In 1962 Stommel wrote a paper “On the smallness of sinking regions in the ocean”. Even though that was a very simple model that couldn’t approach the complexities of the real ocean he was well aware of how incredibly small these regions in fact are. The deep water of the world oceans is supplied by narrow overflows from the Nordic Seas and from several sites around Antarctica, especially the Weddell and Ross Seas. The outflow of warm, very salty water from the Mediterranean is another well-known example. But due to earth’s rotation these overflows don’t run down a slope like a cataract, instead they slide sideways along the bottom increasing in depth, at first rapidly but soon adjusting and thereafter only very gradually. We used RAFOS floats to study overflow from the Nordic Seas through the Faroe Bank Channel.
This 800 m deep channel connects the Nordic Seas to the deep North Atlantic. It is like a weir making it easy to keep track of how much water is flowing through it. To see what happens after the water leaves the weir, we deployed acoustically tracked floats in the flow just where it exits the channel. To make them flow along the bottom their weights were trimmed so that they were slightly buoyant to float a thin 100 m line tag line that would just touch the bottom. We stretched large sheets of fabric to the glass pipe floats to increase their ‘size’ and drift with the water to offset any potential drag by the tag line. This worked well in principle, but the majority of floats got stuck at the bottom somehow. The floats also measured pressure and temperature. Soon after the floats exited the channel their descent was arrested by earth’s rotation and they curved to the right and drifted along the bottom only very gradually increasing in depth as they flowed NW along the southern slope of the Iceland-Faroe ridge and SW along the eastern slope of the Reykjanes Ridge where the surfaced after 45 days. The pressure record shows that their descent wasn’t monotonic, the floats could ‘meander’ up and down the slope as they drifted with the overflow. The temperature record showed violent variations in temperature as the very cold overflow water encountered the warmer water of the North Atlantic.
The fact that most floats got stuck was discouraging. To address this, we had in mind to redesign the floats with an acoustic altimeter to measure distance to bottom, and a buoyancy controller to maintain a fix distance. But the complexity and cost of this technology put a damper on our thinking since a key feature of using floats is to deploy them in numbers so we can get an ensemble of trajectories that give us insight into spatial variability. We have yet to tackle this problem in earnest. My gut feeling is to do away with the tag line and just let the float drift along the bottom using a little thruster (propellor) to lift it from the bottom on a regular schedule to get a profile of bottom boundary layer temperature and thickness.
There is much we don’t know about overflows, how stable they are, to what degree they are continuous or break up into filaments or coherent boluses (eddies along the bottom). And most important, where, and how entrainment takes place, and how much. It’s important to note that in all overflows stirring and mixing with the ambient water is incomplete. Amazingly, the Nordic Seas overflow between Greenland and Iceland can be followed thousands of kilometers along the bottom. There is a distinct core of cold water flowing along the deep western margin of the North Atlantic at 4000 m depth.
The overflow in the Weddell Sea from the Filchner Ice shelf may belong to a separate category: that water is so cold that as it slides down the slope it may accelerate because being so cold it is more compressible than the ambient water and thus gains in density relative to the surrounding water. Bottom-following RAFOS floats would be a good tool to explore these overflow processes in detail.
Prater, Mark D., and T. Rossby, 2005. Observations of the Faroe Bank Channel Overflow using bottom-following RAFOS floats. Deep-Sea Research, Part II,52,481-494