Study Improves Drifter-Based Estimates of Near-Surface Ocean Currents

Using 40 years of observations from the NOAA’s Global Drifter Program Array, the authors depict the annual mean speed of ocean currents at 15 meters depth. The vectors highlight the general direction of the large-scale circulation. Image provided by Lucas Laurindo, University of Miami Rosenstiel School of Marine and Atmospheric Science.

Using 40 years of observations from the NOAA’s Global Drifter Program Array, the authors depict the annual mean speed of ocean currents at 15 meters depth. The vectors highlight the general direction of the large-scale circulation. Image provided by Lucas Laurindo, University of Miami Rosenstiel School of Marine and Atmospheric Science.

Scientists developed a new approach to improve near-surface (15 meters depth) ocean circulation estimations derived from drogued and undrogued drifters (drogues extend below the surface, providing stability) used in the NOAA Global Drifter Program. The proposed new estimation method improves the definition of smaller-scale circulation details relative to traditional methods. Additionally, the team compared the new approach to altimeter-derived velocity data (subsampled at the drifter locations), which revealed that traditional methods likely underestimate the statistical uncertainty of the mean current estimates by a factor of two. The comparison also showed that the corrected slip of undrogued drifters produces velocity measures similar to drogued drifters, doubling the amount of usable data. These combined operations recovered large-scale circulation features that previous drifter-based assessments had not defined well (core speeds for Florida/Gulf Stream Currents were 50% larger) and resolved coherent structures at mesoscale ranges whose visualization was only possible by inferring surface velocities from satellite observations.

The researchers published their findings in Deep Sea Research Part I: Oceanographic Research PapersAn improved near-surface velocity climatology for the global ocean from drifter observations.

“A statistical description of the surface ocean circulation is useful for a variety of applications,” explained study author Lucas Laurindo. “In addition to its value for academic studies, circulation estimates can be used to predict the transport of fish larvae and pollutants such as oil, plastic, and marine debris. In conjunction with satellite observations, it can also help define the most fuel-efficient ship routes across the oceans and serve as a support tool for search-and-rescue operations.”

The NOAA Global Drifter Program provides the most accurate ocean current measurements. However, its observations are scattered in space and time, and its data must be averaged area-by-area to calculate circulation properties, which can smooth out relatively narrow features such as the Gulf Stream. Also, more than half of the data came from drifters that lost their drogues, making them more sensitive to winds and waves and rendering their velocity measurements unusable.

Due to the significant slip of undrogued drifters, previous studies recommended not using their data for calculating flow statistics. However, not including data from undrogued drifters significantly reduces the observational density in extensive oceanic regions. This study sought to extend previous research for correcting the undrogued drifters’ downwind slip and evaluated the advantages and biases of this practice for calculating the mean ocean velocities.

A common approach involves ensemble-averaging data selected within spatial bins. However, uncertainty lies in the choice of bin size. Laurindo described their how they addressed this challenge, “To obtain enough data for trustworthy estimates, these bins tend to be too large. We can think of the bins as pixels in a picture: the larger the pixels, the less details we are able to resolve. We were able to reduce this effect by using 1-D polynomials to model variations of the circulations at spatial scales smaller than the bin size. This approach allowed us to resolve finer details of the ocean circulation than previous assessments.”

The team analyzed a Gulf Drifter Program dataset that comprised more than 29 million, six-hour position/velocity estimates scattered throughout the world’s ocean (1979 to 2015). About 56% of the available data points were from undrogued drifters.

Regarding the likelihood that traditional approaches underestimated real errors by a factor of two, Laurindo said, “We think that this result is important, because it means that marginally significant estimates obtained by previous studies may actually be indistinguishable from zero.”

Laurindo noted that their methods can be extended to study the influence of climate phenomena on ocean circulation, such as the El Niño/La Niña cycle and other forms of multi-year variations.

The new near-surface circulation estimates produced by this study are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at DOI:10.7266/N7SJ1HN5, and from NOAA’s Atlantic Oceanographic and Meteorological Laboratory at https://www.aoml.noaa.gov/phod/gdp/mean_velocity.php.

The study’s authors are Lucas C. Laurindo, Arthur J. Mariano, and Rick Lumpkin.

By Nilde Maggie Dannreuther. Contact maggied@ngi.msstate.edu with questions or comments.

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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 support: The National Science Foundation (OCE Grant 1434198) and NOAA’s Atlantic Oceanographic and Meteorological Laboratory and the Climate Program Office.

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/.

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