The Life-Cycle of High Frequency Internal Waves in a Continental Shelf Sea: Generation, Propagation and Dissipation”.

High-frequency internal waves (HFIW) are particularly important to internal mixing in the shelf seas, where they contain an enhanced fraction of the available baroclinic energy. The origin, generation mechanism, propagation and spatial distribution of these waves are unfortunately still poorly understood since they are difficult to measure and simulate, and are therefore not represented in the vast majority of ocean and climate models.

In this study we aim to increase our understanding of HFIW dynamics in shelf seas through a combination of observational (moorings, gliders, OMGs) and modelling methods (MITgcm), and test the hypothesis that “Solitary waves are responsible for driving a large fraction of the vertical diffusivity at the shelf edge and adjacent shelf region”. Our analysis of two separate sites, both situated ~20km from the continental shelf break, shows that the energetics (KE and APE) of low frequency internal waves (IWs) are of similar magnitude with subtle differences explained through variable local and remote forcing. Baroclinic energy distribution at high frequencies is shown to be near constant at both sites, independent of low frequency forcing. There is however a significant difference in energy levels between sites, one being enhanced by ~60%.

A new high-resolution (50m horizontal) MITgcm configuration is validated using the observed IW characteristics and employed to identify the generation and propagation of IWs in the Celtic Sea. We identify how energy is transferred to higher frequencies and subsequently identify likely mixing hotspots on the Celtic Sea. These predictions are then compared to turbulence data collected using an Ocean Microstructure Glider and VMP to assess the impact of the identified IW characteristics on internal mixing. Lastly, we force the model with different density structures to assess the likely impact of changing climate forcing scenarios on IW generation and internal mixing on the continental shelf.