The effect of a cyclonic eddy on the distribution of lithogenic particles in the southern East China Sea

Off northeastern Taiwan, the collision between the Kuroshio Current (KC) and the East China Sea (ECS) shelf forms a branch current (Kuroshio Branch Current) on the shelf, and the counterclockwise deflection of the subsurface Kuroshio water annually generates a cyclonic eddy, about 70 km in diameter, over the shelf-slope in summer. In this study, the total suspended matter collected from varying water depths over the eddy during August 1994 has been analyzed for Al, which is used as an indicator element to trace lithogenic particle transport. Simultaneously, the measurements of current field, using the Shipboard Acoustic Doppler Current Profiler, and hydrography have also been conducted on the same cruise during the seawater sampling. These offer an opportunity for direct examination of the relationships between circulation patterns and distributions of lithogenic particles to further establish a transport model for lithogenic particles. However, we must emphasize that this is a representative case for the export of terrigenous suspensions delivered from rivers, although the observation was made only in summer. The spatial distribution of particulate Al (PAl) in the upper water displays an Ω shape with high concentrations of PAl close to the Taiwan coast and around the eddy center. It suggests that Taiwan-derived riverborne particulates are the main source for the lithogenic particles in the upper water of the study area. Moreover, the PAl in the upper water is essentially constrained by the cyclonic eddy, and the entrainment and subsequent accumulation of lithogenic particles occur at the eddy center. The 6 µg/l contour of PAl in the upper water almost matches the western boundary of the main stream and branch of the KC across the ECS shelf-slope, and two concentration provinces of PAl can be identified from this contour: one to the west is larger and variable and the other to the east is smaller and uniform. This indicates that the main stream and branch of the KC may confine terrigenous particles to the inshore area where mixing of the lithogenic particle-rich coastal water and the clearer Kuroshio water occurs and may act as a barrier to the offshore transport of terrigenous particles to regions farther away from the coast, toward the north and east. In this study, a conceptual model for the transport of lithogenic particles in the water of the southern ECS is proposed according to the spatial distributions of PAl. This model suggests that the cyclonic eddy at the turning site of the KC may be the important conduit for the indirect transport of terrigenous particles from the ECS shelf and slope to the southern Okinawa Trough or the Pacific Ocean. This is also a valuable example of the role of the eddy system in regulating the distribution and transport of terrigenous particulates