Analysis of Surface Sound Duct in the Northern Shelf of the South China Sea

Abstract EN

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.