Satellite drifters allow us to study the movement of water over the long term over wide areas, and model what would happen to a passive particle "trapped" in a current. Each satellite drifter consists of a drogue, a fabric sock and a length of chain which make sure the buoy at the surface of the water is more influenced by the water currents than the wind (we want to know where the water is going, not where the wind is pushing the buoy!). Within the buoy is a battery and electronics which periodically sends the buoy's GPS position to a satellite. From the satellite, the information is sent to a base station and then to us for later analysis.
Below you can see the result from one such drifter; you can see, if you try to follow the line that the drifter spins around and around the channel for many months and noticeably is not rapidly carried south. This corresponds with a water flow regime that consists of eddies (spinning masses of water) than straight currents (like in a river).
Drifter data from the drogue in the pictures
Drogue in the water. The blue fabric soon sinks and just the buoy is left on the surface. This is one of our early model drifter buoys, with the electronics in the blue housing attached to the float. Later models incorporate the electronics within the float itself.
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