The discovery of the Mediterranean Salt lens off the Bahamas (which we later argue was not from the Mediterranean – see post Feb. 17, 2025) set off a search for similar lenses in the eastern Atlantic. And indeed, it didn’t take long before oceanographers began to recognize them in the hydrographic database. Of course, we started to ask questions about their structure, their longevity and how it was possible for a meddy to travel more than 5,000 km from off Portugal to the Bahamas. This led to a multi-year multi-institutional program to locate and probe such a meddy in detail. Larry Armi at SIO was the driving force in this endeavor.
So, in fall 1984 we set out from the Azores to find a meddy - it took a week of dropping XBTs until we found evidence for a lens of hot water near 1 km depth. We called it ‘Sharon’ in honor of the woman who fired the discovering XBT. The Pegasus profiler became a key player in mapping out the structure of the lens. We first located the center of the lens, and then set up a radial set of transponders so that with Pegasus we could map out its radial and vertical structure. Combining the velocity data with concurrent CTD profiles we could also estimate its eddy potential and kinetic energy.
To revisit the lens the following year, which we assumed would travel some distance west we tagged the lens with SOFAR floats and deep drifters (Richardson et al., 1989). To our immense surprise the lens did not drift west but almost due south nearly 500 km by the time we returned to it the following summer. Taking the same approach as the previous fall, a CTD and Pegasus survey of the lens showed that it had shrunk considerably. The Pegasus profiles showed that the meddy rotated as a solid disk (6-day period) with velocity increasing linearly out to a certain radius, beyond which the velocity field decayed exponentially with increasing distance. The sharp positive potential vorticity jump at the transition is an expression of little exchange between the core of the lens and the surrounding waters. The lens is like a ‘vorticity bottle’. This helps explain why the SOFAR floats and deep drifters remained trapped in and drifted with the lens. These findings were reported by Schultz-Tokos and Rossby, 1981.
A third survey the following year (CTD only), even farther south, revealed only scraps of the original meddy. The southward drift and the rapid decay led us to question as mentioned above whether the original meddy near the Bahamas really came from the eastern Atlantic.
The Pegasus technology was transferred to Benthos of North Falmouth, MA to make it available for wider use by the oceanographic community. While Pegasus was accurate and straightforward to use, having to put acoustic transponders on the bottom limited its usefulness – it worked best for repeat sampling such as we did in the Indian Ocean and the Gulf Stream, not for expeditionary use. The most extensive example of repeat sampling is the study of equatorial currents in the Pacific by Eric Firing where he profiled currents at 13 sites spanning the equator repeatedly over a 16-month period (Firing, 1987). It is an extraordinary piece of work. Eventually, techniques and algorithms were developed so that one could profile currents even from cable-lowered CTDs equipped with ADCPs. Measuring currents from a package ‘dangling’ from a rolling surface vessel would seem like a crazy idea, but it has been shown to work quite well; a method in widespread use today. Looking back, Pegasus had a brief but varied career profiling ocean currents, all thanks to a phone call from Ants Leetmaa.
Firing, Eric (1987). Deep zonal currents in the central equatorial Pacific. J. Mar. Res.,45,791-812.
Richardson, P. L., D. Walsh, L. Armi, M. Schröder and J. F. Price (1989). Tracking three meddies with SOFAR floats. J. Phys. Oceanogr.,19,371-383.
Schultz-Tokos, K. and T. Rossby (1991). Kinematics and Dynamics of a Mediterranean Salt Lens. J. Phys. Oceanogr., 21, 879-892.