Wind, rain, and breaking waves in tropical cycles observed by underwater ambient sound

The air–sea interface beneath tropical cyclones is studied using underwater ambient sound measured by hydrophones onboard air-deployed Lagrangian floats. We obtained ∼200 h of broadband sound data at 1–50-m depth with wind speed 15–45 m/s. We detected acoustic signatures of wind, rain, and breaking waves by decomposing the sound data into three components according to their timescales. There is a complex dependence of sound level on wind speed due to the competing effects of sound generation by breaking waves and attenuation by quiescent bubbles. Sound level increases monotonically with increasing wind speed for low frequencies <200 Hz. At frequencies >200 Hz, sound level first increases and then decreases with increasing wind speed. Rain-generated sound fluctuations have spectra around 10 kHz. At high speeds, rain- and wind-generated sound levels are comparable. Our rain detection algorithm is based on 5–30-kHz sound with periods >20 s and <30 min, and the detected rain events usually last for a few minutes. Our fast sampling rate (two samples per second) revealed sound bursts caused by breaking waves, which usually last for 6–8 s and do not change much with wind speed. At low winds, the sound bursts cover frequencies from 50 kHz down to 200 Hz. At high winds, the sound bursts are disrupted around 1 kHz, with asynchronous high- and low-frequency sound bursts.