Mode-coupling behavior in continental-shelf internal solitary waves

Abstract

Acoustic propagation through continental-shelf internal solitary waves (ISWs) is analyzed using the framework of acoustic modes. Propagation is modeled using both a numerical parabolic equation solution and a simple sharp interface approximation (SIA) matrix formulation. Analytic expressions for mode-coupling strength are also considered. In the coupled-mode (nonadiabatic) regime there is close agreement between the two sets of numerical results. The boundary between adiabatic and coupled propagation, and the relative propensity for various mode pairs to couple, are well predicted by analytic theory. Analysis of the SIA expressions shows that relative mode phases are the dominant parameters governing mode-coupling activity in the nonadiabatic regime. The roles of the ISW amplitude and horizontal scale in determining the relative phasings is examined. Using the phasing results and mode refraction analysis, the traditional two-dimensional acoustic model is extended to handle mode propagation through ISWs when the incident acoustic field is oblique to the ISW wavefront. The extension is valid for a wide range of incidence angles. [Work supported by ONR.]