Paired eddy currents change how sound waves travel through the ocean

Sound waves travel through different types of matter, including liquid water. Importantly, the movement of ocean water can greatly affect how sound waves travel from one point to another.

Mesoscale eddies are large, swirling ocean currents that are kilometers in diameter. These eddies aren’t permanent, compared to the ocean Gulf Stream, but can last on the order of weeks to months and have a large influence on climate, circulation, and the mixture of heat, salt and nutrients in the ocean. Given their temporary nature, mesoscale eddies can also affect how sound waves travel, or propagate, depending on their size, speed, current direction and location. Understanding these effects are critical for accurate underwater target detection and long-range communication.

The effects of individual mesoscale eddies on sound waves have been studied in great detail both in the ocean and through various models. In contrast, the effects of two diametrically opposed mesoscale eddies on sound wave propagation have not been directly measured.

The northeast South China Sea has a high probability of developing anticyclonic eddies (AEs) that rotate in a clockwise direction due to local currents and wind stress. When an AE develops in this region, it is often accompanied by a cyclonic eddy (CE) that rotates counter-clockwise, forming dipole eddies. A group of researchers from the Chinese Academy of Sciences and Shandong University of Science and Technology in Qingdao, China studied three sets of dipole eddies in the South China Sea to better characterize their effects on sound waves. The team published their research on March 21 in Ocean-Land-Atmosphere Research.

“This research, for the first time, systematically analyzes the acoustic effects of three pairs of dipole eddies in the northeastern South China Sea, revealing their unique sound-speed structures and acoustic propagation patterns,” said Lu Lu, researcher at the Institute of Oceanology of the Chinese Academy of Sciences and first author of the research paper.

Specifically, the scientists were interested in learning how the changes in ocean temperature and salinity caused by dipole eddies can alter the structure of the sound-speed profiles, or the speed of sound in water at different depths, which can affect how sound waves travel in the ocean. The team used satellite altimeter data to accurately measure eddy current velocity, assimilated reanalysis data combining numerical model, and a BELLHOP ray-tracing model to predict sound wave propagation through dipole eddies.

Overall, the researchers discovered that AEs tended to sink in the center, which decreased ocean temperature, salinity and sound-speed contours in the AE side of the dipole. On the other hand, CEs demonstrated an upwelling of water that consequently increased temperature, salinity and sound speed. The conditions in the dipole eddies varied from the surrounding water, which the team calculated as temperature, salinity and sound speed anomalies based on average data recorded from the years 2000 to 2020.

“The study found that warm-core AEs and cold-core CEs cause positive and negative sound speed anomalies, respectively, significantly altering the position of convergence zones (CZs) and acoustic transmission loss (TL). The findings demonstrate that the acoustic effects of dipole eddies are markedly different from those of single eddies, providing new perspectives for ocean acoustic research through their complex sound speed structures and propagation patterns,” said Lu.

Through modeling, the team found that the presence of the dipole eddies changes the CZs, or regions of focused sound intensity, and TL regardless of whether the sound source is in the middle of the dipole eddies, in the center of either the AE or CE, or outside of the dipole eddies.

While the current research has enhanced the field’s knowledge of how dipole eddies affect sound wave propagation in the ocean, the team acknowledges the limitations of their study design. The dipole eddies studied moved southwestward in the South China Sea, which may have influenced acoustic propagation. Simulation experiments were also performed assuming flat-bottomed terrain, which may also affect sound wave travel.

“Future work will explore the effects of dipole eddies on acoustic propagation during their evolution, as well as their influence under realistic terrain conditions. The ultimate objective is to investigate the influence of multiple mesoscale vortices on acoustic propagation, thereby summariz[ing] the general principles of dipole edd[y] effects on acoustic propagation,” said Lu.

Feng Nan and Fei Yu from the Key Laboratory of Ocean Observation and Forecasting and the Key Laboratory of Ocean Circulation and Waves at the Institute of Oceanology and the Center for Ocean Mega-Science in the Chinese Academy of Sciences in Qingdao, China, the Laoshan Laboratory in Qingdao, China, and the College of Earth Science at the University of Chinese Academy of Sciences in Beijing, China; Anqi Xu from the Key Laboratory of Ocean Observation and Forecasting and the Key Laboratory of Ocean Circulation and Waves at the  Institute of Oceanology in the Chinese Academy of Sciences in Qingdao, China; and Xiangqian Liang from the School of Mathematics at Shandong University of Science and Technology in Qingdao, China also contributed to this research.

This work was supported by the National Key Research and Development Program of China (2022YFB3205300), the National Natural Science Foundation of China (42376010 and 42306024) and the Taishan Scholar Project of Shandong Province (tsqn202306282).