Different types of subsurface acoustic ducts in the North Pacific

Subsurface acoustic ducts are essential for underwater communication by enabling long-range acoustic propagation with minimal interaction with the sea surface or seafloor. This study performs an in-depth analysis of subsurface ducts in the North Pacific using a large set of Argo profiling floats. By applying the Gaussian mixture model, we classify the ducts into distinct types linked to specific water masses and oceanographic processes. Results show that subsurface ducts show strong regional variability. The subarctic front, defined as the southern boundary of subarctic stratification, serves as a key boundary in duct distribution. North of the front and south of the Aleutian Islands, subsurface ducts are nearly permanent. South of the front, subsurface ducts are driven by the intrusion of warmer subtropical water beneath colder subarctic water in the subarctic-tropical frontal zone. In the subtropical region, the isothermal layer of subtropical mode water can also give rise to subsurface ducts with a duct axis at 150-200 m. Additionally, sound simulations across 50-1000 Hz reveal the frequency-dependent characteristics of these ducts, with critical frequencies varying by duct type.