Drifters

The world’s oceans are constantly on the move. Ocean currents flow in complex patterns and are affected by many forces. Influencing the speed and direction of the water on our planet are the Earth’s major land masses, the bathymetry (bottom topography) of the oceans, the Earth's rotation, and atmospheric changes around the globe.

Critical Currents

Ocean circulation patterns contribute to the heat transport from the tropics to the poles, helping to equalize Earth’s surface temperatures. Surface currents have a great effect on weather patterns around the globe. In addition to important meteorological effects, ocean currents also play an important role in the global transfer of organisms, nutrients, potential pollutants and sediments. At the same time, they also affect the routes taken by ships as they carry goods and people across the seas.

Using devices known as drifters, scientists are beginning to grasp the complexities of global ocean currents, and, in turn, the many systems that they influence. With advances in technology, drifters now provide researchers with information about ocean circulation patterns in real time. The data garnered from these devices will allow scientists to design models of climate and weather patterns, such as El Niño and hurricanes, as well as predict where pollutants, such as oil or sewage, will go if they are dumped or accidentally spilled into the ocean. Information from drifters can also be used to learn more about the distribution and abundance of marine life with early life stages that are planktonic. Plankton are freely floating organisms that travel with oceanic currents.

Catching Liquid Wind

For a device that provides scientists with such a wealth of complex data, drifters are actually quite simple in design. Weighing only about 20 lbs, the “shallow water” or “Davis drifter” only has four major components: the body, “sails,” floats, and a data collection/transmitter package. The body is a waterproof tube approximately 3 ft long and 10 in wide that contains the data collection/transmitter package. Extending out from the sides of the tube are four pairs of arms, each of which has a “sail” stretched between them. Made of treated cloth or vinyl, each sail has an area of about 3 sq ft. These underwater sails act in the same way that sails on a boat do; but rather than “catching” the wind, they “catch” the water, moving the drifter along with the prevailing ocean currents. The drifter doesn’t actually float directly at the water’s surface, but is suspended several feet beneath it by ropes attached to four small floats. If the drifter were to float directly at the surface, it could be blown along by the wind, instead of transported by the currents. For this reason, the floats have relatively little surface area exposed to the wind, and a subsurface sail that causes the drifter to move in tandem with the average flow of water.

The “shallow water” drifter can be deployed from a ship or an airplane. Once the drifter is floating in the water, the transmitter sends a signal to an Earth polar orbiting satellite which calculates its position and relays the information to a receiving station. This data is then processed and distributed. Drifters may also house sensors that measure other ocean properties, such as surface temperature, wind, ocean color, pressure, or salinity, and these data can also be transmitted through the satellite link. A typical drifter will transmit data for approximately 1 to 1.5 yrs. The accuracy of ocean drifter positions averages approximately 500 m, a reasonable level of error considering the scale of the open ocean.