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Study Finds Currents One Centimeter Below Ocean’s Surface Greatly Affect Material Transport

June 14, 2018
Current magnitudes are shows at depts. Of (A) 10 m, (B) 1 meter, and (C) 0.05 m. Green dashed lines indicate the “zooming in” progression from A to C. From figure 2 in the study, provided by Nathan Laxague.

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.

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