Stationary phase internal waves generated by flow along sloping topography

Oikonomou, Emmanouil Konstantinou (1997) Stationary phase internal waves generated by flow along sloping topography. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The phenomenon of stationary phase IWs (Internal Waves) generated by flow along sloping topography is studied in this thesis by means of theoretical analysis, laboratory experiments, ray tracing models and SAR (Synthetic Aperture Radar) imagery interpretation.

Initially the expressions for the IW group velocity, flow fluctuation and energy flux were derived. Then, the theoretical predictions were tested in two laboratory layouts in which a stratified fluid contained in a conical tank was rotated from rest. In the first experimental set-up the surface layer flow was monitored from a camera placed above the centre of the tank and the trajectories of submerged particles were tracked. In the second experimental layout the circulation patterns of the tank interior were monitored from a camera facing the transparent walls of the tank. The shadowgraph technique was applied and particles of specific density revealed IW activity and the IW amplitude was obtained. Although both sets of experiments were in good agreement with the theory, it was the second layout that demonstrated visually the existence of such IWs.

The mathematical formulate of the IWs under investigation were then implemented into eight ray tracing models that provided qualitative information on the propagation of their energy fluxes from their origin up to the sea surface or an interface and/or to where they cross the shelf break. Typical oceanic values of continental slope geometry, stratification and flow regimes were considered. Additionally, in situ measurements over the Malin shelf collected by others were processed and incorporated in a ray model. The Malin simulation indicated the possible generation of stationary phase IWs under realistic conditions and that their rays could radiate up to the seasonal thermocline carrying significant amount of energy flux. Finally, ERS-1 and ERS-2 SAR imagery over the Malin-Hebrides shelf for a 3-year period was analysed with respect to IW surface manifestation. The SAR observations revealed the random presence of IWs travelling along slope that had distinctive properties compared to the frequently occurring cross shelf groups of rank order solitons. It was then proposed that these IWs were generated by the arrival at the seasonal thermocline of energy flux due to stationary phase IWs, in a similar mechanism as observed in some of the laboratory experiments in which continuous stratification was surmounted by an homogeneous layer.

In conclusion, it is suggested that stationary phase IW may represent a potentially important source of energy for both the upper layer and the seafloor processes over the continental slope area. However, the work presented in this thesis implies that such IW phenomena seem to be restricted to the vicinity of the shelf break and extend only over a local scale of the order of a few km.

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