Near-surface underwater noise generated by internal solitary waves in the South China Sea

Abstract

Underwater noise measurements were made over the steep continental slope east of Dongsha Atoll in the South China Sea during a field experiment from 23 May to 25 June, 2019. A single hydrophone was mounted at a depth of 20 m on a mooring deployed at 300 m water depth; temperature/pressure sensors and current meters were also deployed to monitor the water column temperature and velocity structure. Anticipated sources of underwater noise included ship traffic, wind waves and associated near-surface bubbles. Unique to this deep water environment are currents generated by the world’s largest observed internal solitary waves (ISWs) which generate noise associated with flow and bubbles subducted to depths in excess of 30 m. The observed noise variability primarily consisted of tidal- and subtidal-scale fluctuations, superimposed upon which were short timescale fluctuations generated by the ISWs. The noise levels observed during typical ISW passage exhibited a non-linear relationship with frequency, increasing by 5–10 dB at mid-frequencies (>1 kHz) and by 10–20 dB at low frequencies (<1 kHz).