Oceanography as a modern science came of age in the 1970s during a period known as the International Decade of Ocean Exploration or IDOE for short. Major surveys of oceanic variability with moored instrumentation were initiated, MODE and polyMODE were two such endeavors. The SOFAR float group contributed substantially to both. In MODE the floats were deployed at sound channel depth, which also corresponds closely to the zero crossing of the first baroclinic mode. This is not a trivial point as we now know that at the meso- and larger scales the vertical structure of ocean currents can to a large extent be accounted for by the barotropic and first baroclinic mode - meaning the floats were reflecting the action of the barotropic field. In polyMODE a second layer of floats at 700 m to explore action in the main thermocline. Sometimes you’re in for a very big surprise.
Immediately after deployment one of the thermocline floats exhibited a very rapid clockwise motion with a period of less than 4 days. This orbital motion continued uninterrupted for the next two months clearly suggesting a stable feature that might have been formed long before. The float trajectory implied an anticyclonic eddy or lens of some kind, that was all we could say. Fortunately, we had scheduled a cruise to recover, and redeploy some floats. Being aware of this energetic little spinner and knowing where it was, we took the time conduct a detailed hydrographic survey of the area before retrieving the float. And what a treat it was. I don’t need to give all the details; you can read the paper by Riser et al. (1986). But a couple of things can be noted. It was a thin little sliver of water, 20 km overall, and less than 300 m thick embedded in the main thermocline. Probably the smallest one that has been documented. And obviously very stable, something that had become evident in an earlier blog about spinning disks in the ocean. We don’t know where it was formed, but the nearest area with waters of similar T-S-O2 properties appeared to be east of the Lesser Antilles. This isn’t unreasonable given that the Gulf Stream is advecting water from the Caribbean and farther afield. But where was this spinner or lens itself formed? The easiest speculation might be that it was split off from a larger feature in the highly sheared Gulf Stream. We didn’t and couldn’t know. I suppose one could go through all hydrographic stations taken in the warm side of the Gulf Stream to see if any reveal water properties similar to what we found.
Curiously, and this is pure speculation, from a new program scanning currents in the western Caribbean and Gulf of Mexico with a hull-mounted ADCP on the bulk ore carrier Bulk Xaymaca, we saw similar small scale features in the loop current. This at least suggests the possibility that the lens we saw was formed farther south in the tropical Atlantic. But, again, how?! What is the agent for setting up a thin rapidly spinning disk within the water column? I’m not aware that a lens can develop out of nowhere.
We know meddies (Mediterranean salt lenses) are formed where the Mediterranean overflow becomes unbalanced after it passes the Cape St Vincent corner at 1000 km depth. But what about all the thin spinning disks in the northern North Atlantic, both cyclonic and anticyclonic, discussed in the earlier blog, were they all formed this way, somewhere along topographic slopes? Is it possible the Lesser Antilles island chain can act like a grating breaking up the flow through them into smaller features? Or are these spinning disks splinters from collisions between larger energetic mesoscale features? The energetic mesoscale eddy field is typically effectively accounted for in terms of a barotropic and lowest order baroclinic mode. Given this then such collisions must be like car crashes leading to a lot of debris, i.e. spinning disks? Perhaps all of the above? This lens is, so far as I know, still the smallest one documented in such detail. They smaller they get the harder they will be to spot. Is there a smallest possible size to spinning lenses? If left alone how long can they last?
Riser, S., W.B. Owens, H.T. Rossby and C.C. Ebbesmeyer. The structure, dynamics and origin of a small-scale lens of water in the western North Atlantic thermocline. J. Phys. Oceanogr., 16, 572-590, 1986.