Data Keeps Drifting In

Author: Laura Bracken, CARTHE Outreach Manager

After a month at sea conducting a variety of experiments, the Ronald H. Brown has returned home but data continues to pour in. As the ship circled the Gulf of Mexico (GoM), conducting cross-shelf transects, scientists onboard released 25 custom-made, GPS-equipped, biodegradable CARTHE drifters, described in the post “New biodegradable surface drifters to survey the ocean currents of the Gulf.” The drifters can transmit their location every 5 minutes for 1-3 months, providing scientists at the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) with accurate tracks of the ocean currents.

This experiment is unprecedented and will provide a much needed picture of how currents behave near the shelf.  CARTHE has conducted 3 large scale experiments in the northern GoM but by adding the full range of this vast body of water they will gain a better understanding of Gulf-wide dynamics.

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Figure 1. Cumulative map of ship track, drifter tracks, and hurricane tracks

The above map shows the ship track in green. The individual drifter tracks in black, with red dots indicating their last known position. The purple line that crosses Mexico represents Hurricane Franklin, while the purple line that stretches from the Bay of Campeche to Texas represents Hurricane Harvey. Luckily the ship was already home before Harvey developed, but was required to alter its course to avoid Franklin. Hopefully analysis of the drifter tracks near these storms will provide some information about how storms impact ocean currents.

As expected, there are several areas where drifters were retained over the shelves, mainly moving slowly across the shelf, rather than moving into the body of the Gulf. Of particular interest is the Yucatan Shelf.

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Figure 2. Yucatan shelf

The historical drifter database of the GoM shows a gap on the Yucatan Shelf/Campeche Bank (cf. Miron et al 2017).  These deployments will contribute to filling that particular gap. On the Bay of Campeche (west of the Campeche Bank) there are observations of a quasi-permanent cyclonic gyre (called the Campeche Gyre) that models have trouble to represent in the mean.  The drifters deployed in the region are expected to sample this cyclonic circulation, though this has not been seen yet.

CARTHE scientists will continue to track the progress of the drifters, to compare to previous drifter data, and to work towards better understanding how material in the Gulf of Mexico is transported by the ever changing surface currents.

Thank you to the crew of the Ronald H. Brown and the scientists and students who facilitated the release of the CARTHE drifters.

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New biodegradable surface drifters to survey the ocean currents of the Gulf

A collaborative effort with the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE)

Author: Laura Bracken, CARTHE Outreach Manager

Over the next few weeks, researchers on the Ronald H. Brown will release CARTHE drifters throughout the Gulf in areas that we have not had the opportunity to study until now. We are excited about this collaborative project and can’t wait to see where the drifters go!

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A schematic of the planned drifter deployment. Available for the deployment are 25 CARTHE drifters. The drifters will be deployed (blue dots) during each of the planned CTD cruise stations (red dots).

About CARTHE:

During the Deep Water Horizon (DWH) oil spill, BP committed to fund $500 million in broad, independent scientific research in the Gulf, through the Gulf of Mexico Research Initiative (GoMRI). The Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) was funded by GoMRI for the purpose of studying the oil spill and its impact on the Gulf’s delicate ecosystems. CARTHE’s scientific work focuses on the physical distribution, dispersion and dilution of petroleum, its constituents and associated contaminants under the action of physical oceanographic processes, air-sea interactions and tropical storms.  Simply put, CARTHE studies ocean currents to be able to predict where oil or other toxins may go in the event of a future spill.

In order to do this, we use GPS trackers on specially designed buoys called drifters.  The drifters move with the ocean surface currents and transmit their precise location via satellite to the scientists on land. By knowing where the drifters go and how fast they move, we can estimate how the currents are moving and where oil might go in the event of a future spill.

CARTHE

CARTHE has used many different types of drifters over the years but none of them were quite right for the large scale experiments (1000+ drifters released over a small area within a few weeks) that we had planned, so the team decided to develop our own. Over 2 years, Guillaume Novelli and Cedric Guigand at the University of Miami created 20+ prototypes, did extensive testing in both the ocean and the SUSTAIN wind-wave tank, and eventually met their criteria:

  1. Accurately follow the surface currents
  2. Hold GPS unit and batteries for accurate reporting of position over several months
  3. Compact and easy to assemble on a ship
  4. Biodegrade – a key requirement when conducting such a large release.

Considering the worldwide marine debris and ocean plastics problem, we felt it was imperative that such large experiments be conducted using biodegradable materials so that we were not contributing to the problem, only the solution. CARTHE drifters are made of PHA, a bioplastic that will degrade over time in the marine environment.  The finished drifters consist of two interlocking panels that can be assembled quickly onboard a research vessel, a donut-shaped float that houses the GPS and batteries in the center, and a chain that links the two sections together.

In January-February 2016, CARTHE released 1100 custom-made, biodegradable, GPS-equipped drifters into the northern Gulf of Mexico, near the site of the DWH oil spill, in the largest oceanographic experiment of its kind ever conducted. Then in April 2017, we returned to the Gulf, this time just west of the Mississippi delta near Grand Isle, LA, with 500 drifters to study how oil gets from offshore, across the shelf, and onto shore.  Additionally, other researchers have used the CARTHE drifters to study the currents in the Arctic Ocean, and to study how animals like mahi mahi and sea turtles use ocean currents. The possibilities are endless.

For a glimpse into the adventure of designing the CARTHE drifters, please watch Drifting into the Gulf by Waterlust.

To learn more about CARTHE drifters, please visit Pacific Gyre.

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CARTHE drifters after deployment on GOMECC-3. Photo by Leticia Barbero