The diel vertical migration of the sound-scattering layer in the Yellow Sea Bottom Cold Water of the southeastern Yellow sea: focus on its relationship with a temperature structure

Abstract

Using the hydroacoustic method with a 200 kHz scientific echo sounding system, the diel vertical migration (DVM) of the sound-scattering layer (SSL) in the Yellow Sea Bottom Cold Water (YSBCW) of the southeastern Yellow Sea was studied in April (spring) and August (summer) of 2010 and 2011. For each survey, 13–27 hours of acoustic data were continuously collected at a stationary station. The acoustic volume scattering strength (Sv) data were analyzed with temperature profile data. In the spring of both 2010 and 2011, the SSL clearly showed the vertical migration throughout the entire water column, moving from the surface layer at night to near the bottom during the day. Conductivity, temperature, and depth data indicated that the entire water column was well mixed with low temperature of about 8°C. However, the SSL showed different patterns in the summers of 2010 and 2011. In the summer of 2010 (≈28°C at the surface), the SSL migrated to near the bottom during the day, but there were two SSLs above and below the thermocline at depth of 10–30 m at night. In the summer of 2011 (≈20°C at the surface), the SSL extended throughout the entire water column at night, possibly owing to an abrupt change in sea weather conditions caused by the passage of a Typhoon Muifa over the study area. It was concluded that the DVM patterns in summer in the YSBCW area may be greatly influenced by a strengthened or weakened thermocline.