Comparison of decadal low-frequency ambient noise trends measured in the northern Pacific Ocean: ATOC and CTBTO systems

Abstract

Trend estimates from 25 Hz to more than 50 Hz collected from 1994 to 2018 in the north Pacific Ocean are compared. The majority of trends derived from Acoustic Thermometry of Ocean Climate (ATOC) systems over nearly two decades, starting at roughly 1994, suggest a decrease in ambient noise levels of up to 1 dB/year. This is observed on both coastal and deep ocean systems. (Datasets from the remaining systems show either no change or an increase.) Measurements from the Comprehensive Test Ban Treaty Organization (CTBTO) system at Wake Island from 2008 to present provide an overlap from 2008—2013. The CTBTO data streams are essentially continous, and can be averaged and/or aggregated over a variety of time scales. Daily averages show a distinct seasonal cycle. From 2008 to about 2014, the underlying trends are decreasing by as much as 0.55 dB/year, corroborating some of the ATOC observations; from 2014 to the present, the trends are essentially flat. The mechanisms driving these trends appear to be more subtle than the local wind speed or merchant fleet size.