Modal Analysis of Acoustic Energy Periodic Fluctuations Due to Internal Tides in the Yellow Sea

Abstract

Measurement data of the internal-wave and acoustic-fluctuation experiments conducted in the Yellow Sea in summer 2009 are analyzed herein. The internal-wave activity dominated by semidiurnal tides is observed. For the source and 16-element vertical linear array with fixed distance, the received energy fluctuations of 260-Hz continuous wave signals reach 20 dB in a single channel of the receiving array, and the fluctuation period is about 2–3 h across 10-h continuous acoustic measurement. The energy depth distribution varies with time, and the energy fluctuations are less than 10 dB post-depth-integration, evidencing that part of the energy transfers between channels. Acoustic normal modes are calculated according to the temperature chain observation results obtained at the receiving station and subsequently used to analyze the acoustic energy fluctuations: the difference between the average energy depth distribution in 0–5 h and 5–10 h indicates that the energy fluctuations of the single channel may be related to the dominant-mode alternation occurring between the first- and second-order modes; the modal decomposition results of the received acoustic field prove such alternation and explain the single-channel and depth-integral energy fluctuations caused by the first two modes’ amplitude variations.