www.elsevier.com/locate/pocean On the origin of the warm inflow to the Nordic Seas

Warm and saline waters enter the Nordic Seas from the south as part of the warm-to-cold water transformation of the thermohaline circulation of the northern North Atlantic. One explanation for the origin of the Nordic Seas Inflow is a “shallow source hypothesis ” under which the Inflow waters are a modification of upper ocean subtropical waters. Warm waters from the subtropical gyre are carried to the eastern North Atlantic by the North Atlantic Current and branch northwards, joined by poleward upper thermocline flow along the upper continental slope, to provide the Nordic Seas Inflow. Along this pathway the upper water column is progressively cooled and freshened by winter convection, the subpolar mode water transformation process, and this sets the Inflow characteristics. A “deep source hypothesis ” provides an alternative explanation for the characteristics of the Nordic Seas Inflow and the pathway delivering the waters to the Inflow. Under this hypothesis Inflow waters originate from the core of the Mediterranean Overflow Waters in the Gulf of Cadiz carried northward at mid-depth by the eastern boundary undercur-rent in the subtropics, continuing into the subpolar gyre along the eastern boundary, and rising from depths near 1200 m in the Rockall Trough to less than 600 m to cross the Wyville-Thomson Ridge into the Faroe-Shetland Channel and thence to the Nordic Seas. The deep source hypothesis focus is on lower thermocline source waters beneath the sill depth for the Nordic Seas Inflow, in contrast to the shallow source hypothesis focus of transformation of upper thermoc