The Investigation Of The Effects Of Wind And Density Gradient On Cross-Isobath Water Exchange In The Bohai Sea

Water exchange plays an important role in marine self-purification,and impacts the variations of water properties,pollutant diffusion and biological migration.However,due to the frequent influence of human activities,the semi-closed inland Bohai Sea is under great environmental pressure,threatening the sustainable economic development of its surrounding area.Therefore,it is of great significance to study the water exchange process and its driving mechanisms for marine ecological protection and economic development.In this thesis,based on a three-dimensional unstructuredgrid Finite Volume Community Ocean Model（FVCOM）,the characteristics of water exchange processes in the nearshore and offshore Bohai Sea were investigated by crossisobath volume transport,as well as the driving mechanisms of wind and baroclinic processes.In addition,there are many contaminants entering the Bohai Sea through river discharge every year.Based on FVCOM,a Lagrangian particle tracking model was further applied to investigate the riverine particles transport,which could provide reference for land-based pollutant control in nearshore Bohai Sea.The main conclusions are as follows.（1）The cross-isobath water exchange in nearshore and offshore Bohai Sea was strongest during summer and weakest during fall,showing surface offshore and bottom onshore transport.The weakest water exchange across 10-m isobath(E₁₀)in nearshore Bohai Sea occurred in the inner part of each bay.The E₁₀ was strongest in Laizhou Bay and weakest in Bohai Bay,while the strongest cross-20-m-isobath water exchange(E₂₀)in offshore Bohai Sea occurred in Bohai Bay and the weakest one was found in Laizhou Bay.Additionally,the most significant interannual variations of cross-isobath water exchange occurred in Laizhou Bay,while those in Bohai Bay and Liaodong Bay were nonnegligible.（2）Wind forcing could either promote or prohibit the upper-layer E₁₀ in the Bohai Sea via the wind-induced advection,and the volume transport driven by wind-induced advection could be offset by wind-induced mixing.During the stratified period,the wind-induced mixing inhibited the robust offshore E₂₀ in the depth of 5-10 m through the weakening of baroclinic pressure gradients.Although wind forcing was stronger during fall,the wind driven flows adjacent to the 20-m isobath were generally alongshore,resulting in limited impacts of wind on E₂₀ in the Bohai Sea.During spring and summer,baroclinic processes induced by the horizontal thermal gradient enhanced cross-isobath volume transports in Bohai and Liaodong Bays,and the E₁₀ in Laizhou Bay were accelerated by the density driven transport induced by the horizontal salinity gradient.The four-layer vertical structure of Laizhou Bay’s E₂₀ was generated by the collective effects of wind and density gradient during summer.As the volume transports induced by thermal and salinity gradients offset each other,baroclinic processes had little impact on E₁₀ and E₂₀ during fall.（3）The longest transport distance and the largest influencing areas of riverine particles in the Bohai Sea appeared during summer,especially at the surface.The influencing areas of particles from Yellow River,Haihe,Luanhe and Liaohe were concentrated in the western part of Laizhou Bay and north Yellow River estuary,the northwest of Bohai Bay,the west coastal Bohai Sea,and the north of Liaodong Bay,respectively.The transport of particles from rivers in the Bohai Sea was closely related to wind direction.Rivers enlarged the influencing areas of riverine particles.Under the conditions without tide forcing,the transport of particles from rivers presented significant monthly variations.