The mesoscale eddy field in the Lofoten Basin from high-resolution Lagrangian simulations

Abstract

Abstract. Warm Atlantic-origin waters are modified in the Lofoten Basin in the Nordic Seas on their way toward the Arctic. An energetic eddy field redistributes these waters in the basin. Retained for extended periods, the warm waters result in large surface heat losses to the atmosphere and have an impact on fisheries and regional climate. Here, we describe the eddy field in the Lofoten Basin by analyzing Lagrangian simulations forced by a high-resolution numerical model. We obtain trajectories of particles seeded at three levels – near the surface, at 200 m and at 500 m depth – using 2D and 3D velocity fields. About 200 000 particle trajectories are analyzed from each level and each simulation. Using multivariate wavelet ridge analysis, we identify coherent cyclonic and anticyclonic vortices in the trajectories and describe their characteristics. We then compare the evolution of water properties inside cyclones and anticyclones as well as in the ambient flow outside vortices. As measured from Lagrangian particles, anticyclones have longer lifetimes than cyclones (16–24 d compared to 13–19 d), a larger radius (20–22 km compared to 17–19 km) and a more circular shape (ellipse linearity of 0.45–0.50 compared to 0.51–0.57). The angular frequencies for cyclones and anticyclones have similar magnitudes (absolute values of about 0.05f). The anticyclones are characterized by warm temperature anomalies, whereas cyclones are colder than the background state. Along their path, water parcels in anticyclones cool at a rate of 0.02–0.04 ∘Cd-1, while those in cyclones warm at a rate of 0.01–0.02 ∘Cd-1. Water parcels experience a net downward motion in anticyclones and upward motion in cyclones, often found to be related to changes in temperature and density. The along-path changes in temperature, density and depth are smaller for particles in the ambient flow. An analysis of the net temperature and vorticity fluxes into the Lofoten Basin shows that while vortices contribute significantly to the heat and vorticity budgets, they only cover a small fraction of the domain area (about 6 %). The ambient flow, including filaments and other non-coherent variability undetected by the ridge analysis, hence plays a major role in closing the budgets of the basin.

Heat Transport by Mesoscale Eddies in the Norwegian and Greenland Seas

“…Eddy heat flux from the NwASC cannot fully close the heat balance of the Lofoten Basin (Dugstad et al, 2021;Spall, 2010). The required amount, according to Vesman (2021), should be three times the heat flux estimated here.…”

“…The required amount, according to Vesman (2021), should be three times the heat flux estimated here. Eddies generated to the south of the basin and in the NwAFC, as well as other transient dynamic patterns (jets, filaments, submesoscale diffusion), should be added, but they also cannot fully close the regional heat budget (Dugstad et al, 2021;Spall, 2010). Analysis of SAR images and the results of high-resolution FESOM numerical model suggest that eddies with radii of 2-5 km are the most frequent in this region (Bashmachnikov et al, 2020).…”

“…During its lifetime such eddy propagates only a few kilometers and, accounting for its relatively small size, we may expect two to three orders of magnitude less efficient heat transport compared to a single mesoscale eddy analyzed here. However, model studies suggest that, due to a large number of submesoscale structures, their integral effect on the heat transport could exceed that of mesoscale eddies (Dugstad et al, 2021;Spall, 2010).…”

“…Both branches of the NwAC, as well as the WSC, meet the instability criteria and can be regarded as eddy generation regions (Ghaffari et al, 2018;Koszalka et al, 2011;Trodahl & Isachsen, 2018). The area of the largest heat loss along the NwAC is also the region of the most intensive generation of eddies by the NwASC, along the steepest section of the continental slope of Norway (Dugstad et al, 2021;Isachsen, 2015;Koszalka et al, 2011;Raj et al, 2020). However, Lagrangian modeling suggests that eddies may be not the only factor in redistributing heat in the basin.…”

“…However, Lagrangian modeling suggests that eddies may be not the only factor in redistributing heat in the basin. Strong pulses of current developed in jets crossing the basin, filaments and Ekman transport may also be of high importance (Dugstad et al, 2021;Fedorov et al, 2021;Spall, 2010). NwASC is the Norwegian Atlantic Slope Current, NwAFC is the Norwegian Atlantic Front Current, WSC is the West Spitsbergen Current, EGC is the East Greenland Current, NIIC is the North Icelandic Irminger Current, NCC is the Nordcape Current, LV is the Lofoten Vortex, LI is Lofoten Islands, JMF is the Jan Mayen Fracture zone, BSO is the Barents Sea Opening, Sv is Svinoy section, JM is Jan Mayen section and island, BI is Bear Island section and island, Srk is Sorkapp section, FS is Fram Strait section.…”