A DETERMINISTIC APPROACH TO UNDERSTANDING THE SENSITIVITY OF SURFACE DUCT PROPAGATION TO SOUND SPEED FEATURES IN THE UPPER OCEAN

Abstract

A key sound speed feature of the upper ocean is the Mixed Layer Acoustic Duct (MLAD). Acoustic propagation effects due to mean properties of the duct and spatial and temporal variability are thought to be strong functions of acoustic frequency. Key physical mechanisms at work are diffractive leakage and mode coupling. Using both analytic theory and direct numerical simulation, this thesis will examine 400 and 1000 Hz MLAD propagation characteristics by calculating the sensitivity of duct propagation to various ocean perturbations with horizontal scales ranging from 0.5 to 15 km. As a starting point, sound speed profiles (SSP) typical of the spring-summer transition in the North Atlantic are considered. Tools used are first-order mode scattering theory originally developed for shallow water propagation, and direct numerical simulation. Numerical simulations are compared to theory with the goals of 1) evaluating the utility of the shallow water analytic approach for deep water MLADs and 2) putting forward a metric for estimating MLAD stability as a function of frequency and perturbation scales. Results show that while the shallow water analytic approach is not accurate enough for the MLAD due to higher order mode interaction, first-order mode energy equation motivates the non-dimensional interaction matrix, Γmn, which showed strong correlation between multiple scattering events and increased acoustic variability when Γmn > 1.