Author(s): Gavrilov Alexander, Mccauley Robert, Zhang Zhi Yong
The northern part of the northwest shelf of Australia is a meteorologically and oceanographically dynamic marine area. It is also an important feeding and resting ground for great whales on migration. The area has a climate with two distinct seasons: the summer monsoon and its dry winter phase, with transitional periods between the seasons of a couple of months. Most of the annual rainfall of about 780 mm occurs between December and March and is associated with storm activity. Underwater noise data at two sites separated by 150 km were collected over several years near the shelf edge and noise sources were identified from their temporal and spectral characteristics. The major sources were wind (wave breaking), whale vocalisations and fish choruses, shipping traffic and offshore seismic exploration. Monthly statistical variability over the frequency range of 10–3 kHz was computed as power spectral density percentiles. Omura’s whale vocalisations dominate at 20-30 Hz from September to April. Evening fish choruses produce moderate spectral peaks around 150 Hz from September-November and are present less than 10 percent of the day. Above 200 Hz the noise follows approximately the typical variation with frequency for wind noise, indicating noise from rain was insignificant in comparison to noise generated by wind. Wind noise was lower during the transition periods between summer and winter seasons. There was significant correlation between the noise level and the wind speeds recorded at a weather station about 150 km away. Using these wind speeds as proxy for the in-situ wind speeds, the noise data were compared with other measurements in Australasia and the Northern Hemisphere.
Name: Dr Zhi Yong Zhang