On the origin of the warm inflow to the Nordic Seas

Abstract 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 undercurrent in the subtropics, continuing into the subpolar gyre along the eastern boundary, and rising from depths near 1200m in the Rockall Trough to less than 600m 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 thermocline waters above the sill depth. On the basis of regional water mass distributions, geostrophic shear, and direct current measurements, we reject the deep source hypothesis in favor of the shallow source hypothesis. Rather than a flow of deep Mediterranean Overflow Water along the eastern boundary rising from depth to feed the Nordic Seas, the Inflow is supplied directly by transformed North Atlantic Current waters from the same depth range as the Inflow. Mediterranean Overflow Water is a constituent of the Inflow, but only through its contribution to defining the temperature-salinity relationship of the interior of the subtropical gyre from which the North Atlantic Current draws its water, rather than by direct northward advection of Mediterranean Outflow Water via the eastern boundary undercurrent crossing from the subtropics into the subpolar domain. Instead, the eastern boundary undercurrent wholly expels its transport of Mediterranean Overflow Water into the eastern edge of the subtropical gyre, creating the mid-depth westward extending subtropical salinity plume. For the lower thermocline isopycnal range of the core of the Mediterranean Overflow Water, measurements show that at the Wyville-Thomson Ridge the flow is southward and descending (rather than northward and rising), representing cold spillover from the Faroese Channels entraining warm upper ocean waters as the spillover plume descends and warms into the northern Rockall Trough. No evidence is found in Rockall Trough northward of 54°N (Porcupine Bank) for direct advection of Mediterranean Overflow Waters via an eastern boundary undercurrent extension, and the very dilute Mediterranean Overflow Waters in the Trough reflect the interaction of the Wyville-Thomson Ridge plume descending into the Trough from the North, and lower thermocline waters delivered to the Trough from the south.