Wave chaos and travel time effects of ocean mesoscale structure at global ranges

Abstract

A range-dependent ray-trace model and the broadband UMPE model are used to model sound propagation at fixed bearing through a field of mesoscale baroclinic modes in order to study the effects of the mesoscale structure on travel time at long ranges. The ray model exhibits chaos at ranges beyond a few Mm as manifested by an exponentially increasing number of triplications of the wavefront. In addition, the ray model predicts a mesoscale travel time bias in the direction of later time (‘‘cold bias’’) of the last axial arrival that is about 50–150 ms/Mm. At center frequency 75 Hz and bandwidth 50 Hz, the full-wave UMPE model qualitatively confirms the ray-trace predictions of chaos, that is now called ‘‘wave chaos’’ because it represents apparently random and unstable behavior of finite frequency wavefields. Also, the UMPE model quantitatively confirms the above mesoscale bias of the last axial arrival. Theories based on the adiabatic approximation are used to explain the modeled mesoscale bias in magnitude and sign. The steeper, early raylike arrivals are relatively stable in the presence of mesoscale structure and should be useful for long-range tomography. [Work supported by ONR.]