Direct Measurements of Pathways of Overflow Water in the Nordic Seas

Cold and relatively low salinity mode water is formed in the Iceland and Greenland Seas. These waters may exit directly to the North Atlantic through the Denmark Strait or cross the ridges into the Norwegian Sea and contribute to the overflow waters that spill through the Faroe Bank Channel. In the Lofoten and Norwegian Basins these waters are seen as a salinity minimum at intermediate depth.

In an earlier Lagrangian observational study it was shown that the intermediate circulation in the Nordic Seas is under remarkably tight topographic control. Flows can be narrow and intense along slopes and weak in the interior, and that exchange of intermediate waters between basins takes place along topographically controlled routes. It was also shown that the Faroe Bank Channel overflow waters must come from along the slope north of the Faroes, and not from the interior of the Norwegian Sea.

In recent studies, acoustically tracked subsurface drifters, RAFOS floats, indicate that intermediate waters in the Lofoten Basin along the Mohn Ridge may be transported along topography in the western Norwegian Basin directly to the Faroe Bank Channel. However, the data also show that some of the RAFOS floats are diverted into the interior. There is also intermediate transport eastward from the Iceland Sea directly to the overflow, but only one RAFOS float deployed in the Iceland Sea did this. Several of the floats deployed north of Iceland were on their way toward the Norwegian Sea, but due to low mean flow speeds in the Iceland Sea only two made it as far as the Norwegian Basin during their mission (280-650 days). On the other hand, four RAFOS floats in the western Iceland Sea at intermediate depth revealed a rapid pathway along the Greenland slope into the Irminger Sea. These drifted from 70°N to the Denmark Strait in about 100 days, indicating a much more rapid transport in the western part of the Iceland Sea, compared to the very slow eastern export.