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.
The principal characteristic of a surface sound duct (SSD) is that the sound speed increases monotonically with the depth below the sea surface. This upward-refractive sound speed structure acts as a duct in which acoustic energy may be trapped and propagated to long ranges without bottom interaction. In continental shelf regions, the seafloor is a lossy boundary and hence the SSD is an effective duct for the sound propagation over a long range.
Variability in water temperature, salinity, and hydrostatic pressure affects SSD formation through the structure of sound speed. In general, water temperature and pressure are considered to dynamically vary with depth and have a strong influence on sound speed structure. On the contrary, salinity remains relatively constant over ocean zones and thus has little effect on sound speed [
The freshening process is primarily controlled by several factors including local freshwater flux, river discharge, and salt transport by large-scale circulation or oceanic eddies [
In this paper, an advanced diagram tool is proposed. The first improvement is that the new method can be applied to fine sampled data, such as the CTD. 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. The NSSCS is characterized by seasonally varying precipitation [
The northern South China Sea annual open cruise is a long-term investigation program that has been conducted every autumn since 1992. The open cruise is carried out by the South China Sea Institute of Oceanology, Chinese Academy of Sciences. To understand the physical, chemical, and biological aspects of the northern South China Sea, measurements included water mass property, ocean circulation, atmospheric structure, and chemical and biological elements [
Spatial distribution of shipboard CTD observations during the open cruises. Some sites were not measured in each year due to the actual investigation plan and in some cases bad weather conditions.
According to the actual salinity of the NSSCS, low-salinity water contains salinity lower than 32 psu [
Occurrence statistics of SSD formation and freshening process in the 8-year open cruise.
A1–A9 | B1–B9 | C1–C9 | |
---|---|---|---|
Normal salinity/low salinity | 16/27 | 11/22 | 16/16 |
Duct formation in low salinity (Case I) | 27 | 22 | 15 |
No duct formation in low salinity (Case II) | 0 | 0 | 1 |
Duct formation in normal salinity (Case III) | 15 | 11 | 14 |
No duct formation in low salinity (Case IV) | 1 | 0 | 2 |
All of the CTD profiles are referred to as Cases I–IV. As shown in Table
Four typical examples are plotted in Figure
Typical cases in the NSSCS.
Accordingly, the aim of the following experiment was to provide an analysis of the relationship between the SSD and the temperature-salinity structure in the NSSCS and to explore the response of the duct to some oceanographic phenomena.
The formation and characteristics of SSD are the combined product of temperature, salinity, and hydrostatic pressure. To assess the contributions of these different factors, an improved
The sound speed
As shown in (
The corresponding critical
The straight-line equation of
Cold surge case.
To address the ocean’s physical mechanisms governing SSD formation, the colored area can be further subdivided into four areas as follows.
As a result of turbulent mixing, a fine sampled profile may show many points of inflection or drastic slope changes so that the
The improved
The data measured on October 3, 2004, at A3 (22°41′N, 113°58′E) was selected for the cold surge case (Figure
In the
Likewise, most freshening-induced SSD cases in the NSCS were similar to the cold surge case. As the main part of the diluted water was located at the upper 10 m, and with a maximum thickness of not more than 20 m [
On August 17, Typhoon Nuri formed and then emerged into the South China Sea. The data measured on August 16, 2008, at B2 (22°5′N, 114°56′E) was selected for the typhoon genesis case (Figure
Typhoon genesis case.
In the
The data measured on August 16, 2008, at C1 (22°41′N, 116°17′E) was selected for the offshore upwelling case (Figure
Offshore upwelling case.
In the
Compared with previous surveys [
The data measured on September 19, 2009, at B2 (22°4′N, 114°56′E) was selected for the low-salinity lens case (Figure
The low-salinity lens case.
Sectional distributions of salinity.
Table
Acoustic characters of duct cases.
