Abstract
The northern shelf of the South China Sea (NSSCS) is characterized by surface low-salinity water due to discharge from the Pearl River. In such an environment, the surface sound duct (SSD) is the most important duct for near-surface sonar applications. Nevertheless, the mechanism of SSD formation is very complicated and is influenced by salinity, temperature at the air-sea interface, and various additional marine phenomena. In this study, an 8-year conductivity-temperature-depth (CTD) profile of the NSSCS was used to analyze the SSD formation. An advanced diagrammatic method is proposed to provide a quantitative analysis of the contribution of salinity, temperature, and hydrostatic pressure on SSD formation. Large salinity gradient (0.25 psu/m) was shown to play a crucial role in SSD formation when a mixed layer exists. As representative examples, the sea under cold surges, typhoon genesis, and low-salinity lenses were studied. Conversely, the absence of SSDs in low-salinity water was also observed in upwelling regions. This study further showed that highly negative temperature gradients affect SSD formation even in low-salinity water. Furthermore, although the duct depth of a low-salinity SSD is usually less than 10 meters, it still can serve as an effective duct for acoustic propagation.
Highlights
The principal characteristic of a surface sound duct (SSD) is that the sound speed increases monotonically with the depth below the sea surface
Hydrostatic pressure is the dominant factor in duct formation, corresponding to the area enclosed by the threshold line, Sz = −0.016/γ and Tz = −0.016/β
The SSD is different from the sonar duct in spatiotemporal variability, it is still an excellent waveguide for long-range propagation
Summary
The principal characteristic of a surface sound duct (SSD) is that the sound speed increases monotonically with the depth below the sea surface. The effect of salinity on duct formation is not negligible in the freshening process, that is, when freshwater mixes with seawater due to river discharge into the sea. Bulgakov et al studied the mechanism responsible for the SSD in the tropical Atlantic Ocean This area contains a strong freshening of oceanic waters by continental discharge and rainfall. Since the mixed layer in the tropical ocean is relatively stable, temperature contributes little, and as such, the formation of the haline duct was due to a sharp change in salinity. The second improvement focuses on distinguishing the general contributing factors of the different types of SSDs by valuating the contribution of salinity, temperature, and pressure This improved diagram tool was used to study the SSD in the northern shelf of the South China Sea (NSSCS), and the quantitative characteristics of the duct are discussed. A quantitative analysis of the SSD formation in the NSSCS is presented here
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
