Cross-Shelf Penetrating Fronts of Buoyant Coastal Currents Around the Headland

Buoyant coastal current induced by riverine outflow often propagates along the coast in the direction of coastal trapped wave. However, it is uncertain how the buoyant coastal current interacts with irregular coastal bathymetry. Measured surface salinity distributions indicate that the buoyant coastal current, which is induced by the Changjiang River plume, frequently penetrates offshore around the curved headlands in the Zhoushan Islands. This signal can also be seen in satellite chlorophyll data, reflecting its importance in cross-shelf exchange both physically and ecologically. To understand the variation and underlying dynamic mechanisms of these penetrating fronts, realistic and idealized numerical experiments were conducted. The results indicated that the penetrated water salinity was about 30–32 psu, with a horizontal penetration scale of tens of kilometers. Curvature and self-organizing map analyses suggested that it was a frequent event with an approximately 2-week spring-neap cycle from generation to decay. It more likely occurred in transition seasons such as spring and autumn, owing to the runoff accumulation and gentle wind conditions. Smoothing the topography would substantially eliminate the penetrating fronts. Idealized model experiments suggested that headland-induced river plume penetration was a type of curved topography-induced current separation. The criterion for this to occur is u/fR_c > 1, in which u is the current velocity, f is the Coriolis parameter, and R_c is the curvature radius. In this case, the headland is too sharp that the Coriolis force could not balance the centrifugal force. Once penetrated, the buoyant water was further transported by the ambient Taiwan Warm Current.