Throughout the 20th century oceanographers have been mapping out the water masses of the oceans. Long ago we learned that the Atlantic is saltier than the Pacific, and the Mediterranean even more so. We know that the cold waters of the global abyss originate in the Antarctic and that during interglacial times also come from the Arctic by spilling into the deep North Atlantic. We’ve excelled at measuring temperature, salinity, oxygen, and other water properties, but we have not been able to measure currents! Only much later, barely 50 years ago, did we begin to map out currents at depth by tracking instruments that drift with the water. It is these floats that really opened our eyes to how the ocean below the surface behaves.
These instruments, known as Swallow floats after its inventor John Swallow, were originally tracked acoustically from a ship overhead. They were later redesigned to be tracked over many hundreds of kilometers thanks to a property of the ocean called the deep sound channel or SOFAR (sound fixing and ranging) channel that traps sound to spread horizontally. The first-generation floats would emit a signal that could be heard at distant underwater listening stations we had set up around the western North Atlantic and later from moored autonomous listening stations. Later we developed floats that would listen for timed signals from anchored acoustic beacons serving as an underwater GPS system. These RAFOS (ranging and fixing of sound) floats were developed by our group in the 1980s and have been used by many in almost all oceans since then. The float tracks reveal all kinds of flow patterns we had never seen before, one of which we might call ‘spinning disks.’
One of the very first we discovered was a case of pure luck. On a cruise northwest of Puerto Rico, we were taking a hydrographic station with an instrument that allows us to see and record temperature and salinity while it is being lowered. Suddenly, near 1000 m below the surface, the instrument entered a 200-meter thick layer of very warm and salty water. This came as a complete surprise to us; the only source of such water that we knew of was the Mediterranean more than 5000 km to the east - this didn’t make sense. But just to be sure we deployed another instrument (an XBT) to confirm the existence of the warm water layer. To our great fortune we had several SOFAR floats onboard that could be tracked from the remote listening stations we had set up. My colleague Doug Webb at the Woods Hole Oceanographic Institution, the designer of the floats, was able adjust the weight of the floats so they would drift in the warm layer near 1000 m depth. We quickly learned how all three orbited this layer of warm water clockwise, the fastest one with a 6-day period. We called this 20 km ‘spinning disk a ‘meddy’ thinking it was an eddy filled with water from the Mediterranean. Learning of this discovery, oceanographers started a search for and found numerous such meddies in the eastern Atlantic.
A major appeal of these neutrally buoyant floats is how even a single instrument can trace out fluid motion at depth in the ocean. This capability to map out or visualize fluid motion where you can’t see or be everywhere at the same time is incredibly valuable. It is thanks to these that we now know the ocean is full of ‘spinning disks, i. e. lenses or coherent vortices as we sometimes call them (Shoosmith et al., 2005). If a float is deployed near the center of a lens it may remain trapped in it for years along with the water in it. These lenses of spinning water appear to be quite stable and can drift for thousands of kilometers. Some will decay over time, others will meet a violent end when they are ripped apart by other lenses or currents. If left alone they might continue spinning for a decade, perhaps longer? They can spin clockwise or anti-clockwise. They can be as large as 50-100 km in diameter or only a few, but they all have in common that they are thin, very, very thin, typically 50-100 times wider than they are thick. We know they can exist just about anywhere, in the upper ocean or at depth, but we still know very little about their populations and their role in transporting ‘bottled’ water from one region to another. A curious feature is that clockwise spinning eddies in the northern hemisphere tend to drift equatorward whereas anticlockwise spinning eddies will try to drift poleward. In the southern hemisphere it is the other way around.
Shoosmith, D. R., P. L. Richardson, A. S. Bower and H.T. Rossby, 2005. Discrete eddies in the northern North Atlantic as observed by looping RAFOS floats. Deep-Sea Research II,52,627-550.