Acoustic ray chaos induced by mesoscale ocean structure

Abstract

It has previously been shown that some acoustic ray trajectories in ocean models with periodic range dependence exhibit chaotic behavior, thereby imposing a limit on one’s ability to make deterministic predictions using ray theory. The objective of the work reported here is to quantify the limitations of ray theory to make predictions of underwater sound fields in the presence of realistic mesoscale structure. This is done by numerically investigating sound ray propagation in an analytically prescribed sound speed model consisting of Munk’s canonical profile perturbed by a randomly phased superposition of several baroclinic modes of the linearized quasigeostrophic potential vorticity equation. The ray equations used are consistent with the parabolic wave equation. To investigate ray chaos, power spectra are calculated and Lyapunov exponents are estimated. For realistic strengths of the mesoscale field, near-axial ray trajectories are found to be chaotic with characteristic e-folding distances (inverse Lyapunov exponents) of several hundred km. Steep rays, on the other hand, are found to be stable. This suggests that, in the presence of mesoscale structure, deterministic predictions of near-axial underwater acoustic energy using ray theory are limited to ranges of one to two thousand km, while steep rays should be predictable over longer ranges.