Experimental and Numerical Modelling of Boundary Effects on Internal Solitary Waves

Abstract

In the ocean, waves exist not just at the surface, but also on internal density interfaces. Driven by energy from the tide and wind, these internal waves are responsible for horizontal transport, and vertical mixing of heat, nutrients, and water properties. Large Internal Solitary Waves (ISW) can travel long distances without significantly changing. Until that is they interact with boundaries. This thesis explores two such cases of boundary effects on ISWs, firstly when ISWs travel up slopes (shoaling), and secondly when they interact with floating bodies, such as sea ice. Numerical and laboratory methods are utilised in the thesis. The first two chapters of this thesis investigate shoaling ISWs, and the effects of stratification on this process. Whilst previous studies have identified that ISWs shoal via one of four mechanisms (namely plunging, collapsing, surging and fission), depending on a combination of the slope, and wave steepness (wave amplitude/wavelength), here it is identified that the presence of density gradients in the upper layer and lower layer suppress the plunging and collapsing dynamics respectively. In Chapter 4, it is discussed how dense pulses produced by shoaling over a shallow (geophysically representative) slope compare under different stratifications. Chapter 5 presents a new diagnostic tool for understanding mixing in these numerical models, by tagging water parcels based on their fluid properties. The final chapter investigates ISW-sea ice interactions using laboratory experiments with floats of varying sizes. The motion of these floats are successfully modelled simply as the average velocity of fluid under the float, and the relationship between float motion, float length and wavelength is explored. Flow features induced by larger floats form a pair of counter-rotating vortices, which are explained with the relative velocity of the flow, relative to the float. This indicates a complex dynamic in a system with many non-stationary components.