Mode coupling in a coastal wedge with a sloping thermocline

Abstract

Internal waves populate multiple stratified aquatic systems and are one of the most energetic high-frequency events in shallow water and deep ocean, causing strong currents and turbulence. In this paper, propagation of a sound signal is considered in a wedge-like area in the presence of an internal Kelvin wave, which causes periodic diurnal changes of the thermocline depth along the acoustic track. This, in turn, leads to periodic inclination of the thermocline in the vertical plane, which causes sound field fluctuations at the vertical line array. Analysis of the experimental data and numerical modeling suggest that physical reasons for the sound field variability are 1) mode coupling in the area, where time-dependent normal mode eigenvalues approach each other (i.e., quasi-cross-section), 2) fluctuations of the normal modes’ attenuation coefficients. Numerical modeling is carried out within the framework of the Parabolic equation and normal modes theory.