Mr. Tom's Blog

Discovery of the two parallel streams of the Irminger Current.

The Nuka Arctica was a container vessel of the Royal Arctic Lines that operated out of Nuuk in Greenland to Aalborg in Denmark on a 3-week schedule, occasionally making a stop at Reykjavik on its westbound legs. Starting in 1999 we operated a 150 kHz acoustic Doppler current profiler (ADCP) for four years. Thanks to the high density of zooplankton backscatterers it reached to O(300) m depths. Starting in late 2012 we operated a 75 kHz ADCP that reached much deeper. Despite traversing some of the worst seas in the North Atlantic, the ADCPs worked well, in no small part thanks to the deep draft the vessel. Thanks to this operation we discovered that the Irminger Current flowing north on the western side of the Reykjanes Ridge consisted in fact of two separate yet parallel flows!

The Nuka Arctica operation became a wonderful collaboration between us here at GSO, the Faroe Marine Research Institute (FMRI) and the University of Bergen (UiB). FMRI made the key contribution with the ADCP, it is what got this whole enterprise started! We contributed with the sea chest, a structure that is welded into the hull of the ship to house the ADCP. The UiB contributed with data loggers, technical, and operational support. In addition, the University of Stockholm funded the purchase of the GPS compass; this was key to knowing the exact heading of the vessel so its velocity vector could be subtracted from the ADCP data to yield accurate O(1) cm/s water velocities. We, Charlie Flagg, George Schwartze and I, installed the equipment while the ship was in drydock at Landskrona in fall 1999. Just as with the earlier Oleander installation, everything worked as intended right from the start.

A graduate student at the UiB, Øyvind Knutsen, joined us for a few months to learn from Sandy Fontana, our ADCP guru, how to process the ADCP data, and to initiate a first analysis of the material, it was an exciting time. One of the striking things to emerge was the discovery of two parallel flows of the Irminger Current, a very robust finding that illustrated the power of the vessel-based ADCPs: sampling the horizontal dimension at high resolution. He also documented the southward flow on the eastern side of the ridge. See also the paper by Chafik et al. (2014) for more detail on the vertical structure of these currents. The data from the Nuka Arctica operation have contributed significantly to our knowledge of the subpolar North Atlantic circulation. I’ll try to write more on this later.

Knutsen_figure


Knutsen, Ø., H. Svendsen, S. Østerhus, T. Rossby and B. Hansen (2005). Direct measurements of the mean flow and eddy kinetic energy structure of the upper ocean circulation in the NE Atlantic. Geophys. Res. Letters,32. L14604, doi:10.1029/2005GL023615

Chafik, L., T. Rossby and C. Schrum (2014). On the spatial structure and temporal variability of poleward transport between Scotland and Greenland. Journal of Geophysical Research doi: 10.1002/2013JC009287