The sound speed structure of an observed lens-shaped subsurface-intensified eddy and its effects on acoustic propagation

Mesoscale eddies significantly influence underwater acoustic propagation by modifying sound speed distributions. Existing studies have predominantly focused on the acoustical effects of surface-intensified eddies detectable via satellite altimetry. However, the acoustic impacts of subsurface-intensified eddies remain unclear due to the difficulty of observation. This study investigates a lens-shaped subsurface-intensified eddy (LS-SSE) observed by a spray glider off Mindanao in 2010, systematically examining its oceanographic and acoustic characteristics. The LS-SSE contains warm, salty water, exhibiting maximum temperature and salinity anomalies of 1 °C and 0.2 psu near the 26.75 σ θ isopycnal. These thermohaline anomalies substantially alter the local sound speed field, creating a subsurface acoustic duct (LS-SSE-Duct) between 250–350 m. Dynamic decomposition indicates that this duct is primarily attributed to the “spice sound speed” component, associated with water masses advected by the LS-SSE along isopycnals. Numerical simulations using decomposed “tilt” and “spice” sound fields reveal their distinct effects on acoustic propagation. In the tilt field, the upper sound energy turning points shoal by ∼50 m relative to the unperturbed field, and the first and second convergence zones shift forward by ∼4 km. In the spice field, the LS-SSE-Duct could trap sound energy with weak attenuation over ∼220 km. Acoustic modes within the LS-SSE-Duct shift to higher orders with increasing horizontal range away from the sound source, leading to the transfer of ducted energy to the shallower water layers. Beyond 80 km, average transmission loss (TL) increases with frequency. Frequency-dependent TL variations are mainly associated with mode coupling, and the contribution of diffractive effect is negligible.