An exciting day! In summer 2010 I was on a cruise with my Bergen colleague Henrik Søiland in the Norwegian Sea. After fulfilling the primary objectives of the cruise, we had a day to spare to take a closer look at an area where we knew from past surveys that the water is notably warmer and saltier, and to much greater depths than the surrounding ocean. This is interesting because a pool of warm water reaching to great depth must exhibit clockwise motion, most likely in the form of an anticyclonic eddy. We frequently see eddies near strong currents like the Gulf Stream or the Kuroshio Current in the Pacific Ocean. These eddies result from pinch-offs of the meandering current. But why a pool of warm water all by itself over the deepest part of the Lofoten Basin in the Norwegian Sea? What did it look like? In just one day we learned so much, it felt like a discovery, all thanks to an instrument called the acoustic Doppler current profiler (or ADCP for short).
Today almost all research vessels are equipped with ADCPs. These instruments transmit short acoustic pulses or pings in four oblique directions to profile the velocity structure underneath and then listen for backscattered sound from zooplankton at various depths in the water column. The Doppler shift of the backscattered echoes gives us the velocity component in each beam. From these we can determine the horizontal components of motion relative to the ship. The ship’s motion can be accounted for quite accurately thanks to the global positioning system leaving us with water velocity with ~1 cm/s accuracy. The ADCP is a very powerful tool. Some models profile currents in detail near the surface, others can profile to beyond 1000 m depth.
As we steamed into the pool of warm water, we could immediately see the clockwise motion and that it had a well-defined center. Knowing this, we set up a survey to map out the velocity structure while making a few stops to profile temperature, salinity, and oxygen of the upper ocean. The survey revealed strikingly coherent and circular eddy out to about 50 km radius with an 80 cm/s maximum swirl velocity about 18 km from the center. The profile near the center revealed an extraordinarily well-mixed layer to 1000 m depth. The surface water was warm due to summertime heating, but thin because it floats on top with little downward mixing thanks to the quiet weather in summer. We call this feature the Lofoten Basin Eddy or LBE for short (Søiland and Rossby, 2013). The ADCP could profile currents to much greater depths in the LBE than outside thanks to a high concentration of zooplankton backscatterers.
It may seem a bit contradictory but the deep pool of warm water results from intense cooling in winter that leads to intense vertical mixing. So where does the heat come from? It comes from smaller anticyclonic eddies that are spun off the Norwegian Atlantic Current where it goes unstable off the Lofoten islands of Norway. These eddies drift west into the deeper part of the Lofoten Basin where they ‘collide with’, ‘get sucked into’, or are ‘ablated by’ the LBE. I’m not sure how well we know how this merging takes place. But merge they do for the heat that comes with these eddies is distributed into the LBE and in so doing helps maintain the density contrast that keeps it spinning. These eddies arrive all year round helping to build up the heat content of LBE which in wintertime is lost to the cold atmosphere. At the same time the deep mixing in winter helps maintain the density contrast between the eddy and surrounding water, and thus the pressure difference that balances the anticyclonic motion of the eddy.
A curious consequence of this heat loss is that as water becomes denser, it becomes warmer and saltier than the water around it at depth! The warm surface water is more saline than the water at depth. Thus, as it cools and settles on a deeper density surface, it must be warmer than the surrounding water for it to have the same density. This condition of being warmer and saltier is often referred to as being ‘spicier’, an apt description thanks to Walter Munk.
We found in the ICES (International for the Exploration of the Seas) data archives several Soviet hydrographic sections through the LBE from the 1960s. These strengthened our suspicion that the LBE is a permanent feature of the Norwegian Sea. It shows up quite clearly in a Norwegian hydrographic section ‘Sotra’ from 1930. Since we have every reason to think that the Norwegian Atlantic Current is a permanent feature of the Nordic Seas circulation (since the end of the last glacial epoque), it is tempting to think that eddies have continually been breaking off where the current goes unstable, and that these feed and maintain the LBE. Who knows, if it was there 80 years ago, perhaps it has been in existence for 100s maybe 1000s of years?! Since it doesn’t wander far from 70°N, 4°E I can’t but help wonder if the high zooplankton concentration in the LBE has left a sedimentary record of cliimate for the region? This might well be worth exploring. I am not aware of any other mid-basin eddy like the LBE in the world ocean. But it is more than a curiosity for the heat loss associated with the LBE is considerable, and as a result the water at depth is substantially spicier in this region than anywhere else in the Nordic Seas.
Søiland, H., and T. Rossby (2013), On the structure of the Lofoten Basin Eddy, J. Geophys. Res. Oceans, 118, 4201–4212, doi:10.1002/jgrc.20301.