The Svalbard branch of the West Spitsbergen Current: Hydrography, transport and mixing

Abstract

Data from a 10 days shipboard survey in August 2015, northwest of Svalbard, are used to investigate the transport, structure and mixing of Atlantic water (AW) along the Svalbard branch and Yermak branch of the West Spitsbergen Current. Using the common definition of AW, the volume transport south and north of the Yermak Plateau (YP) is estimated to be 3.6 (3.4, 3.7) Sv and 1.1 (1.1, 1.3) Sv respectively, where the upper and lower bounds are inferred from the sensitivity to the choice of streamtubes. The current south and north of YP is horizontally symmetric, with surface-enhanced geostrophic velocities. On YP the symmetry breaks down, and the current spreads out and weakens over the flat plateau, with intensified currents near the steep continental slope. Volume transport across YP is 0.8 (0.5, 1.3) Sv. In addition to a well-defined Svalbard branch, current measurements show recirculation north of the Molly Hole. At the time of the survey, AW temperatures and salinities north of 79◦N are found to be significantly higher than shown by the Monthly Isopycnal and Mixed-layer Ocean Climatology. Microstructure measurements show a net surface heating of 1-2 W m^−2 of the AW layer. The downstream temperature and salinity gradients show cooling and freshening rates of 0.15◦C and 0.016 g kg^−1 per 100 km along path distance. The observed cooling rates cannot be accounted for by the vertical turbulent heat flux. Isopycnal diffusion in an eddy field north of YP is capable of cooling the current at observed rates, and can generate lower Arctic intermediate water in the process.