Mid-depth mixing under the subpolar front at the Mid-Atlantic Ridge

Finescale parameterizations of diapycnal mixing are now widely used to expand the data base of ocean mixing observations for a better spatial coverage than direct microstructure measurements. Further, they are also a useful tool to assess temporal changes of the strength and distribution of internal wave energy and mixing in the ocean, and thereby identify the processes responsible for these distributions. Here, we use an 8 year data set to examine the temporal variability of mixing estimated from internal wave finestructure at the part of the Mid Atlantic Ridge (MAR) where the subpolar front crosses the ridge from west to east Atlantic. Earlier estimates of finescale mixing at this location (based on 3 realizations) revealed a temporal variability of mixing strength at mid-depth below the NAC. The observed mixing strength was linked to the presence of an NAC core, and the pattern of energy dissipation depended on the position of the subpolar front. The extended data set now covers the years from 2008 to 2015, with seven repeats of a hydrographic section reaching roughly from 47° N to 53° N along the MAR, west of the ridge crest, and confirms the presence of enhanced mixing below the NAC/subpolar front. The coverage of years with strong negative (2010) as well as positive (2012) winter North Atlantic Oscillation indices and associated fluctuations in NAC position and volume transport allows to examine the effects of variations in external parameters like strength (volume transport) and structure of NAC interacting with the topography, stratification, presence of eddies and surface eddy kinetic energy on the distribution of mixing in the water column, especially at mid-depth where it potentially impacts deep water transformation.