Study of horizontal multipaths and ray chaos due to ocean mesoscale structure

Long-range ocean acoustic propagation in the presence of idealized mesoscal e structure is studied by first deriving a two-dimensional horizontal-plane parabolic wave equation that follows from the adiabatic mode approximation . In the geometric limit, a nonautonomous Hamiltonian dynamical system havi ng one degree of freedom is derived. A stochastic formalism is developed to analyze this nonintegrable dynamical system. The main result is that on av erage two rays that are initially separated by an infinitesimal amount dive rge exponentially at a rate given by the Lyapunov exponent that has been ca lculated theoretically and compared to numerical experiments with agreement to two decimal places. The practical implication of this result is that to mographic inversions based on assumed pointwise accurate ray predictions mi ght not be possible beyond the "predictability horizon" of many thousands o f kilometers, due to horizontal-plane multipaths induced by naturally occur ring mesoscale activity. (C) 2000 Acoustical Society of America. [S0001-496 6(00)05201-2].