Case | Case | Case | |
---|---|---|---|
Duct depth | 43 m | 7 m | 6 m |
Maximum grazing angle | 4.65° | 2.73° | 8.35° |
Minimum cutoff frequency | 247 Hz | 2591 Hz | 840 Hz |
Duct depths can be obtained from the
Energy emitted within the maximum grazing angle will be trapped in the duct, whereas steeper rays leave the duct and propagate via deep bottom-reflected paths. The maximum grazing angle is calculated by [
A simplified expression for the maximum grazing angle is
In fact, the duct ceases to trap energy when the acoustic frequency is below the lower bound frequency, called the minimum cutoff frequency. Based on the variable
It is obvious from Table
To illustrate the feature of acoustic transmission in the SSD in the NSSCS, simulations were performed based on the four reprehensive cases. Referring to the bottom of the NSCS [
The acoustic rays were solved by the beam tracing program BELLHOP [
Acoustic ray for different cases.
The number of rays trapped in the duct for different cases was 4, 2, 0, and 8, in full agreement with the calculation results in Table
The mode amplitude was solved by the normal mode program KRAKEN [
Mode amplitudes for Case
The mode amplitude of the lower mode was shown to be more depth-dependent, whereas the energy distribution of the higher mode was relatively uniform with depth. In the SSD transmission, only a fraction of lower modes was effective and their amplitudes were strongly depth-dependent. For example, the first three modes of large amplitudes could not propagate in the surface duct owing to zero amplitude in the surface duct. The fourth mode, which had the largest amplitude in the duct, became one of the dominant modes. Additionally, a remarkable amplitude was generated in the fourth mode when the source was located at 3.4 m, but the amplitude was nearly zero in the remaining depth intervals of the duct. These data indicate that an appropriate source depth can improve the performance of sound transmission in SSD.
The transmission loss (TL) was computed by KRAKEN for each case. To highlight the duct effect, the sound source was located at a depth where the amplitude of the dominant mode was a peak in the upper water, that is, 3.4 m, 3.7 m, 2.9 m, and 9.8 m for the following cases. The TLs for the four cases are shown in Figure
TLs for different cases.
Duct propagation was observed in Cases
For a quantitative description of duct propagation, horizontal TLs were calculated, as shown in Figure
Depth-average TLs for different cases.
Although the SSD is different from the sonar duct in spatiotemporal variability, it is still an excellent waveguide for long-range propagation. For the diluted area in the NSSCS, it is the only effective acoustic duct. Knowing the formation and acoustic characteristics of the SSD can improve the performance of near-surface sonar in both naval and fishing applications.
In situ CTD observations confirmed that surface ducts are almost universal in the NSSCS during autumn, especially in the diluted area. However, various marine phenomena contribute to the complicated relationship between duct formation, salinity, and temperature gradient. For example, the SSD cannot be formed due to a large negative temperature gradient in the offshore upwelling area, although the salinity gradient shows a large positive value. Even if the SSD formed, it is difficult to ascertain the physical mechanisms that govern a waveguide formation. Therefore, an improved
Based on the In the presence of the mixed layer, salinity became a dominant cause of duct formation for the diluted area. In the absence of the mixed layer, the formation of the surface duct was affected by large gradients of temperature and salinity; the relative values between them determined whether the duct formed or not. In the NSSCS, a duct may consist of two different types of ducts at different depths. In general, the upper diluted duct had a larger value of the sound speed gradient than the lower hydrostatic duct. The upper duct was more conducive to long-range propagation. Marine phenomena, such as river plume, cold surge, typhoon, upwelling, and low-salinity lens, can strongly affect the oceanographic parameters in the upper ocean. This observation raised an interesting question as to whether the duct exists. The Duct depth is an important characteristic that affects the performance of sound propagation in the duct. As the thinness of the diluted layer is usually no more than 10 meters, the duct depth of a freshening-induced duct is small. However, large salinity gradients significantly increase the maximum grazing angle and decrease the cutoff frequency, resulting in a more effective duct. For example, as shown in Figure
It should be noted that the results presented in this paper are based on a status of fixed source depth and frequency. Some results present here, such as Figures
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This study is supported by the National Natural Science Foundation of China (41406041), the Natural Science Foundation of Guangdong Province (2014A030310256), and the Project of Enhancing School With Innovation of Guangdong Ocean University (GDOU2016050246). The authors would like to acknowledge the data support from “South China Sea and Adjacent Seas Data Center, National Earth System Science Data Sharing Infrastructure, National Science & Technology Infrastructure of China (