High-Resolution Observations of Upwelling and Front in Daya Bay, South China Sea

Field observations of coastal regions are important for studying physical and biological features. Observations of high-resolution coastal phenomena were obtained by using a tow-yo instrument and a turbulence profiler at Daya Bay in the South China Sea in October 2015. Details of coastal phenomena, including warm water from a nuclear plant discharge, as well as an upwelling, and front, were obtained. The upwelling, with a width of 2 km, resulted in saltier and more turbid water near the bottom, with low chlorophyll-a and dissolved oxygen contents being transported upward to the surface layer and changing the local water environment. The front, with the lateral salinity variations as large as 0.7 psu across 1 km, was active at the water intersection of the South China Sea and Daya Bay. Such events commonly form during weak stratification periods in autumn. Continuous measurements from VMP-250 profiler over circa 22 h revealed active fronts and an averaged dissipation rate of 8 × 10−8 W/kg and diffusivity of 5.8 × 10−5 m2/s (i.e., one order of magnitude larger than in the open ocean) in the thermocline. The front was accompanied by strong mixing, indicating that it had formed at the intersection of different water masses and played an important role in energy dissipation in Daya Bay, further affecting the distribution of ecological elements.