of Mexico Research Initiative (GoMRI) to the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment II (CARTHE II) and Consortium for Advanced Research on Transport of Hydrocarbon in the Environment III (CARTHE III). Other support included the Office of Naval Research for CALYPSO (N00014-18-1-2461).
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 https://gulfresearchinitiative.org/.
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