The influence of ocean fronts on the vertical spatial characteristics of ambient noise

The unique sound speed structures of ocean fronts have a significant influence on the vertical spatial characteristics of ocean ambient noise. This study establishes a three-dimensional ocean ambient noise field model using ray theory, based on in-situ ocean front data from the Northwest Pacific. Numerical simulations are performed to analyze the vertical spatial properties of the noise field under ocean front conditions. The results show that above the critical depth, ocean fronts reduce both the horizontal notch width and notch depth in the vertical directivity of noise. As hydrophone array element spacing increases, the vertical noise coherence undergoes a pronounced shift, transitioning sequentially from positive to negative correlation and back to positive correlation. Numerical analysis reveals that changes in the depth of the sound channel axis induced by ocean fronts modify the arrival elevation angles of acoustic rays. Specifically, a subset of refracted-refracted rays is converted into refracted-surface-reflected (RSR) rays, thereby increasing the energy contribution from sea-surface noise sources to the receiver. This conversion process explains the observed reductions in notch width and depth. Further analysis of acoustic ray paths demonstrates that RSR rays primarily modulate the oscillation amplitude of the vertical correlation function. Surface-reflected-bottom-reflected rays govern the oscillation period of the vertical correlation function. The enhanced prevalence of RSR rays is the main reason for the changes in noise vertical coherence.