Late spring thermocline and chemoclines in the area off the Rizhao–Lianyungang coast, western South Yellow Sea

Conductivity–temperature–depth (CTD) measurement and continuous current observation with an Acoustic Doppler Current Profiler (ADCP) were synchronously conducted in May, 2016, at five hydrographic stations off the Rizhao–Lianyungang coast, western South Yellow Sea. The intraday evolution of thermocline and chemoclines is analyzed and the relation of the periodical changes in temperature and salinity of the deep water with tidal currents is discussed. Results show that the thermocline and chemoclines of dissolved oxygen (DO) and pH have formed as early as in May. The thickness of thermocline usually varies between 2～4 m. It mostly dwells in the depth range from 4～7 m to 7～10 m. The maximum gradient reaches 0.80 °C/m. DO and pH chemoclines dwell at the depths of 10～14 m, which are deeper than the thermocline. To some degree, the chemoclines of DO and pH occur synchronously at the same depths and have no relations to the thermocline. Above the chemocline DO fluctuates markedly and maintains at a high level or even oversaturated in daytime. Its peak concentrations do not occur in the surface layer (0～2 m) but in the sub-surface layer (2～14 m). Below the chemocline it invariably remains about 4 mg·L⁻¹ and slowly drops downward. The pH chemocline is characterized by vertical rapid jump, including downward positive and negative jumps with a maximum strength of 0.03～0.04 pH units. In the neap tides thermocline is stable with a large gradient, whereas in the spring tides the gradient and sustainability is reduced apparently. These results suggest that the enhancement of tidal currents undermines the sustainability of thermocline. The periodical changes in temperature and salinity of deep waters are consistent with that of the tidal level, indicating the consequence of advection driven by tidal currents.