Turbulent diapycnal mixing in the Nordic Seas

The distribution of turbulent diapycnal mixing in the Nordic seas is mapped from observations of internal wave density and velocity fine structure. The uppermost 500–1500 m host two distinct mixing regimes. In the eastern basins, the diapycnal diffusivity (K) straddles 10^-5 m^2 s^-1, whereas in the weakly stratified Greenland and Boreas basins it is raised by an order of magnitude. Below 2000 m, low stratification is associated with intense turbulent mixing across the Nordic seas, with diffusivities in the range 3 x 10^-4 – 10^-2 m^2 s^-1. These mixing rates agree within uncertainties with three tracer-based diffusivity estimates in the region and are associated with turbulent dissipation rates (E) that are at most moderately enhanced above typical open ocean values. A minimum in both E and K is commonly found at ~1500 m, a depth level that is most efficiently sheltered from shallow and bottom energy sources for the mixing. Available evidence points to wind work on upper ocean inertial motions as a shallow source, with semidiurnal internal tides generated at different levels of the topography contributing to both shallow and deep turbulence. While the closure of the North Atlantic meridional overturning circulation in the Nordic seas appears to be primarily driven by air-sea interaction, turbulent mixing has the potential to play a critical role in shaping the stratification and ventilation of the region via a range of complex interactions with convection.

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