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Discussion papers | Copyright
https://doi.org/10.5194/os-2018-40
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Apr 2018

Research article | 16 Apr 2018

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Ocean Science (OS) and is expected to appear here in due course.

Overflow of cold water across the Iceland-Faroe Ridge through the Western Valley

Bogi Hansen1, Karin Margretha Húsgarð Larsen1, Steffen Malskær Olsen2, Detlef Quadfasel3, Kerstin Jochumsen3, and Svein Østerhus4 Bogi Hansen et al.
  • 1Faroe Marine Research Institute, PO Box 3051, FO-110 Tórshavn, Faroe Islands
  • 2Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen, Denmark
  • 3University of Hamburg, Bundesstrasse 53, 20146 Hamburg, Germany
  • 4Uni Research Climate, Nygårdsgata 112, N-5008 Bergen, Norway

Abstract. The Iceland-Faroe Ridge (IFR) is considered to be the third-most important passage for dense overflow water from the Nordic Seas feeding into the lower limb of the Atlantic Meridional Overturning Circulation with a volume transport on the order of 1Sv (106m3s−1). The Western Valley, which is the northernmost deep passage across the IFR, has been presumed to supply a strong and persistent overflow (WV-overflow), contributing a large fraction of the total overflow across the IFR. However, prolonged measurements of this transport are so far missing. In order to quantify the flow by direct measurements, three instrumental packages were deployed close to the sill of the Western Valley for 278 days (2016–2017) including an Acoustic Doppler Current Profiler at the expected location of the overflow core. The average volume transport of WV-overflow during this field experiment was found to be less than 0.03Sv. Aided by the observations and a two-layer hydraulic model, we argue that the reason for this low value is the inflow of warm Atlantic Water to the Norwegian Sea in the upper layers suppressing the deep overflow. The link between deep and surface flows explains an observed relationship between overflow and sea level slope as measured by satellite altimetry. This relationship, combined with historical hydrographic measurements allows us to conclude that the volume transport of WV-overflow most likely has been less than 0.1Sv on average since the beginning of regular satellite altimetry in 1993. Our new direct measurements do not allow us to present an updated estimate of the total overflow across the IFR, but they indicate that it may well be considerably less than 1Sv.

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Bogi Hansen et al.
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Short summary
The Western Valley is one of the passages across the Iceland-Scotland Ridge through which a strong overflow of cold, dense water has been thought to feed the deep limb of the Atlantic Meridional Overturning Circulation (AMOC), but its strength has not been known. Based on a field experiment with instruments moored across the valley, we show that this overflow branch is much weaker than previously thought and that this is because it is suppressed by the warm current in the upper layers.
The Western Valley is one of the passages across the Iceland-Scotland Ridge through which a...
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