Acoustic mode coupling due to subaqueous sand dunes in the South China Sea: Extension of the adiabatic criterion to waveguides with bedforms

Abstract

The large subaqueous sand dunes on the upper continental slope of the South China Sea (SCS) create a range-dependent ocean acoustic waveguide within which acoustic energy is expected to couple between propagating normal modes. Here, the criterion of adiabatic invariance is extended to the case of a waveguide possessing bedforms. The morphological features of the bedforms modeled in this theoretical and numerical investigation are based on echosounder observations of the SCS sand dune field during a research cruise in the spring of 2012 on the Taiwanese R/V Ocean Researcher 2 (OR2). Using the extended criterion for adiabatic invariance to examine mode propagation over these bedforms, results demonstrate that bedforms increase mode coupling strength such that the criterion for adiabatic propagation is exceeded for waveguides with small bedform amplitude to water depth ratios; increasing bedform amplitude enhances mode coupling. Physically, initially bottom-trapped mode 1 energy abruptly couples to higher adjacent modes, with most of the energy preferentially settling into a few select modes. The scattered energy fills the water column to near-surface depths downrange of the bedforms. Numerical simulations confirm the extended criterion parameterization. [This work was supported by National Science Council of Taiwan.]