An acoustical method of determining the relative vorticity of oceanic motions is advanced. By transmitting signals in opposite directions around a closed loop, for example a 3 km triangle, one can obtain directly the circulation, which by virtue of Stoke's theorem is equivalent to the average vorticity of the "enclosed" fluid times the area....
Rossby, T.. 1975. "An oceanic vorticity meter." Journal of Marine Research 33, (2). https://elischolar.library.yale.edu/journal_of_marine_research/1318