On The Simulation Of The Long-term Change Of The Physical Environment In East China Sea

The East China Sea (ECS) has a complex physical environment, since it is infected by the ocean, the rivers, as well as the driving elements on its surface boundary.. Meanwhile, for its enormous span of geography, there are complicated mass and energy exchange in different areas. Then, using the long-term numerical simulation is a better way to eliminate the temporal and spacial limitation on its structure, to describe its circulations objectively, and to make sure its authenticity and continuity.In the present paper, The Regional Ocean Modeling System is used to simulate the physical circulations of the ECS for 48 years. The SODA data is used to construct the initial states and boundary conditions. And the data for surface boundary is come from NCEP. Besides, the forcing factors on the open boundary not only include the tide, and also the fresh flux from the Yangtze River and Huanghe River.In the section of Bohai Sea strait, in winter time, the wind forcing has stronger influence in the shallower area, so the water exchange flows in from the middle part and out from the southern and northern part; however, in summer time, the baroclinic effect is obviously stronger than wind forcing, the sea level decreases from the coast to the middle part, consequently, the pressure gradient force makes the water flow out from the north and in from the south. In addition, the topography in the section of the Bohai Strait is rugged, the dynamic process caused by topography restrict the current conformation. The flux of the strait distributes like cosine curve. The outflow forced by wind in winter has a biggest value as 1*104m3/s in January, while, the inflow forced by baroclinic effect in summer has a biggest value as 1.5*104m3/s in August. In spring and autumn the water exchange is weaker, and with the smallest value as 0.1*104 m3/s。The yearly change of the net transport through the Bohai Strait section is consistent with change of salinity at the observation station, that means the water transport through the Bohai Strait is a important factor conducing the yearly change of the salinity in the Bohai Sea.The Kuroshio water has different character with shelf water. With the boundary at isobath of 120 meter from the north of the Taiwan island to the Cheju island, the exchange and mixture of the Kuroshio water and the shelf water is discussed. The water exchange is stronger at the intrusion of the Kuroshio, and decreases along the latitude increasing. The shelf water flows through the isobath into the Kurishio current in the area norther than 28 degree. On seasonal changes, the Kuroshio current intrudes to the shelf having a biggest value in the autumn and winter, and smaller value in spring and summer. For the temperature at shelf is lower compared to the water brought by the Kuroshio in winter, the baroclinic current is strengthened. Meanwhile, the Ekman current forced by wind is also forward to the shelf. The whole structure is opposite in summer. The shelf water intrudes to the outside having a bigger value in spring and summer, and smaller value in winter and autumn, that is opposite with intrusion of the Kuroshio. The seasonal distribution of salty exchange shows that the salty has a higher value in the area 600km norther than the Taiwan island in winter, that can demonstrate that the intrusion from the Kuroshio to the shelf, while, the salty has the lowest value as 33psu at the Cheju island. In summer, with the intrusion of shelf water to the outside, the salty value decreases 1psu in upper 50 meter layers and 0.2psu in lower layers. And the water with higher temperature and lower salinity is mainly in the upper layer. Besides, since the flux of the Yangtze River is biggest in summer, the area with lower salinity increases obviously near the Cheju island.From the EOF of energy, we find that the changes of climate in the late seventies have the biggest influence on the distribution of kinetic energy. The temporal distribution of kinetic energy is 39 months behind it of PDO, while the temporal distribution of the Kuroshio current abnormality is 3 years behind the latter, so we can conclude that it takes around 2 to 3 month that the signal of the Kuroshio current is introduced to the ECS and infects the redistribution of energy in the whole area. With the first EOF mode of potential energy, the temporal distribution of it is 44 month behind the temporal distribution of PDO. That means the changing potential energy more depend on its seasonal changing. The baroclinic potential energy is mainly infected by wind forcing, since the wind directly infects the mixture of the surface layer and forces the redistribution of density of the sea.The seasonal changes of energy show that there is always higher kinetic energy in the outer of the Yangtze River. But when the tidal effects are unconsidered, that subdivision disappears. Then, which can be considered a product of tides. However, the tidal experiment demonstrates that the tidal energy is not efficient reason for that subdivision, since the distribution of tidal energy is not exactly like consistent with it in kinetic energy. In fact, the kinetic energy is a product of movements of circulations, and the tidal current ellipse is the most direct behaviour of the period movements of tidal current. The distribution of kinetic energy is very similar with the sum of the square of long axis and short axis, except in the area of estuary of the Yangtze River. When the topography changes, the kinetic energy and tidal current ellipses change homogeneously and have the same distribution. So the water revolving is the directly reason to induce the higher kinetic energy in the outer of the Yangtze River.The structure of Lagrange current is similar with that of the Euler current. The big difference between them is shown in the area from the center to the south part of the Yellow Sea, since the Euler current flows directly to the shelf of the East Sea, however, the Lagrange current is infected by the current toward north at the estuary of the Yangtze River, both of them flow east and separate at the west of Cheju island, then, one of them turns north and forms a closed circulation with currents from the north, the other flows east-south and through the outside of Yangtze estuary the East Sea. When the tidal current flows along a cape, the water particles are subjected to a strong centrifugal force due to the large curvature of the streamline. The centrifugal force makes the water flow out, and the sea level falls in the vicinity of the cape, then the water on both sides flows towards the cape forming two rings with different rotating senses. The shallow topography makes the tidal current weaker, then the current changes in a tidal period, which result in a net displacement. Thereby, the Lagrange current can represent the character of tidal current better than the Euler current.