Oblique wave-wave interactions of nonlinear near-surface internal waves in the Strait of Georgia

Although nonlinear interactions that occur when two large internal waves collide at oblique angles are often observed in the natural world, quantitative and theoretical aspects of these interactions are only poorly understood. The available analyses are generally theoretical or the result of limited numerical experimentation, with few (if any) quantitative field measurements. Here we describe four cases of internal wave interactions, two of which involve the fundamentally non-steady generation of ``Mach stems{''} at the site of the interaction, observed in the Strait of Georgia, Canada. Details of the interactions are quantified using time sequences of photogrammetrically rectified oblique images obtained from a circling aircraft, while simultaneous observations are made of water column properties from a surface vessel. The imaging technique allows us to estimate the speed and direction of each wave, as well as details of phase shifts induced by interactions. An existing small-amplitude theory is applied but is found to overestimate the likelihood of Mach interactions at large amplitude.