Distribution and cross-front transport of suspended particulate matter over the inner shelf of the East China Sea

An obvious shear front between the Taiwan Warm Current and the Zhe-Min (Zhejiang-Fujian) Coastal Current exists over the inner shelf of the East China Sea (ECS) in winter. Although the cross-front spread of surface water in the ECS has been reported based on satellite images, the cross-front transport of suspended particulate matter (SPM) has not been well studied. To reveal the detailed characteristics of SPM in vertical direction, in terms of different fractions, and to study its cross-front transport and mechanism, high-vertical resolution profiles of SPM concentrations observed by Laser In-Situ Scattering and Transmissometry (LISST) and derived from water samples at various levels, together with salinity and temperature, were measured in February 2007, from 34 stations in the ECS. Results showed that volume concentration (VC) of SPM finer than 280.07 µm has a good correlation with mass concentration (MC), because coarser factions were thought to be biological components. The VC was then converted to the MC of different fractions at high resolution, by reducing the influence of biological sources. Both temperature and salinity data showed an obvious front along 50 m-isobath in the bottom layer, tilting seaward toward the surface layer. Within 50 m-isobath, the MC was higher than 2 mg/L in the surface layer and 10 mg/L in the bottom layer. The SPM could be transported across the front to the sea in the bottom layer, even to the east of 100 m-isobath, where the MC of SPM was less than 1 mg/L in the surface layer and 2 mg/L in the bottom layer. The SPM across the front was mainly silt fraction with some fraction of clay. An idealized numerical model was established. Numerical experiment results showed that bottom Ekman transport produced by the northerly wind in winter is the key factor controlling cross-front transport, and strong northerly wind is responsible for its expansion. This study helps our understanding of material exchange in the coastal and shelf seas.