February 8, 2003

Drifters determine where we go
by Jon Hare, Chief Scientist

(drawing by David Reinert, Oregon State University)
Drifters are oceanographic equipment and are designed to follow pieces, or parcels, of water. The idea is that the drifter conveys information about location at time A and at time B. The change in location (motion) of the drifter between time A and B can then be estimated, as can the speed (change in location divided by time). Thus, the basic purpose of a drifter is to provide information on ocean circulation.

There are a variety of drifter designs, ranging from the simple to the complex. Simple drifters can be cards that are released at a known point with a return address. When the cards are found by somebody, they are returned and the release point and time and the recovery point and time provide information about ocean circulation that moved the card from release point to return point. As an aside, containers of merchandise are occasionally lost from ocean going ships. A well-publicized example is thousands of Reebok sneakers lost in the North Pacific. As the merchandise washes up on shore, information is gained about the ocean circulation from where the container was lost (release point) to where the merchandise washed up (recovery point).

From this very simple drifter, a piece of paper with a return address, a number of more complicated designs have arisen. One of the most complicated designs has the drifter changing its buoyancy and dropping below the surface to a certain depth, say 500 m. On the descent, the drifter measures temperature, conductivity and depth, among other things. In essence the drifter acts like a CTD. Once the drifter returns to the surface, it transmits its data (temperature, conductivity, and depth) along with its location to a land based receiving station via satellite. This data is then combined with the data from hundreds of other similar drifters to provide a large-scale picture of the temperature, salinity and density in the ocean. Information about ocean circulation is also gained from the changes in the drifter’s position over time. These drifters can operate for months to several years.

(below: a WOCE-SVP-B courtesy of Techocean)
The drifters we are using to track the Charleston Gyre are a standard design: WOCE-SVP. The basic premise is similar to the drifter just described, minus the change in buoyancy and CTD profiling. The drifter consists of three parts. The globe is at the surface and contains the instrumentation, in our case a surface temperature probe. The drogue is what ‘catches’ the water and causes the drifter to follow the parcel of water. The drogues we are using are about 5 m long and centered at 15 m. The idea is that the drifter is following a parcel of water at 15 m. The tether connects the globe to the drogue and is a thin but strong wire. The globe transmits data every 90 seconds. Satellites receive this data and calculate a position. The position and data are then relayed to a land-receiving station where the data is emailed to us on the ship, as well as stored to be sent back to the laboratory at the end of the cruise. We receive these locations every 3-5 hours and they are 3-5 hours old when we receive them. Most of the drifters we have released are this type.

The drifter we are following, however, has a slight difference. In addition to everything described above, the drifter has a GPS receiver. The Global Positional System is made up of multiple satellites from which receivers can triangulate their position. There is a receiver in the drifter’s globe that determines position every 30 minutes. These positions are good to within ~ 50 m. The globe transmits data ever 90 sec including the GPS determined positions. Like the other drifters, satellites receive these data and transmit to a land-based receiving station and these are emailed to the ship and stored for later analysis. When we receive these positions via email, they are several hours old. We also have a receiver on the ship that picks up the data transmission from the drifter. The receiver passes the data to a computer where the data is decoded and the real-time position calculated and displayed. This way we know exactly where the drifter is and we can follow it, make MOCNESS tows and deploy the CTD.

The following image shows the paths of 5 of the satellite-tracked drifters and the GPS-drifter. The satellite drifters are shown as different colored lines. As you can see, they were placed in the gyre which carried them in a counter-clockwise direction until they got near the Gulf Stream. The Gulf Stream took them northward. The satellite-tracked drifter shown in red got caught in the Gulf Stream immediately after deployment and didn't go around the gyre. These drifters will be tracked for a total of 80 days.
The black dots show the position of the GPS-tracked drifter. The ship followed this drifter during the mission. At one point we retrieved this drifter so we could anchor near shore and wait out a storm. We redeployed this drifter once the storm had passed.