Our work developing the SOFAR float program brought me to Tudor Hill, an underwater acoustics laboratory in Southampton, Bermuda where they had a SOFAR hydrophone we could use for float tracking. I soon learned that they had some hydrophones at 4500 m depth about 70 km south of the island. Being at that point quite familiar with Doug Webb’s work with Swallow floats, it occurred to me that I could use the hydrophones to get vertical profiles of currents by tracking a float acoustically as it slowly sank through the water column. So far as I know profiling currents this way had never been attempted. It is the oceanic analogue to tracking balloons as they ascend into the atmosphere.
The ocean bottom hydrophones made this a very attractive setup, all we needed to do was build the float (and a spare since the ocean bottom - aka Davy Jone’s locker - sometimes becomes the ultimate destination for an instrument!). A day trip from the island to the hydrophone site, release and recovery of the float made for a cost-effective discovery program. The float emitted short acoustic pulses (pings) on a precise schedule, which were picked up at the hydrophones and stored along with timing data on a tape recorder at Tudor Hill. The next day I’d play back the tape and digitize the acoustic timing data. So, from two slantwise travel-times which gave me distance from the float to the hydrophones as it sank along with pressure (and temperature) recorded in the float, I could determine the float’s movements down and up through the water column. The lateral movements of the float as it descended gave us horizontal velocity as a function of depth.
The vicissitudes of weather prevented us from making as many trips to the hydrophone site as planned for, but the 8 profiles we obtained showed currents to be highly variable in all directions. To quantify their structure, we adapted code previously developed by Henry Perkins to decompose the profiles into a set of orthogonal modes defined by the density structure, what is known as the Brunt-Väisälä frequency profile. Having seen many wiggly profiles of temperature and salinity I wasn’t surprised to see plenty of little wiggles in the velocity profile. But I was surprised to learn that nearly 50% of the kinetic energy is accounted for by the barotropic (depth-independent) mode and another ~40% in the first baroclinic mode. The remaining residual kinetic energy revealed no distinctive patterns, it was distributed throughout many higher order modes (Rossby, 1974).
A year later in May 1971, Tom Sanford, Henry Stommel and I ran a 4-day inter-comparative study profiling currents using my acoustic profiler, Tom’s electro-magnetic profiler (EMP) and hydrocasts for the geostrophic method. Initially, the objective was to validate the EMP profiler by comparing it with the acoustic method. In fact, it worked so well Tom combined the acoustic and EMP profiles to examine the energetics of inertial wave propagation (Rossby and Sanford, 1976). Tom and his students went on to make many truly unique and valuable contributions with the EMP.
The SOFAR float program took so much of our time that we had to put this activity rest. Further, while the hydrophones made for a convenient first test of the technology, it wasn’t portable. It would be another 8 years before we took the next step, what would become the Pegasus project.
Rossby, H.T. (1974). Studies of the vertical structure of horizontal motions as observed near Bermuda. J. Geophys. Res., 79, 1781-1791.
Rossby, H.T. and T. Sanford (1976). A study of velocity profiles through the main thermocline. J. Phys. Oceanogr., 6(5), 766-774.