Study Finds Small-Scale Flows Alter Transport Pathways on the Ocean’s Surface

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The blue tracer patch shows the tracer advection by the full velocity field (large and small-scale flows) while the red tracer patch shows the tracer advection by the mesoscale-only field. The mesoscale transport barriers are in gray in the background. Image credit: A.C. Haza, T.M. Ozgokmen, and P. Hogan.

Scientists analyzed model simulations of tracer dispersion in a Gulf of Mexico eddy to find out if small-scale flows surrounding the eddy influenced where the tracer went. The researchers observed that the small-scale flows disrupted the eddy’s large-scale flow patterns enough so that 20-50% of the tracer that was entrained in the eddy leaked out, crossed into surrounding areas, and followed flows of nearby eddies. The researchers published their findings in Ocean Modelling: Impact of submesoscales on surface material distribution in a Gulf of Mexico mesoscale eddy.

Understanding particle movement at the ocean’s surface is important for several applications, one being the tracking of buoyant pollutants such as oil spills that require rapid forecasts to reduce environmental and socioeconomic damage. However, this is a challenge because of highly variable surface currents, and wide-ranging spatial scales on the ocean’s surface.

Ocean circulation models can reproduce the meso or large-scale (greater than 10s of km, lasting days to months) circulation features using the assimilation of satellite altimetry data.  However, submesoscale (100 m to 10 km scales, lasting hours to a day) circulation features have different dynamics and little is known about how they interact with mesoscale circulations and affect transport of buoyant tracers. This study’s authors used the Hybrid Coordinate Ocean Model to simulate an eddy that exhibited bursts of small-scale flows along its rim and applied filters to isolate the mesoscale from the submesoscale features. The researchers compared the entrainment of a tracer by the eddy with and without submesoscale features and then analyzed its dispersion properties on the ocean’s surface.

“We have been relying on the mesoscale field to predict the distribution of oil spills, biogeochemical tracers, oil and plastic in the upper ocean,” explained study author Angelique Haza. “This study helps us better understand how the submesoscales dramatically alter the surface tracer distribution predicted by the mesoscale-only field.”

Haza said that the results show that while small-scale surface flows increase the overall dispersion of a tracer, they also generate areas of high and low tracer concentrations by accumulating the tracer into lines of convergence that intertwine with the mesoscale pathways.  “In practical applications, this patchiness in oil concentration may help responders to locate pockets of surface oil for possible recovery for a limited time.”

The authors suggest that future research address the spatiotemporal variability of small-scale flows (more pronounced during winter), the influence of strong winter winds and summer wave action and latitude on transport, and modelling these transport processes.

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N7BC3WJC.

The study’s authors are A.C. Haza, T.M. Ozgokmen, and P. Hogan.

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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). Other funding sources included the Office of Naval Research (Grant #N000141110087).

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 2010-2017 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).