Modulation of Sea Ice on the Low-Frequency Acoustic Propagation over the Gakkel Ridge, Arctic Ocean

Rapid changes in Arctic sea-ice coverage influence the propagation of low-frequency underwater sound, but a lack of near-surface sound speed data beneath dense sea ice limits our mechanistic understanding. To address this, we deployed 102 disposable sonobuoys along ~400 km of profiles across the Gakkel Ridge to record acoustic signals emitted by a ship-towed air-gun array. Near-surface sound speeds ranging from 1438 to 1443 m s^-1 were estimated at each sonobuoy, producing a spatially continuous, high-resolution map of acoustic conditions. Integration of these sound-speed fields with in situ hydrographic profiles and a Sea Ice Coverage Index derived from SAR grayscale imagery revealed that increasing ice concentration elevates the mean near-surface sound speed while reducing its spatial variability. This relationship indicates that dense ice stabilizes the upper ocean, weakens vertical sound-speed gradients, and promotes nearly linear acoustic propagation, whereas reduced ice cover freshens the surface layer, amplifies thermohaline variability, and increases propagation complexity. Complementary OASES–Bellhop simulations further show that, at 10–25 Hz, sea ice shortens acoustic ray paths and advances first-arrival times primarily through non-specular scattering and supercritical refraction at the rough ice–water interface. These findings highlight the role of sea ice in shaping Arctic acoustic environments and demonstrate the utility of active acoustic sensing for monitoring upper-ocean structure and characterizing sea-ice conditions.