Monthly and Seasonal Trends in Statistical Characteristics of Underwater Noise in Chukchi Plateau

Abstract

In Arctic Ocean, the sea ice concentration, sea surface temperature and wind speed will change significantly with months and seasons, and then affect the characteristics of underwater noise. In this paper, the measured underwater noise data over a year period from October 1, 2018 to September 30, 2019 (local time) in the Chukchi Plateau are presented and discussed to study the monthly and seasonal statistical characteristics of underwater noise. Due to the transient sound signals generated by sea-ice activities, the underwater noise spectrum levels have a greater fluctuation during ice-cover period, and there are a large number of discrete values in the upper part of the empirical probability density diagram of noise spectrum level, which have deviated from the main distribution areas of noise spectrum level. For temperature increasing and ice-melting, the ice concentration decreases rapidly with time in summer, and the sea surface of Chukchi Plateau is in ice-free conditions for a few months. During the open-water conditions, the underwater noise of Chukchi Plateau should be dominated by wind-generated noise and it is almost unaffected by transient sounds, therefore, there are few discrete values which can be discovered on the empirical probability density diagram of noise spectrum level. The mean and median values of noise spectrum levels of the four seasons have been studied and compared. The spring reaches the lowest level in mean and median values for the gradually rising temperature and lower wind speed, and it is considered to be the quietest season in a year in Arctic Ocean, which is in agreement with the view of Sagers and Ballard. The attenuation trends of the mean and median values with the increasing frequency in summer are different from that of the other three seasons. The other three seasons exhibit a steeper attenuation slope than summer. The median values of noise spectrum levels measured in this paper demonstrate a higher attenuation slope compared with the results of Sagers and Ballard. The measured underwater noise is dominated by wind-generated noise in summer, therefore, the attenuation law of median values with the increasing frequency in summer is compared with that of Wenz. The noise spectrum levels decrease 4 dB/octave with the increasing frequency between 630 Hz and 2 kHz in this paper, and the result of Wenz is approximately decreasing 5 dB/octave.