Physical mechanisms for the offshore detachment of the Changjiang Diluted Water in the East China Sea

Abstract

Physical mechanisms for the summertime offshore detachment of the Changjiang Diluted Water (CDW) into the East China Sea are examined using the high-resolution, unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM). The model results suggest that isolated low salinity water lens detected west of Cheju Island can be formed by (1) a large-scale adjustment of the flow field to the Changjiang discharge and (2) the detachment of anticyclonic eddies as a result of baroclinic instability of the CDW front. Adding the Changjiang discharge intensifies the clockwise vorticity of the subsurface current (originating from the Taiwan Warm Current) flowing along the 50-m isobath and thus drives the low-salinity water in the northern coastal area of the Changjiang mouth offshore over a submerged plateau that extends toward Cheju Island. Given a model horizontal resolution of less than 1.0 km, the CDW front becomes baroclinically unstable and forms a chain of anticyclonic and cyclonic eddies. The offshore detachment of anticyclonic eddies can carry the CDW offshore. This process is enhanced under northward winds as a result of the spatially nonuniform interaction of wind-induced Ekman flow and eddy-generated frontal density currents. Characteristics of the model-predicted eddy field are consistent with previous theoretical studies of baroclinic instability of buoyancy-driven coastal density currents and existing satellite imagery. The plume stability is controlled by the horizontal Ekman number. In the Changjiang, this number is much smaller than the criterion suggested by a theoretical analysis.