In the middle of the Iceland Basin there’s a region with very high eddy kinetic energy (EKE). Curiously, it is quite localized, it doesn’t connect or relate to boundary currents, it doesn’t seem to be fed by a series of eddies as with the Lofoten Basin Eddy, i.e. eddies being shorn by the unstable Norwegian Atlantic Current (LBE, see January 11, 2024 blog). While it sits in the middle of the North Atlantic Current (NAC), the NAC is not a well-defined baroclinic current that meanders and breaks off eddies like a Gulf Stream, say. Sometimes there is a well-defined anticyclonic eddy, as such known as the Prime Eddy. It can last for months, maybe even longer, but so far as I know there is no evidence that it is permanent in the sense of the LBE. See Martin et al., 1998 for an excellent description of what has become known as the Prime Eddy (Plankton Reactivity In the Marine Environment). So what’s going on?
There was a time when I assumed the area might be fed and maintained in a fashion similar to the LBE, but I couldn’t find any evidence for eddy shedding from the NAC where it flows north just west of Hatton Bank. What could be inducing the high EKE there? One way or another it must be connected to the NAC.
First, a little background. The NAC in the Iceland Basin transports warm water north toward the Iceland-Faroe-Scotland ridge. Some of this warm water will continue across the ridge into the Nordic Seas, the remainder will turn west at the Iceland-Faroe ridge and SW along the eastern slope of the Reykjanes ridge forming a cyclonic loop around the basin. But there is also a well-defined deep cyclonic circulation in the Iceland Basin such that the north-flowing NAC is not strictly baroclinic but has a parallel deep barotropic component as well. In fact, both the shallow NAC and the deeper flow appear to be located over a SW to NE trough in the Iceland Basin known as the Maury Channel. This may be relevant because channel shoals significantly from about 3400 m depth at 55°N to 2600 m at the Prime Eddy.
Water flowing NE in the shoaling channel will be squeezed by roughly 25% over this distance inducing a corresponding negative trend in relative vorticity. I don’t know how this would work, but if this is added to the baroclinic NAC it would presumably reduce the cyclonic shear and increase the anticyclonic shear - perhaps in such a way as to render it baroclinically unstable. This is interesting for it suggests a mechanism to excite locally a high EKE from the ‘mean flow’ of the NAC. The Prime Eddy may be the best or most extreme outcome of this process. It can apparently remain in place and be quite long-lived (many months if not more) such that during winter months it can develop a very deep mixed layer. Deepening the mixed layer in the center enhances the baroclinicity of the eddy like what happens to the permanent Lofoten Basin Eddy. But unlike upper ocean or intra-thermocline lenses the Prime Eddy extends to the bottom reinforcing the supposition that the shoaling Maury Channel plays a fundamental role in its formation. Numerous writers have noted that compared to the rest of the Iceland Basin this region is conspicuously eddy active, but there is no evidence, so far as I know, that the Prime Eddy is a truly permanent feature like the LBE or the Mann Eddy. The biggest threat the Prime Eddy would not be dissipation (as with Meddies) or bottom friction, but rupture by the NAC. If, for example, it develops a strong velocity gradient (for whatever reason), that may be enough to tear the Prime Eddy apart leaving a scattering of smaller eddies in its place. But if so they either stay nearby and/or break further apart for they there is no trail of high EKE which you might expect if they were advected to the NE by the NAC.
As the title Prime implies, it appears to be a biologically active region. If preferentially so, perhaps the sediments have some interesting information on the presence/absence of a NAC between present and past glacial époques.
In any event, it might be quite informative to conduct a XBT systematic survey of the area to hopefully locate a fully developed Prime Eddy and with floats study its cohesiveness and longevity. Kind of like what we did with Meddy Sharon (Armi et el., 1988). It is such a localized area that with guidance from altimetry one day of research vessel time would suffice to localize and map it. We could also seed it with non-acoustic (i.e. not tracked) floats that remain at depth as long as the temperature remains above a certain value indicating that it is within the bowl of deep mixed warm water. But once the temperature drops below some threshold value (meaning either that it is slipping out of the bowl or the bowl is weakening) the float surfaces and reports its location and temperature history. We used this technique successfully with Meddy Sharon.
Martin, A.P., Wade, I.P., Richards, K.J., Heywood, K.J. (1998). The Prime Eddy. Journal of Marine Research, 56(2), 439-462.
Armi, L., D. Hebert, N. Oakey, J. Price, P. Richardson, T. Rossby and B. Ruddick (1988). The History and Decay of a Mediterranean Salt Lens, Nature, 333(6174), 649-651.