The Polar Front in the northwestern Barents Sea: structure, variability and mixing

Abstract. In the northwestern Barents Sea the warm and salty Atlantic Water meets the cold and fresh Polar Water, forming a distinct thermohaline front, the Barents Sea Polar Front. Here we present the structure of the front, its variability and associated mixing using observations from two cruises conducted in October 2020 and February 2021 during the Nansen Legacy project in the region between the Hopen Trench and the Olga Basin. Ocean stratification, currents and turbulence data were obtained during seven ship transects across the Polar Front near 77° N, 30° E. These transects are complemented by four missions using ocean gliders, one of which was equipped with microstructure sensors to measure turbulence. Across the front, we observe warm (> 1 °C) and salty (> 35.0 g kg−1) Atlantic Water intruding below the colder (< 0 °C) and fresher (< 34.6 g kg−1) Polar Water, setting up a baroclinic front and geostrophic currents reaching 25 cm s−1, with estimated eastward transport of 0.2±0.6 Sv (1 Sv =1×106 m3 s−1). We observe anomalous warm- and cold-water patches on the cold and warm side of the front, respectively, colocated with enhanced turbulence, where dissipation rates of turbulent kinetic energy range between 10−8 and 10−7 W kg−1. Short-term variability below the surface mixed layer arises from tidal currents and mesoscale eddies. While the effects of tidal currents are mainly confined to the bottom boundary layer, eddies induce significant shifts in the position of the front and alter the isopycnal slopes and the available potential energy of the front. Substantial water mass transformation is observed across the front, likely a result of eddy-driven isopycnal mixing. Despite the seasonal changes in the upper layers of the front (0–100 m) influenced by atmospheric forcing, sea ice formation and brine rejection, the position of the front beneath 100 m depth remained relatively unperturbed.