The life-cycle of high-frequency internal waves in a continental shelf sea : generation , propagation and dissipation

Internal waves (IWs) have been recognised as one of the main drivers of climate controlling circulation, sustaining fisheries in shelf seas and CO2-pump system. High frequency IWs 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 high frequency IWs 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 ~20 km from the continental shelf break, shows that while the energetics (KE and APE) of low frequency IWs differs slightly between sites, which can be explained through variable local and remote forcing. Baroclinic energy distribution at high frequencies is shown to be near constant at both sites, however, there is a significant difference in energy levels between sites. A new high-resolution (50 m horizontal) MITgcm configuration is employed to identify the generation and propagation of IWs in the Celtic Sea and subsequently identify likely mixing hotspots on the Celtic Sea by using calculated Froude number and body force maps. The Froude number maps have shown that the majority of 3D model domain has Fr>>1, which leads to generation of Lee waves. 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.