Oceanographers use two very different approaches to describe fluid motion. One, the Eulerian method, considers the speed and direction of fluid motion at a point. We use current meters to capture this information. The longer the record, the more we can say about the temporal properties of fluid motion at that point. This point may be representative of flow in nearby regions, but it will tell us little about how fluid motion varies across space. The other method, the Lagrangian method, follows a fluid parcel that has been tagged with a marker of some kind. In our case we use a neutrally buoyant float to follow its movement or trajectory, giving us information about flow patterns in the ocean. This approach gives us a wealth of information about the spatial structure of currents. Some trajectories are easy to interpret, others are not. Such floats were first developed by John Swallow in the early 1950s. They were tracked from a ship.
But ships are expensive. Some years earlier Henry Stommel had proposed to use the SOFAR channel ( https://dosits.org/science/movement/sofar-channel/history-of-the-sofar-channel/ ) to track subsurface drifters over great distances. In 1966 he invited me to work with Doug Webb at WHOI to develop and implement a SOFAR float tracking system. Doug was in charge of the float development, a major challenge. The idea was to build a free drifting neutrally buoyant device that would transmit an acoustic signal on a precisely timed schedule. My task was to implement the tracking system that would use the arrival times of these signals at three or more sites to determine the float’s movement through the ocean. As with any technical development, there were many unknowns and challenges to be met. One of the early ones concerned acoustic signal loss in the ocean – how far away could we reasonably expect a signal to be detectable? In fact, quite far, many 100s of km. In 1973 Doug deployed 20 SOFAR floats as part of a successful field program called MODE (Mid-Ocean Dynamics Experiment). Doug describes his work very nicely in a documentary called ‘The Turbulent Ocean’ (youtube.com). A few years later, 1978, he and I collaborated on another project, this time with the U.S.S.R. called polyMODE. The SOFAR float was becoming a significant contributor to ocean science but could only be used where we had shore based SOFAR hydrophones. To relax that limitation Doug’s colleagues at WHOI developed the autonomous listening station (ALS) that could be deployed anywhere on moorings. For example, thanks to the ALS, French oceanographers used SOFAR floats to study how the mid-Atlantic ridge influences currents overhead (TOPOGULF).
Here at GSO we eventually took another approach, namely, to reverse the entire system: we would anchor the SOFAR floats to establish an underwater navigation system. The drifting floats, now small glass pipes equipped with accurate clocks, would listen for, and determine the arrival times of the transmitted signals. These would be logged in their micro-processor memory. At the end of mission the float would drop ballast, return to the surface, and transmit the accumulated data to a satellite (Systeme Argos), which would then forward the data to us. We call these underwater drifters RAFOS floats (SOFAR backwards) for the sound goes in the ‘opposite’ direction, from stationary transmitters to drifting receivers (Rossby et al., 1986). They offered two advantages: the small glass pipes instead of huge aluminum tubes (needed to carry the battery packs for the acoustic transmitter) allowed for much lower fabrication costs. The other advantage is that they can be made to remain on an isopycnal surface regardless of depth and thereby track fluid motion more realistically whereas the aluminum SOFAR floats by default are constant depth devices.
The first study with the RAFOS float took place in the Gulf Stream in 1984, nearly 40 years ago. The very first deployments were made by a fisherman who would motor out into the Gulf Stream at Cape Hatteras. But the seas were often too severe. Fortunately, we found a freighter service between Norfolk, VA and Bermuda that could help us. The captain, a former NOAA officer, would take XBTs to locate the center of the Gulf Stream – at which point he would release a float. It was an immensely successful collaboration (Bower and Rossby, 1989). Since then, RAFOS floats have been used by scientists in almost all ocean basins except the Arctic.
Rossby, T., D. Dorson and J. Fontaine (1986). The RAFOS system. J. Atmos. and Oceanic Tech., 3, 672-679.
Bower, A.S. and T. Rossby (1989). Evidence of Cross-frontal Exchange Processes in the Gulf Stream Based on Isopycnal RAFOS Float Data. J. Phys. Oceanogr., 19, 1177-1190.