The Nordic seas are commonly described as a single basin to investigate their dynamics and sensitivity to environmental changes when using a theoretical framework. Here, we introduce a conceptual model for a two-basin marginal sea that better represents the Nordic seas geometry. In our conceptual model, the marginal sea is characterized by both a cyclonic boundary current and a front current as a result of different hydrographic properties east and west of the midocean ridge. The theory is compared to idealized model simulations and shows good agreement over a wide range of parameter settings, indicating that the physics in the two-basin marginal sea is well captured by the conceptual model. The balances between the atmospheric buoyancy forcing and the lateral eddy heat fluxes from the boundary current and the front current differ between the Lofoten and the Greenland Basins, since the Lofoten Basin is more strongly eddy dominated. Results show that this asymmetric sensitivity leads to opposing responses depending on the strength of the atmospheric buoyancy forcing. Additionally, the front current plays an essential role for the heat and volume budget of the two basins, by providing an additional pathway for heat toward the interior of both basins via lateral eddy heat fluxes. The variability of the temperature difference between east and west influences the strength of the different flow branches through the marginal sea and provides a dynamical explanation for the observed correlation between the front current and the slope current of the Norwegian Atlantic Current in the Nordic seas.

The water masses exiting the Labrador Sea, and in particular the dense water mass formed by convection (i.e. Labrador Sea Water, LSW), are important components of the Atlantic Meridional Overturning Circulation (AMOC). Several studies have questioned the connection of the LSW production to the AMOC variability. This is partly due to the limited understanding of how this locally formed water mass leaves the interior of the Labrador Sea. In this study, the pathways and the timescales of the water masses exiting the Labrador Sea via the boundary current are investigated by Lagrangian particle tracking. This method is applied to the output of a strongly-eddying idealized model that is capable of representing the essential physical processes involved in the cycle of convection and restratification in the Labrador Sea. The Lagrangian trajectories reveal that prior to exiting the domain the water masses follow either a fast route within the boundary current or a slower route that involves boundary current-interior exchanges. The densest water masses exiting the Labrador Sea stem from this slow route, where particles experience strong water mass transformation while in the interior. In contrast, the particles that follow the fast route experience water mass transformation in the boundary current at the western side of the domain only, yielding a lighter product. Although both routes carry roughly the same transport, we show that 60% of the overturning in density space is associated with the volume transport carried by particles that follow the slow route. This study further highlights that the export of dense water masses, which is governed by the eddy activity in the basin, yields export timescales that are usually longer than a year. This underlines the necessity of resolving the mesoscale features required to capture the interior–boundary current exchange in order to correctly represent the export of the LSW.

Atlantic Water takes various pathways through the Nordic Seas, and its transformation to denser waters forms a crucial connection to the lower limb of the Atlantic Meridional Overturning Circulation. Circulation maps often schematize two distinct pathways of Atlantic Water: one following the Norwegian Atlantic Slope Current along the continental slope of Norway and one following the Norwegian Atlantic Front Current along the Mohn and Knipovich Ridges. In this paper, the connectivity between the northward flow along these ridges is investigated. Analyzing trajectories of surface drifters and ARGO floats, we find that only 8% of the floats that travel near the mid-ocean ridges take the frontal pathway to the north. Indeed, by tracing numerical particles in a realistic numerical simulation, part of the water mass traveling along the Mohn Ridge follows the 2,500 m isobath eastward and joins the slope current, instead of flowing north along the Knipovich Ridge. Furthermore, north of 74°N, frequent exchange between the slope current and the front current is observed. Therefore, the slope current and front current are less isolated than often schematized. Additionally, the observational data set reveals substantial cross-ridge exchange; 31% of the floats that travel within 60 km from the mid-ocean ridges cross it. Results from numerical simulations indicate that the cross-ridge exchange leads to cooling and freshening of the Atlantic Water along the front. Deployments of floats near the mid-ocean ridges are needed to investigate the pathway of Atlantic Water and its exchange across the ridge in more detail.