Importance of large-scale coastal circulation on bay-shelf exchange and residence time in a subtropical embayment, the northern South China Sea
Abstract A nested ocean model with a particle tracking module is used to investigate the bay-shelf exchange and the residence time in Daya Bay (DB) in the northern South China Sea in the presence of coastal circulation with a prominent seasonal cycle. The model reasonably reproduces the key features of the seasonal mean circulation in DB and its adjacent coastal waters. The summertime longitudinal circulation is characterized by a two-layer structure, with a surface seaward flow above a landward, slope-climbing flow at the bottom. In winter, however, it is characterized by a vertically well-mixed, cyclonic, lateral circulation, with a landward (seaward) flow on the eastern (western) flank. Therefore, the residence time in the upper layer of DB is much shorter during summer than winter. Numerical experiments were conducted to quantify the relative roles of the large-scale coastal circulation and other processes (e.g., local winds, tides, river and cooling water discharge) on the residence time. The large-scale coastal circulation and the local winds are the main drivers of the upper-layer residence time. The large-scale coastal circulation controlled the seasonal pattern of bay-shelf exchange and the residence time in DB, whereas the local wind effect was comparable with the large-scale coastal circulation and had a greater effect in winter. The local wind reduces residence time by intensifying the upwelling (downwelling) circulation and has substantial effects on the spatial distribution of the residence time within DB. In addition to the large-scale coastal circulation and local wind, the residence time is regulated by tides, river and cooling water discharge. Moreover, during the extreme weather conditions (e.g., tropical storm and typhoon), the residence time is significantly reduced by the intensified bay-shelf exchange. The spatial and temporal variation of residence time in DB is of great significance in the regional coastal management for marine environmental protection.