A weak planetary vorticity gradient and weak density stratification in the northern North Atlantic and Nordic seas lead to time-mean currents that are strongly guided by bottom topography. The topographic steering sets up distinct boundary currents with strong property fronts that are prone to both baroclinic and barotropic instability. These instability processes generate a macroturbulent eddy field that spreads buoyancy and other tracers out from the boundary currents and into the deep basins. In this paper we investigate the particular role played by baroclinic instability in generating the observed eddy field, comparing predictions from linear stability calculations with diagnostics from a nonlinear eddy-permitting ocean model hindcast. We also look into how the bottom topography impacts instability itself. The calculations suggest that baroclinic instability is a consistent source of the eddy field but that topographic potential vorticity gradients impact unstable growth significantly. We also observe systematic topographic effects on finite-amplitude eddy characteristics, including a general suppression of length scales over the continental slopes. Investigation of the vertical structure of unstable modes reveal that Eady theory, even when modified to account for a bottom slope, is unfit as a lowest-order model for the dynamics taking place in these ocean regions.

The Lofoten Basin in the Nordic Seas plays a central role in the Atlantic overturning circulation by acting as a reservoir for the warm and saline Atlantic Water flow toward the Arctic Ocean. The mass and heat exchange between Atlantic Water and the Lofoten Basin impacts the water mass transformations and the surface heat loss, but the processes governing this exchange are not well understood or quantified. Here we study the circulation in the Nordic Seas and the heat transport in the Lofoten Basin using a combination of Lagrangian and Eulerian methods. We analyze the trajectories of about 150 surface drifters, augmented with a set of about 47,000 surface trajectories calculated using the output from a regional numerical simulation, to investigate the drifter pathways and their exchange with the Lofoten Basin. The drifters reveal that water parcels with long residence time inside the basin contribute substantially to the heat loss and typically enter from south across the outer rim of the Vøring Plateau and, to some extent from east, from the eastern branch of the Norwegian Atlantic Current. The main contributors to the lateral heat transport to the Lofoten Basin are the near‐surface heat transport by the mean flow in the southern sector of the basin and the subsurface eddy fluxes from the Lofoten Escarpment in the east.

Observations from the past decades have promoted the idea of a long-lived anticyclonic vortex residing in the Lofoten Basin. Despite repeatedly recorded intense anticyclones, the observations cannot firmly decide whether the signature is of a single vortex or a succession of ephemeral vortices. A vortex persisting for decades requires some reinvigoration mechanism. Wintertime convection and vortex merging have been proposed candidates. We examine Lofoten Basin vortex dynamics using a high-resolution regional ocean model. The model is initialised from a coarser state with a weak eddy field. The slope current intensifies and sheds anticyclonic eddies that drift into the basin. After half a year, an anticyclone arrives at the center, providing the nucleus for a vortex that remains distinct throughout the simulation. Analyses show that this vortex is regenerated by repeated absorption and vertical stacking of lighter anticyclones. This compresses and - in concert with potential vorticity conservation - intensifies the combined vortex, which becomes more vertically stratified and also expels some fluid in the process. Wintertime convection serves mainly to vertically homogenise and densify the vortex, rather than intensifying it. Further, topographic guiding of anticyclones shed from the continental slope are vital for the existence and reinvigoration of the Lofoten Vortex. These results offer a new perspective on the regeneration of oceanic anticyclones. In this scenario the Lofoten Vortex is maintained through repeated merging events. Fluid remains gradually exchanged although the vortex is identifiable as a persistent extrema in potential vorticity.