Seasonal variation of tidal currents and residual currents in Daya Bay

A comprehensive understanding of the circulation structure within a bay and its formation mechanisms is crucial for predicting the transport and diffusion pathways of pollutants discharged into the sea, and holds significant importance for activities such as sewage discharge planning and emergency response to environmental incidents. Based on in situ current observations at five stations in Daya Bay during spring tides, respectively in early summer, late summer, autumn, and winter, this study analyzed the spatial characteristics of tidal currents, residual currents, and their seasonal variations, with preliminary discussions on their formation mechanisms. Results show that the observed currents were generally weak throughout the investigation period, with flood currents stronger than ebb currents, and current velocities remained comparable across all four survey periods. Harmonic analysis of the tides revealed distinct residual circulation patterns: during the winter spring tide observation period, an anticyclonic circulation pattern was observed within the bay. The combination of wind fields and high-frequency radar observations suggests that this pattern is attributed to the intrusion of the winter offshore current into the bay from the western side of the bay mouth. In contrast, under similar conditions in autumn, the residual flow field was relatively weak, likely influenced by short-term fluctuations in the local wind field. During both early and late summer spring tides, the residual currents exhibited cyclonic circulation patterns. However, their underlying formation mechanisms may differ in that: the early summer pattern was primarily driven by the influence of offshore current entering the bay from the eastern side of the bay mouth, while the late summer pattern was mainly associated with the intrusion of coastal upwelling water. This inference was supported by the distribution characteristics of the bottom-averaged water temperature.