Seasonal Variations Of Mass Transport Between Bohai Sea And Yellow Sea-A Numerical Modeling Study

Water exchange between bay and open ocean plays a key role in the transport and distribution of nutrients,pollutants and sediments.As the sole corridor between the Bohai Sea and Yellow Sea,water exchange and mass transport in the Bohai Strait control the sediments transport and pollutants diffusion in this area.Based on a three-dimensional hydrodynamic model(ROMS)validated by observation data,the seasonality and dynamic mechanism of water exchange and mass transport in the Bohai Strait are studied by using conservative passive tracers and Lagrangian particle tracking method.In addition,numerical experiments were conducted to evaluate the effects of different dynamic factors on seasonal water exchange and mass transport.The simulation of conservative passive tracers indicates that the water exchange in Bohai Strait shows a pattern of ‘inflow from north and outflow from south' both in summer and winter.Tide-induced residual current correlated significantly with topography and coastline leads to the formation of “inflow from north”.And in winter,this process is further amplified by the Yellow Sea Warm Current intrusion at the north of strait.On the south of Bohai Strait,the Bohai Sea water mass was carried by the Western Yellow Sea Coastal Current into the Yellow Sea along the northern coast of the Shandong Peninsula,bypassing the Chengshantou and then entering the South Yellow Sea.In summer,the mass transport along the Shandong Peninsula presents a stratification that surface tracers transport farther than the bottom,which is caused by the bottom compensation inflow and the vertical uneven flow velocity;as the intense vertical mixing of water,the vertical structure of mass transport is uniform in winter.Over all,water exchange shows a pattern of “strong in winter and weak in summer”: the water exchange rate of the Bohai Sea is 18.4% in winter,16.3% in summer.The results of the Lagrangian particle tracking show a structure: wind-drift current in the upper layer and circulations in the lower layer.Particles released in the surface layer migrate northeast driven by southerly wind in summer,and accumulate in the south of the Bohai Sea,then moving along coast under northwest monsoon in winter.After 3-months' movement carried by the coastal current,the particles present a banding distribution surrounding the Shandong Peninsula,and the capacity and efficiency of particle transportation are higher in winter than that in summer.A set of numerical experiments were conducted to evaluate the impact to water exchange and mass transport between the Bohai Sea and Yellow Sea caused by individual factors,including wind,heat flux,temperature-salinity structure and tide.In winter,water exchange and mass transport are dominated by wind,and the effects of temperaturesalinity structure and heat flux are limited.In summer,mass transport is significantly affected by density circulation caused by baroclinic effect,which can be enhanced by heat flux through increasing the vertical temperature gradient of seawater;but the southerly wind-induced compensation inflow is unfavorable to water exchange.In addition,tide plays an important role in water exchange between the Bohai Sea and Yellow sea.The tide-induced residual current correlated with topography and coastline at the Laotieshan channel is a key factor in the formation of the pattern — ‘inflow from north' in the Bohai Strait.