We all know what is meant by the AMOC: the Gulf Stream and the North Atlantic Current transporting warm salty water north, and the two overturning areas where dense water is produced, one in the Irminger/Labrador Sea and the other in the Nordic Seas. The former produces water to 1-2 km depths while the latter spills dense water through the Denmark Straits and the Faroe Bank Channel into the deep North Atlantic. We also recognize that if you want a moderate climate in Europe, warm water must enter and circulate in the Nordic Seas where it will lose its heat to the atmosphere, cool and spill back into the deep North Atlantic. But not only that, this overflow water must continue south to low latitudes and beyond for if we don’t export cold water south, we can’t import warm water to the north. That export takes place via the deep western boundary current. What it looks like depends upon where you look.
At the Grand Banks south of Newfoundland some of the deep water, especially the shallow Irminger/Labrador Water gets split into some that continue west along the Canadian continental slope and some that gets stirred across the Gulf Stream and continues west south of the stream. These separated flows come together again south of Cape Hatteras and continue south, first along the eastern slope at the Blake-Bahama Outer Ridge (BBOR) and then back northward along its western slope. The BBOR is striking sedimentary feature that juts southeast from the Blake Plateau just south of Cape Hatteras. Two papers document the DWBC in this area beautifully, Jenkins and Rhines (1980) and Stahr and Sanford (1999). The former explores the distribution of tritium in the deep ocean resulting from the 1960s atmospheric H-bomb tests. They find elevated tritium levels at 4 km depths of the BBOR. Year-long current meter deployments revealed a strikingly stable flow along the ridge slope. They make a strong case that this tritiated water must have originated in the Nordic Seas. I recommend it highly, it’s an easy yet informative read.
The second paper maps out the DWBC at the BBOR in beautiful detail. Their tool is the absolute velocity profiler (AVP), a free-falling instrument that measures the tiny electric field potentials induced by water (an electrolyte) moving through the earth’s magnetic field. The AVP is also equipped with a high frequency ADCP so it can resolve in greater detail the vertical structure of currents, especially as it approaches the bottom – and thus can resolve the bottom Ekman layer. The AVP also measures temperature and salinity. They define several sections across and along the ridge and thus can classify deep transports according to temperature and hence their origin in the North Atlantic. Somewhat remarkably, they found that the two short sections they set up normal to the ridge could capture the bulk of AMOC return flow at depth and its distribution into the upper (Labrador/Irminger Sea) and deeper (Nordic Seas) origins. This indicates that on average the bulk of the DWBC flows along the western margin of the deep North Atlantic. Both papers show the role of topography is guiding the DWBC south and around the ridge. But they also note that the DWBC is not a ‘pipe’ flow. Jenkins and Rhines note that the tritium concentration has been reduced by a factor 10 from its source water farther north. This lateral (isopycnal) exchange with the surrounding waters must play an important role in maintaining the huge reservoir of cold water in the deep North Atlantic. We still have much to learn about lateral stirring and mixing at great depths. The AVP was an amazing tool in its day. Today the same information can be obtained with ADCP-equipped CTDs, whether cable-lowered or free-fall. With these we can profile right into the bottom boundary layer.
There is little question the BBOR is a sedimentary feature built up by particulates deposited from the DWBC. I’m not sure of the dynamics (I haven’t read up on this), but I imagine that as the Gulf Stream flows off the Blake Plateau into deep water it accelerates and entrains water from below. This entrainment as well as the increasing depth induces a cyclonic trend leading the DWBC to deposit its suspended material away from the continental slope, creating in effect the BBOR. It is not a coincidence that the BBOR is located just south of the Gulf Stream. There is much to be learned and understood about the DWBC, here and elsewhere!
Jenkins, W.J. and P. B. Rhines (1980). Tritium in the deep North Atlantic Ocean. Nature, 286, 877‹880.
Stahr, F. R., and T.B. Sanford (1999). Transport and bottom boundary layer observations of the North Atlantic Deep Western Boundary Current at the Blake Outer Ridge. Deep-Sea Research II, 46, 205-243.