Drifters

“Drifters” is the general term for a device meant to flow with and measure ocean currents. There are a variety of drifter types but the two most popular today are the standard “surface drifter” and “drogued drifter”. The surface drifter, promoted by Russ Davis in the early 1980’s, is designed to track the upper one meter of the sea. While there are several small variations of this design, the basic configuration includes a central mast supporting a set of four sails mounted orthogonally to form at least one square meter of drag surface and a transmitter above the surface with minimized windage.

The drogued drifter standards came out of the World Ocean Circulation Experiment devised to measure subsurface flow. It generally consist of a small surface float that houses the transmitter with a thin wire tether supporting a holey-sock drogue at depth. The objective is to maintain a 40-to-1 drag ratio between the drogue and the surface buoy.

Depending on the process under investigation, researchers use either one of the above or some other configuration to best mimic the flow. There has been a lot of effort recently to devise a smaller surface drifter, for example, to better simulate oil spill trajectories. Some new designs are also underway to make a more eco-friendly drifter from biodegradable materials.

Examples of surface drifter (left, photo provided by Jim Churchill, WHOI) and drogue drifter (right, image supplied by University of Maryland Center for Environmental Science).

US Northeast drifter archive.

Lobster Larvae Settlement

As an example of how drifters are used in the coastal ocean, we describe here an application to study American lobster larval transport. A project began in 2003 to track the New England coastal current and try to explain the interannual changes in larval settlement along the coast. With biologists detecting a variation in the settlement locations (charted in the Figure to the right), the hypothesis was that the coastal current delivers larvae in different places every year (Incze et al 2000; Wahle et al 2004). While there were multiple circulation models under development that might be able to address this question, we needed some data to validate the model simulations. Beginning that year and for several years thereafter, surface drifters were deployed regularly in the late spring months at many places along the coast of Maine, New Hampshire, and Massachusetts. Since these were low-cost units built primarily by local high school students and deployed by local fishermen, nearly 100 units could be deployed each year so that over a thousand multi-month tracks are now in a web-served database.

Lobster settlement locations.

Cold Stunned Turtles in Cape Cod Bay

The sea turtles residing in Cape Cod Bay each fall are often trapped and fail to migrate into warmer waters. As they become “cold stunned”, they are no longer able to swim and often wash ashore stranded. While the number of strandings recorded by volunteer beachwalkers each year was near 100 for a few decades, beginning in 2013 that number had risen to around 1,000 per year. This resulted in new research to understand currents in the bay and involve dozens of drifter deployments. The primary objective has been to validate the local ocean models so that we can hindcast the circulation and then attempt to explain the strandings in the past.

To read more about the strandings that happened in 2012, click here.

These figures depict the mean current based on many years of drifter observations (top) and the same for model estimates (bottom).

Mass Bay Winds

Animations on this page show passive particle trajectories computed using the Maine Coastal Current hindcast circulation fields originally obtained in May 2005 aboard R/V Oceanus (cruise OC412). The specific objective was to find Lagrangian trajectories ending in Massachusetts Bay on May 18 and to map their starting locations on May 7, the time of onset of a major nor'easter wind event. The overall objective was to link the 2005 Alexandrium fundyense outbreak to ECOHAB-GOM program's conceptual models. The at-sea computations involved real-time assimilation of shipboard, moored, and other data then available relative to the domain of computation. Both moored and ship-board ADCP current measurements were assimilated via the inversion of the unknown sea-level open boundary conditions. An objective mapping method was used to generate the model initial condition by merging CTD measurements with the GOM temperature and salinity climatology. The forward model was forced by surface winds observed by GoMOOS buoys (Anderson et al., 2005).

The trajectories shown here are computed for inert particles fixed at 1 m below the sea surface. There is no dispersion. If a drifter is headed across a boundary, it is immobilized for that time step only and lingers in place until moved in a feasible direction by future currents. The region was also subdivided into inshore and offshore donor regions to calculate their contributions to Mass Bay as shown in the pie chart. These Oceanus articles from 2005 and 2006 summarize how this work is related to Harmful Algal Blooms (HAB’s or red tides) in New England waters.

Lagrangian trajectories originating within Massachusetts Bay only. These trajectories are largely retained within Mass Bay by the major nor'easter wind event. The exit path seeding the outer Cape and Nantucket via the Race Point (yellow) is also visible at the tip of Cape Cod.

Particle movement between May 7 and 18. Particles that end up in Mass Bay are shown in green. Particle initiation is synchronized to the wind event on May 7, shown in the upper left corner.

Trajectories of three surface drifters released during cruise OC412 near Cape Ann on May 9, 2005. Several dates are labeled along each drifter track.

Map showing inshore and offshore regions acting as source regions for Mass Bay. This is used to quantify particle contributions given in the pie chart.

Trajectories for particles starting within donor region Inshore 3. 49% of these particles (green) end up in Mass Bay, while the rest (blue) find their way around the Cape.

Pie chart summarizing the relative contributions of donor regions to Mass Bay particle population.

California Current 1993

This is a short look at a fragment of drifter tracking history. As part of the Office of Naval Research Eastern Boundary Current Accelerated Research Initiative (ONR ARI), 63 drifters were released off Northern California between May and November 1993 to study the near-surface flow field in and near the California Current. The drifters followed the standard WOCE SVP (World Ocean Circulation Experiment Surface Velocity Program) design, were tracked by ARGOS and were drogued with holey socks centered at 15 m.
The drifters were released in two arrays. The first one consists of an incoherent array containing 7 drifters deployed along a constant latitude in May, June, August and November by WHOI. The second one consists of deployments of 6 and 16 drifters in small clusters during July and September by NPS (Naval Postgraduate School). The OSU (Oregon State University) released additional 11 drifters instrumented to measure upwelling radiance as part of both arrays.

The movies show the drifter movement for both arrays. The position data has been low-pass filtered to remove inertial and tidal variability. Drifters are distinguished by color: WHOI (black), NPS (white) and OSU (red). The drifter tails are 5 days long. Time is given in day of the year (yd). Movies start on May 6 (yd 126) and end on December 29 (yd 363).
The drifter data shown here were collected and processed by Dick Limeburner (WHOI), Jeff Paduan (NPS) and Mark Abbott (OSU), with funding from ONR. The movies were made by Matthias Johnsen at Dartmouth College. The original clips on VHS tapes were digitized by Cassie Stymiest. More information on the 1993 California current drifter launching effort can be found here.

The overall California current drifter movement between May 6th and Dec 29th, 1993.

The San Francisco area drifter movement between May 6th and Dec 29th, 1993 (zoomed in to show detail).

SCOPEX 1989

This is another short look at the Gulf of Maine drifter tracking history. Satellite tracked drifters were launched as a part of the SCOPEX (South Channel Ocean Productivity EXperiment) effort in the Great South Channel in the western Gulf of Maine in early June, 1989. 13 near surface drogues were centered at 5 m and 6 below-thermocline drogues at 50 m. They were tracked by ARGOS. The tracking results revealed near surface drifters to be trapped in the clockwise gyre around Georges Bank. Eight drifters circled the Bank 16 times with an average circulation time of 48 days and an average speed of 12 cm/s. No below-thermocline drogues were found to circle the Bank.

The drifter data shown here were collected and processed by Robert Beardsley and Dick Limeburner (WHOI). The movies were made by Matthias Johnsen at Dartmouth College. The original clips on VHS tapes were digitized by Cassie Stymiest. More information on the 1989 SCOPEX project can be found in Limeburner and Beardsley (1996).

1989 SCOPEX effort 5 m and 50 m drifter tracks.