Mechanistic insights into wind field heterogeneity effects on vertical directionality of ocean ambient noise.

This study establishes a quantitative framework using field observations and normal mode theory to reveal wind field control mechanisms over ambient noise vertical directionality in shallow water. Acoustic data from a vertical line array in the northern South China Sea, combined with sound speed profiles, seabed properties, and multi-source wind fields (ERA5 reanalysis/Weibull-distributed synthetics), demonstrate: (1) A 20-km spatial noise-energy threshold (>90% energy contribution), challenging conventional near-field assumptions (1-2 km); (2) frequency-dependent distribution: low-frequency (50-200 Hz) directionality depends on near-field sources, while high-frequency (>400 Hz) energy shifts seaward due to modal cutoff variations; (3) model validation shows 0.96 correlation at 100 Hz/100 km (stratified medium accuracy), but seabed interface waves induce 3.8 dB deviation at 50 Hz; (4) wind heterogeneity thresholds: uniform wind approximation causes negligible error (<0.01 dB) under weak gradients (0.36 knots/km) but significant error (1.47 dB at 50 Hz/20 km) under strong gradients (8.23 knots/km); (5) strong wind gradient-frequency coupling yields stable ± 17° grazing-angle biases below 400 Hz versus high-frequency anomalies at large angles. The work revises classical homogeneous-wind noise models and mandates 5-km spatial resolution for marine acoustic monitoring. Future studies will integrate three-dimensional propagation and bubble dynamics for extreme-condition predictions.