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.
Abstract. Observations from moored instruments are analyzed to describe the Norwegian Atlantic Slope Current at the Lofoten Escarpment (13∘ E, 69∘ N). The data set covers a 14-month period from June 2016 to September 2017 and resolves the core of the current from 200 to 650 m depth between the 650 and 1500 m isobaths. The along-isobath current, vertically averaged between 200 and 600 m depth, has an annual cycle amplitude of 0.1 m s−1, with the strongest currents in winter, and a temporal average of 0.15 m s−1. Higher-frequency variability is characterized by fluctuations that reach 0.8 m s−1, lasting for 1 to 2 weeks, and extend as deep as 600 m. In contrast to observations in Svinøy (2∘ E, 63∘ N), the slope current is not barotropic and varies strongly with depth (a shear of 0.05 to 0.1 m s−1 per 100 m in all seasons). Within the limitations of the data, the average volume transport of Atlantic Water is estimated at 2.0±0.8 Sv (1 Sv =106 m3 s−1), with summer and winter averages of 1.6 and 2.9 Sv, respectively. The largest transport is associated with the high temperature classes (>7 ∘C) in all seasons, with the largest values of both transport and temperature in winter. Calculations of the barotropic and baroclinic conversion rates using the moorings are supplemented by results from a high-resolution numerical model. While the conversion from mean to eddy kinetic energy (e.g., barotropic instability) is likely negligible over the Lofoten Escarpment, the baroclinic conversion from mean potential energy into eddy kinetic energy (e.g., baroclinic instability) can be substantial, with volume-averaged values of (1–2)×10-4 W m−3.
The Lofoten Basin in the eastern Nordic Seas plays a central role in modifying the warm Atlantic Water inflow toward the Arctic Ocean. Here, the Atlantic Water experiences increased residence times, cooling, and substantial transformation. In this study, we investigate the Atlantic Water inflow pathways to the Lofoten Basin and their vertical and seasonal variations using 2-D and 3-D Lagrangian simulations forced by a high-resolution ocean model. Atlantic Water enters the basin from all directions, but we find two main inflow pathways at all vertical levels, one close to the Lofoten Escarpment in the southeast, associated with the Slope Current, and another close to the Helgeland Ridge in the southwest, associated with the Front Current. The surface inflow exhibits a stronger seasonal forcing than the inflow at depth as well as a stronger heat loss that is dominated by water masses entering the basin from the south. At deeper levels, the warm inflow from the east cools, while the relatively colder inflow from the west warms. The 2-D and 3-D synthetic trajectories show similar pathways. However, they are affected differently by the seasonal signal, giving different heat exchange patterns. Our results have implications for how results from Lagrangian observations in the region should be interpreted. Plain Language Summary The Lofoten Basin in the Nordic Seas is of fundamental importancefor the modification of the warm northward flowing Atlantic Water. Much of the ocean heat is lost to the atmosphere in this region. This is maintained by warm water inflows from regions around. Here, we study these inflows, their vertical structure, seasonal variability, and contribution to the heat budget in the basin. We apply an ocean model to advect purposefully released particles in the Nordic Seas seeded at 15-, 200-, and 500-m depth and study their pathways and fates. We analyze both a horizontal 2-D (particles are fixed at depth) and a full 3-D (particles can move in the vertical) simulation and compare the two. We find that the water masses mainly enter the Lofoten Basin in two regions, one in the southeast and one in the southwest. However, the vertical structure reveals that water that is cooled enter the basin via different routes at the surface than at deeper levels. The seasonal variations are also larger at surface than at depth. The 2-D and 3-D simulations show overall similar patterns, but the 3-D simulation reveals larger seasonal variations than the 2-D simulation.
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