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 Ocean. This is a main contributor to the production of North Atlantic Deep Water that feeds the lower limb of the Atlantic meridional overturning circulation, which has been predicted to weaken as a consequence of climate change. 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 channel for 1995-2005 with an ensemble hindcast experiment 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 ridge, but this has been compensated for by the effect of changes in sea level. In contrast with earlier studies, 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.