Turbulence in the Pycnocline: Temporal and Spatial Scales

Vertical exchange of nutrients facilitated by small amounts of vertical mixing across the pycnocline in continental shelf seas sustains a substantial proportion of the total oceanic primary production (15-30%). Mixing at this critical interface is sporadic and highly variable, both spatially and temporally, which makes its observation and prediction very difficult. We present preliminary results from a new project PycnMix, which aims to produce a ‘step change’ in the representation of pycnocline mixing processes in regional scale shelf sea models. We present results from the reanalysis of over 20 years of microstructure measurements from across the seasonally stratified North West European Shelf Seas, forming the world’s largest observational database of shelf sea pycnocline turbulence measurements.

We find that the distribution of pycnocline turbulence data relative to proposed stability criteria is highly dependent on the large range of representative turbulent length and timescales observed. Instrument separation and resolution is likely a key factor in the observed distribution of data. Here we present the potential impacts of under resolving turbulence and mixing in the pycnocline and show how this can give a skewed impression of the mechanisms involved. We employ theoretical frameworks of turbulent behaviour to reanalyse data over suitable length scales to assess the impact on empirical solutions. The results from a high-resolution non-hydrostatic model are used to validate methods and to assess the broader impact of our findings.