Wind dependence of deep ocean ambient noise at low frequencies

Abstract

A series of experiments has been carried out using a vertical line array to measure low-frequency ambient noise at deep ocean sites in the Northeast Pacific Ocean. Data in the band from 13 to 300 Hz were processed to obtain the array response directed vertically upward in order to study the noise due to local winds. The results indicate that there are two regimes of behavior depending on the wind speed v, with the transition occurring at the onset of wave breaking, v∼10 kn. The noise level (NL) for a specific wind noise process was related to a power n of the wind speed according to the relationship NL=B+20n log v, and the model parameters were obtained by fitting the data. The average value over the band was n=0.3 for wind speeds <10 kn and n=1.33 for higher speeds. This latter value is significantly larger than the value n=1, which is observed in ambient noise data at higher frequencies above 1 kHz. This result suggests that the noise process in breaking waves may be different at low frequencies from that at higher frequencies. Assuming a uniform distribution of surface dipole sources, the source level for the wind-generated noise was determined over the low-frequency band for wind speeds from 10–30 kn. The values are consistent with those of a recent analysis of available noise data [Kewley et al., ‘‘Low-frequency wind-generated ambient noise source levels,’’ J. Acoust. Soc. Am. 88, 1894–1902 (1990)], and the model provides good agreement with wind noise measurements obtained with omni hydrophones in the southern oceans.