Lateral Heat Transport in the Lofoten Basin: Near‐Surface Pathways and Subsurface Exchange

Abstract

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.

Mechanisms of the time-varying sea surface height and heat content trends in the eastern Nordic Seas

“…2) have served to induce an anticyclonic flow anomaly carrying a larger fraction of AW from the slope current into the Lofoten Basin. This flow anomaly acts to enhance the near-surface heat transport by the mean flow entering the Lofoten Basin from south (Dugstad et al, 2019). In combination with alterations of eddy fluxes from the Lofoten escarpment (Spall, 2010;Chafik et al, 2015;Dugstad et al, 2019) the anticyclonic mean-flow anomaly are plausible mechanisms for the build-up of the Lofoten Basin heat content over the period 1993-2017.…”

“…The circulation in the AW domain consists of a current system of two branches (Orvik and Niiler, 2002); The Norwegian Atlantic Front Current (NwAFC) and the Norwegian Atlantic Slope Current (NwASC), see Lofoten Basin the trends in AW density serve to strengthen the outer NwAFC branch at the expense of the inner NwASC branch. This is expected to augment the mean flow heat transport that enters the Lofoten Basin from south (Dugstad et al, 2019). Potentially, this could also increase the residence time of the AW in the region as an increasing fraction of the AW tends to follow the NwAFC, taking a longer path along the western edge of the Lofoten Basin.…”

Norwegian Atlantic Slope Current along the Lofoten Escarpment

“…The model outputs used here have a horizontal resolution of 800 m, have 60 vertical layers with increased resolution towards the surface (1-3 m at the surface and about 60 m at the bottom) and are stored as 6-hourly outputs. The model fields are described in detail in Dugstad et al (2019b).…”

Norwegian Atlantic Slope Current along the Lofoten Escarpment

“…The Lofoten Basin is affected by the Atlantic Water (AW) transport, and becomes a major heat reservoir that is exposed to large surface heat losses (Rossby et al, 2009b;Dugstad et al, 2019a) and substantial water mass transformations (Rossby et al, 2009a;Bosse et al, 2018). The AW enters the basin both as a broad slab in the upper layers between the two branches (Rossby et al, 2009b;Dugstad et al, 2019a) and by eddies detached from the unstable slope current (Köhl, 2007;Isachsen, 2015;Volkov et al, 2015). The eddy-induced lateral heat fluxes distribute the heat in the basin (Spall, 2010;Isachsen et al, 2012;Dugstad et al, 2019a).…”

“…The AW enters the basin both as a broad slab in the upper layers between the two branches (Rossby et al, 2009b;Dugstad et al, 2019a) and by eddies detached from the unstable slope current (Köhl, 2007;Isachsen, 2015;Volkov et al, 2015). The eddy-induced lateral heat fluxes distribute the heat in the basin (Spall, 2010;Isachsen et al, 2012;Dugstad et al, 2019a). The region is energized, manifested in the average geostrophic eddy kinetic energy (EKE g , see Sect.…”