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Observed and modelled stability of overflow across the Greenland–Scotland ridge


Across the Greenland–Scotland ridge there is a continuous flow of cold dense water, termed ‘overflow’, from the Nordic seas to the Atlantic Ocean1. This is a main contributor to the production of North Atlantic Deep Water2 that feeds the lower limb of the Atlantic meridional overturning circulation, which has been predicted to weaken as a consequence of climate change3,4. The two main overflow branches pass the Denmark Strait and the Faroe Bank channel. Here we combine results from direct current measurements in the Faroe Bank channel5 for 1995–2005 with an ensemble hindcast experiment6 for 1948–2005 using an ocean general circulation model. For the overlapping period we find a convincing agreement between model simulations and observations on monthly to interannual timescales. Both observations and model data show no significant trend in volume transport. In addition, for the whole 1948–2005 period, the model indicates no persistent trend in the Faroe Bank channel overflow or in the total overflow transport, in agreement with the few available historical observations. Deepening isopycnals in the Norwegian Sea have tended to decrease the pressure difference across the Greenland–Scotland ridge7, but this has been compensated for by the effect of changes in sea level. In contrast with earlier studies7,8, we therefore conclude that the Faroe Bank channel overflow, and also the total overflow, did not decrease consistently from 1950 to 2005, although the model does show a weakening total Atlantic meridional overturning circulation as a result of changes south of the Greenland–Scotland ridge.

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Figure 1: The GSR overflow branches.
Figure 2: Observed and computed deep overflow.
Figure 3: Pressure forcing of the FBC overflow.
Figure 4: Modelled long-term changes in the North Atlantic.


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This work was partly supported by the European Commission as part of the MOEN (Meridional Overturning Exchange with the Nordic Seas) project, the Danish Environmental Ministry through the Danish Cooperation for Environment in the Arctic (DANCEA) programme, the Nordic Council in the project Arctic–Atlantic Exchanges (ARATEX), the German Federal Ministry of Education and Research (BMBF) in the North Atlantic Project, and the Research Council of Norway in the projects BIAC (Bipolar Atlantic Thermohaline Circulation) and POCAHONTAS (Polar Climate and Heat Transport). The altimeter products used were produced by Ssalto/Duacs and distributed by Aviso with support from CNES (

Author Contributions All authors contributed equally to this work.

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Correspondence to Steffen M. Olsen.

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Olsen, S., Hansen, B., Quadfasel, D. et al. Observed and modelled stability of overflow across the Greenland–Scotland ridge. Nature 455, 519–522 (2008).

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