The Norröna was the 3rd ship Charlie Flagg and I were to equip with an acoustic Doppler current profiler (ADCP). It operates out of the Faroes to Denmark and Iceland. The beauty of this vessel’s route is that it spans all warm water entering the Nordic Seas and hence the climatologically crucial deep branch of the Atlantic overturning circulation (AMOC). Thus, the case for instrumenting this ship with an ADCP was a no-brainer. The suggestion to do this was likely made easier from our earlier successes on the Oleander and the Nuka Arctica. But as I wrote earlier, we were in for a nasty surprise. The two huge bow thruster openings blanketed the hull with bubbles that wiped out the acoustics. Fortunately, we were able to turn things around thanks to Charlie’s initiative to put the ADCP in a fairing. Although less than perfect we started to get valuable information on currents in the Faroe-Shetland Channel (FSC). Curiously, they didn’t fit with what we expected.
Historically, the FSC was seen as the main route by which warm, salty North Atlantic water entered the Nordic Seas, most of it as a well-defined boundary current flowing north along the eastern slope of Rockall Trough. It is clearly identified in the famous 1909 Helland-Hansen and Nansen book ‘The Norwegian Sea’. The problem was that the ADCP data were indicating a substantial flow south on the western side of the FSC, this was not expected. This bothered me to the point that I wondered whether we had a measurement problem.
We hadn’t fully solved the bubble problem. Thanks to following winds and seas we got much better data returns steaming from the Faroes to Shetland than vice versa. Might there be a bias creeping in due to the uni-directional sampling? But if so why localized to the western side of the FSC? I discussed this with Sandy Fontana, who managed all the data processing, telling her that we had an issue with the data. But she reminded that she calibrated the ADCP whenever the vessel is in shallow water. This is done by integrating vessel speed and heading acoustically over the bottom and comparing that distanced traveled with travel according to the GPS. It’s an extremely accurate procedure. It started to look like our findings were real. With time we also got data steaming westbound, and these gave the same results: there is a substantial flow south in the western half of the FSC. This of course reduces the net amount of water entering the Nordic Seas through the FSC. The reason it came as a surprise is that this flow is less baroclinic and thus had remained ‘invisible’ to standard hydrographic methods. Look at Figures 2 and 5 in our first report (ref. attached below).
As the project continued we started getting velocity data along the Iceland-Faroe Ridge (IFR). These confirmed what was already known: that the lion-share of North Atlantic water entering the Nordic Seas does so here and not in the FSC. And as we noted earlier, all warm water entering the Nordic Seas comes via the Iceland Basin, not along the eastern slope or through Rockall Trough.
But here’s my point, if you think there’s a possibility of the AMOC slowing down, there is no better place to detect any such trend than here at the FSC and IFR. Bogi Hansen and colleagues got that that right with their ongoing project to monitor overflow through the Faroe Bank Channel. Similarly, that was the power of the Norröna. Sadly, for technical and financial reasons this program was shut down. If I were younger and knowing now how to address the bubble problem, I would make every effort to restart it. Standing by!
Rossby, T. and C. Flagg. 2012, Direct measurement of volume flux in the Faroe-Shetland Channel and over the Iceland-Faroe Ridge. Geophys. Res. Letters, 39, L07602, doi:10.1029/2012GL051269