Observations of turbulent mixing and hydrography in the marginal ice zone of the Barents Sea

Measurements of hydrography, currents, microstructure shear, and temperature were made at ice drift stations in the marginal ice zone (MIZ) of the northern Barents Sea. Highly variable mixing regimes were observed within and below the pycnocline. Elevated turbulent dissipation (5-15 x 10(-7) W kg(-1)) was associated with strong vertical shear between the surface layer and the subsurface currents, as well as strong tidal flow over shallow topography. Dissipation in the pycnocline was enhanced at stations with strong wind forcing. During drifts under relatively calm wind and away from strong fronts and abrupt topography, station-mean dissipation values were up to a factor 50 lower and double diffusion contributed significantly to the vertical heat flux where hydrography favored diffusive layering. Independent measures of turbulent length scale from density overturns compared well with those inferred from the dissipation measurements. The variability of dissipation was better captured using a scaling by shear suggested for shelves rather than shear variance models appropriate to the deep open ocean. Sufficiently resolved patches of enhanced temperature microstructure used in combination with dissipation measurements suggest mixing efficiency of R-f similar to 0.2 for patches stable to double diffusion, comparable to the conventional upper bound of R-f similar to 0.17. Mixing efficiency for double-diffusive convection favorable cases is found to be significantly larger, R-f similar to 0.36. Water mass modification and fluxes of nutrients and dissolved carbon were found to have large local variability in accordance with the observed variability of vertical mixing in the MIZ.