Looking Inside Internal Tides – A High Resolution Modelling Study

Shelf-sea models have progressed substantially in the last couple of decades in terms of their predictive capabilities and are now at the point where mesoscale features, such as internal tides, can be resolved. Internal waves are of particular importance as they include mechanisms for transferring matter, such as nutrients for phytoplankton and CO2, between deep water and the surface mixed layer. However, there is still disparity between the small-scale turbulent mixing processes and the large scale driving forces, a link which is vital to understanding the underlying mechanisms behind this exchange of matter. The aspiration of this project, PycnMix, is to simulate (in fine detail) the mixing processes that are observed in the field, to help optimise parameterisations in shelf-sea models.

To bridge this scale gap, a large-eddy simulation (LES) model has been utilised to focus in on the fine-scale (<1m) detail of the flow field, whilst using a novel technique to retain the large scale driving forces. This method permits the simulation of tidally driven turbulent mixing processes. In particular we investigate the turbulent mixing processes due to (i) tides rubbing over a rough sea bed, which result in elevated levels of shear and mixing in the bottom boundary layer, and (ii) the internal tides, generated non locally by interaction between the barotropic tide and bathymetric features, which result in elevated levels of mixing across the pycnocline.