Low-salinity-induced surface sound channel in the western sea of Jeju Island during summer

Abstract

Surface salinity in the western sea of Jeju Island in Korea becomes low due to the inflow of the Chinese coastal waters during summer. One of the characteristics of low salinity water is the formation of a surface sound channel (SSC) due to the decrease in sound speed by salinity. However, a quantitative analysis between low salinity water and SSC has not been fully investigated yet. In this paper, a temperature-salinity (T-S) gradient diagram is introduced in order to assess SSC formation and its acoustic characteristics are also investigated through a case study of low salinity waters. Maximum angles of limiting rays were less than 4.6° and low frequency cutoffs were higher than 2.0 kHz for the SSCs formed in low salinity water. When the salinity gradients were large (>0.5 psu/m), a SSC was formed more efficiently than other cases whose salinity gradients were small. On the other hand, a SSC was not formed in spite of highly positive salinity gradients when the amount of temperature gradients was negatively high enough (<-0.5 °C/m). However, the acoustic energy transfer in the surface ducts was dependent on frequency and position of source.

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

“…Compared with previous surveys [14], the rate of no haline channel formation in diluted water in the NSSCS (1.6%) was far less than the rate in East China Sea (57.1%).…”

“…Among all of the 108 measurements, the duct formed 104 times (96.2%). The probability of waveguides formation was higher than 31.1%, which was observed in the East China Sea [14]. In this case, the isothermal layer was shown to be more stable in lower sea latitudes.…”

“…The straight-line equation of above becomes a threshold line on a two-dimensional plane of and . Thegradient method judges whether the SSD is formed or not by comparing the measurement gradients marked by a point on the plane of and [14]. As shown in Figure 3, the threshold line divided the plane into two areas: the colored areas above the threshold line and the colorless areas under the threshold line.…”

“…It was then pointed out by Kim et al that the mechanism of the haline duct formation is comprehensive for the middle ocean latitude; even in a very strong salinity gradient environment, the haline duct is not formed [13]. A temperature-salinity gradient diagram (diagram) method was developed to analyze the formation of the SSD [14]. This approach combines various factors to determine whether an SSD exists.…”