Study Finds Currents One Centimeter Below Ocean’s Surface Greatly Affect Material Transport
– JUNE 14, 2018
Researchers conducted a first-of-its-kind measurement of the vertical dynamics of water motion near the ocean’s surface in the northern Gulf of Mexico. The team observed substantial shear (decay of current magnitude with depth) in the upper one meter of the ocean. The average current speed at 1-centimeter depth was twice the average speed of currents up to 1-meter depth and nearly four-times the average speed up to 10-meters depth. These findings suggest that shear is an essential factor for predicting the separation and transport of buoyant materials in the upper centimeters of the ocean. The researchers published their findings in Geophysical Research Letters: Observations of near-surface current shear help describe oceanic oil and plastic transport.
Wind forcing and wave dynamics strongly determine motion in the upper few centimeters of the water column. However, neither observations nor operational ocean models can presently resolve this layer of the ocean’s current profile, making it difficult to predict the transport of buoyant materials such as microplastics and oil. Analysis of near-surface dynamics have involved two-dimensional, horizontal studies of transport that generally treat the upper one meter of the ocean, and sometimes the upper 10 meters, as the same.
Researchers sought to address this observation gap by employing a set of cutting-edge sensing technologies for ocean currents (including special cameras, drifting instruments observed by drones, an autonomous underwater vehicle with acoustic current meters, wind and wave gauges) in ways that mitigated each other’s blind spots. “Instruments that only sense the upper few centimeters were used alongside those that only sense below 20 cm depth. This allowed for a complete description of the directional current profile between 1 centimeter and 13 meters depth. The novelty of the effort was the way that multiple platforms and instruments were brought to bear to study the same (small) horizontal patch of ocean,” explained study author Nathan Laxague.
Researchers collected measurements on April 27, 2017, during the SPLASH (Submesoscale Processes and Lagrangian Analysis on the Shelf) campaign under ideal environmental conditions for flow observation. Interface forcing (the sum of microscale wave breaking and skin friction) resulted in a layer of strong turbulence and high shear in the top few centimeters of the water column.
“Substantial shear (in this case, sharp decrease of current magnitude with increasing depth) was found over every segment of the current profile,” explained Laxague. “For example, current speeds decreased by a third over the upper 5 cm, then decreased by half from 5 cm down to 1 m, then decreased by another half from 1 m to 2 m.” Laxague said that the observed shear likely causes differential material transport, meaning that highly buoyant material that lingers in the upper few centimeters, is transported in a different way than neutrally buoyant material that lingers in a deeper layer of the ocean.
The researchers note that modern circulation models do not yet have sufficient resolution to describe the thin layer of the ocean’s upper centimeters and cannot sufficiently estimate how oil and buoyant plastic is transported by wind- and wave-related motions.
“An important general notion to take from this study going forward is that the term ‘surface’ (or ‘near-surface’) is equally applied in the literature to a wide range of depths with drastically different relationships with atmospheric forcing. However, there is a substantial difference between how material in the upper five centimeters reacts to wind- and wave-induced currents and how the upper ten (or even one) meter does,” said Laxague. “My hope is that our study will push this difference a bit further to the forefront as the topic of buoyant material transport is discussed going forward.”
Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N7ZW1JC0.
The study’s authors are Nathan M. Laxague, Tamay M. Ozgokmen, Brian K. Haus, Guillaume Novelli, Andrey Shcherbina, Peter Sutherland, Cedric M. Guigand, Bjorn Lund, Sanchit Mehta, Matias Alday, and Jeroen Molemaker.
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 https://gulfresearchinitiative.org/.
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