In the instruments section there is a brief description of the Pegasus profiler that we developed in the late 1970s. It profiles currents in the ocean much like an ascending balloon profiles winds aloft. Whereas a balloon can be tracked visually or by radar, the Pegasus profiler is tracked acoustically relative to acoustic beacons or transponders that have been placed on the ocean bottom. From the lateral displacements of the instrument as it sinks and rises, we can profile absolute velocity to cms-1 accuracy. At Ants Leetmaa’s request, a physical oceanographer at AOML in Miami, we designed and built the profiler (on a very fast time scale) to study the Somali Current (see ‘Near miss in the Seychelles’). That project was so successful we decided to initiate a similar program to study the Gulf Stream. We got funding to build two more for a campaign of bi-monthly sections across the Gulf Stream over a 3-year period.
On our first cruise September 1980 we defined 7 sites at each of which we deployed a pair of acoustic transponders on the ocean bottom to provide the base line for tracking the free-falling profiler as it sinks toward to bottom, drops ballast, and ascends back to the surface. After the transponders were deployed and surveyed, we took our first section of profiling currents across the stream. It went very well. Sixteen cruises and over 200 drops later we still had both instruments, neither one was lost. But it was exciting. Due to the strong currents in the Gulf Stream, it was important for the ship to be near where the profiler surfaces. While the instrument was equipped with a radio beacon that we could hear with our radio direction finder, the antenna is so close to the sea surface that surfaces waves will block the signal if the distance is too great. (For the same reason, if there is any sea, and there usually is, you can’t see the instrument on the surface unless quite close by – a few hundred m at most.) So the challenge was to navigate the ship to be nearby when it surfaces. We knew the distance to the instrument, but it is constantly changing due to both its horizontal and vertical motion. Tracking it became like an arcade game in slow motion. If you gave the right instruction to the bridge on how to maneuver to stay close, good. But if you made a bad call, we could be steaming away from the instrument. You don’t want to be in this situation just as it is about to surface because of time it takes for the ship to respond to a course correction. In the end it all went well, but at times it got exciting as we anxiously awaited whether the course correction was effective or not. Nighttime recoveries were easier for the Xenon flasher could be seen at great distances. You only needed to see the flash once to know what to do.
These bi-monthly cruises were also vehicles for other activities. We conducted zooplankton net-tows at various depths. We deployed and recovered other investigators’ moored instrumentation. We conducted a first test of the iso-pycnal float concept. We hosted birdwatchers on one or two cruises. And a number of students came along for their first sea-going experience.
Summertime is always attractive for work at sea, which means there’s a lot of competition for ship time. This meant that on one occasion we were allotted only 5 days port-to-port. This hurt for we had a lot to do. Because it was a such short cruise John Knauss, our dean, thought he might slip out of the office and join us. He asked, and I was happy to oblige. The general rule is you leave after breakfast, 8-9 AM, on the first day, and arrive when the tide is right on the last day (day 5 in this case). This made for a very tight cruise as it takes nearly 24 hours just to get to the Pegasus line. I had never done this, but to maximize our ship time I asked for permission to leave 1 minute after midnight. The response was of course no way, but it turned into a reluctant yes when I pointed out that the dean was coming along! Most of these cruises were on the Endeavor, but we also sailed on the Cape Hatteras, out of Morehead City, NC. Both were a pleasure to work on.
The Pegasus data set was the first one to describe accurately and in some detail the velocity structure of the Gulf Stream in deep water. It has been used in several studies over the years. Please contact me if you’d like a copy.