Monitoring coastal ocean dynamics under typhoon conditions using submarine distributed optical-fiber acoustic sensing

Coastal communities face increasing risks from sea-level rise and changing cyclone activity in a warming climate, yet in situ observations remain sparse, particularly, during extreme weather. Instrumenting submarine optical-fiber cables with distributed acoustic sensing (DAS) offers a promising solution by using seafloor compliance noise induced by ocean surface gravity waves (OSGWs) as an environmental proxy. Here, a DAS observation of compliance noise along a 17.7 km cable in the Zhoushan Archipelago, China, during Typhoon Khanun (2023) is reported. Distinct spectrographic features of observed compliance noise (0.05-0.35 Hz) reflect the interactions of OSGWs with tidal levels, currents, and winds. Using a proposed methodology, bathymetry, tidal levels, linear shear current vector profiles, and directional OSGW spectra along the cable were systematically measured. Validations against reference bathymetry, local forecasts of tidal level and currents, satellite imagery, and reanalysis winds confirm the measurement reliability. The measured results reveal the dominance of the M2 tidal constituent in the study area, a periodically enhanced vertical shear of flood currents induced by the local submarine slope, and the local OSGW response to typhoon-intensified winds. This work demonstrates the feasibility of repurposing existing submarine optical-fiber cables for robust monitoring of coastal ocean dynamics and hazards.