Modulation Of Shelf Circulation Under River Discharges And Tides In The East China Sea

Shelf circulation is a major driving force for material transport on the shelf,which has great influences on ocean dynamic,ecological,and sedimentary processes.The shelf circulation is affected by many factors,such as wind,tide,sea surface temperature,salinity,topography,sea surface height,cross-shelf exchange and ambient ocean circulation,which determine the direction and strength of the shelf circulation.The East China Sea is one of the seas most affected by river discharges in the world.The river plumes are strongly affected by the active shelf circulation system.Meanwhile,these rivers also regulate the density structure and dynamic height of the ocean,thus affecting shelf circulation.Previous studies did not pay much attention to the effect of fresh water on the circulation,because the river discharge is much smaller than the circulation in magnitude.On the other hand,the East China Sea shelf is one of the widest shelves in the world,and the tide here is also very strong.Tides have a profound influence on the mixing and transport processes,not only regulating the salinity field but also changing the bottom friction and tidal residual current.Under the strong tidal,the various forms of river plume and shelf circulation and the impact of their interaction are still unclear.Based on the Regional Ocean Model System(ROMS),we developed a numerical model covering the East Asia Marginal Seas to evaluate the effects of river freshwater and tides on shelf circulation in the East China Sea,and the corresponding dynamic mechanism was analyzed.The model considered the effects of many factors such as river discharge,tide,wind,and shelf circulation.Validations against measured data indicated the model is reliable.In this paper,climatological experiments were conducted to compare the dynamics with and without rivers.It was found that both local rivers(such as the Changjiang River)and external rivers(such as the Pearl River)had important regulating effects on shelf circulation.The river freshwater raised the adjacent sea level,changed the density field,weakened the Taiwan Warm Current(0.1m/s)and strengthened the Zhe-Min Coastal Current(0.1m/s).In the vertical,affected by the river water,the core of the Taiwan Warm Current shifted from the surface layer to the bottom.The reason is that there are opposite density gradients on the inner shelf and the middle shelf.After the thermal wind balance was readjusted,the Taiwan Warm Current became the maximum at the bottom.In the cross-shelf direction,the subsurface water of Kuroshio was easier to intrude the coastal area because of the enhanced bottom Ekman transport and cross-shelf exchange,with an increase of 20% within 200 m isobath and 5% within 100 m.Its mechanism was baroclinic and baroclinic regulation under the influence of river fresh water.The fresh water from the Changjiang River also caused the southward coastal current to increase in autumn and winter,and the coastal sea level to rise.The enhancement of the Zhe-Min Coastal Current by the Changjiang River fresh water in winter can be described by the Knudsen relation.Through calculation,the Zhe-Min Coastal Current has increased about 10 times the volume transport of the Changjiang River runoff.This paper compared the climatological experiments with and without tides to study the impact of tides on shelf current.The results show that tides can adjust the circulation in the East China Sea.It slightly reduced the velocity of the Taiwan Warm Current(～0.1m/s),and slightly increased the velocity of the Kuroshio(0.1-0.5m/s).In addition,tides influenced the distribution of the Changjiang diluted water,making the front of the Zhe-Min Coastal Current broaden and the current axis move seaward.After analyzing the results of horizontal momentum,it was found that major momentum terms affected by tide were the pressure gradient force term,the Coriolis force term,and the horizontal advection term.Tidal rectification resulted in significant horizontal advection term near the coast,resulting in a new momentum balance between pressure gradient force and Coriolis force.The influence of tides on the circulation can be divided into direct influence(i.e.,by adjusting the tidal induced residual current and bottom friction)and indirect influence(i.e.,by adjusting the density structure)of the coupling of tides and fresh water.In order to further determine the impact of tides and river fresh water,several numerical experiments with and without tides and rivers were set up,and the two effects were calculated using the experimental results.By analyzing the direct influence of tides,it was found that tides changed the sea level gradient of the East China Sea,and had a certain impact on the Kuroshio,the Taiwan Warm Current and the Zhe-Min Coastal Current.On a large scale,tidal changed the sea level slope of the East China Sea,resulting in geostrophic current changes in the East China Sea,especially the Kuroshio.At the same time,the combined influence of tide and density structure leads to the change of the mixing of the front in the horizontal and vertical directions,thus changing the form of the geostrophic current.The differential mixing and the dispersion of the surface and bottom layers affected by the tide have obvious seasonal changes.In spring and summer,the mixing of the surface layer is strong,while the bottom layer is relatively weak.By comparing the results of the difference in mixing and dispersion between the Zhe-Min coastal water with and without the influence of tides,it was found that the vertical differences between the terms formed a zonal distribution and had significant seasonal fluctuations.These zonal differences were distributed in the middle layer in autumn and winter,and in the surface layer in spring and summer.After analyzing these differences,it was found that the tide readjusts the vertical mixing on the middle-outer shelf and the horizontal mixing on the inner shelf.