The Central Part Of The South China Sea Deep-water Upper Ocean Currents And Circulation Features Analysis And Simulation

Current and temperature observations from three moored Autonomous Temperature Line Acquisition System (ATLAS) buoys are analyzed to examine the seasonal thermocline variability and tidal currents in the center deep basin of South China Sea (SCS). And HYCOM model is used to simulate the seasonal variation of ocean circulation in SCS, and in combination with T/P altimeter data, simulation results were used to analyze the character and mechanism of seasonal variation of ocean circulation and sea surface height (SSH) in SCS deep water area.In the center deep basin of SCS, thermocline with intensity of 0.1℃/m exits for the whole year. The seasonal variation of the thermocline is mainly controlled by sea surface heat and geophysical vorticity. In spring, the double thermocline structure occurs in all the three buoy location in SCS a result of the back-rise of temperature.During the depth of 50m to 500m, there are two remarkable short-term period oscillations in temperature, which is diurnal and semi-diurnal period respectively, especially the diurnal period. These two period oscillation change both in spatial an time. And this indicates that internal tide exits in the central basin of SCS. The center depth of the thermocline is between 50m and 100m, and on that depth the spatial and time variations of semi-diurnal or diurnal period oscillation are in agreement with those in thermocline. In the central depth of thermocline, the diurnal (semi-diurnal) period oscillation in temperature is more obvious in the time zone where the intensity of thermocline is larger.Spectrum analysis of current data shows that diurnal and semi-diurnal tidal constituents play an important role in the total current, especially the diurnal tidal constituents. Inertial period is more evident in the southern part of SCS deep basin. Wavelet analysis reveals that the main period of current varies with depth, which indicates that the current is mainly baroclinic, as the results of EOF analysis. From north to south, the magnitude of diurnal tidal constituents to the total current decrease while that of semi-diurnal tidal constituents remain similar. The magnitude, direction and contribution of most tidal currents, except the semi-diurnal tidal current in the northern part, vary significantly with water depth. This indicates that the tidal current is mainly baroclinic in the SCS deep basin, the same as the result of Wavelet analysis and EOF analysis. The tidal ellipses whirl round along with depth, also with the length of major and minor axis. It has some relation with the structure of the density stratification. The density stratification causes the baroclinity of current, which may yield the energy transmission and dissipation and affect the generation of the internal tides and mixing of SCS at the upper layer. And it can make a large influence on the ocean circulation. Velocity and direction of the mean current varies with seasons at all three buoy location, especially at ST1 and ST2. In general, seasonal variation of the current direction at the three stations agrees with the basin circulation in SCS.Surface circulation of the SCS deep basin has remarkable seasonal variation. Northern part of the SCS deep basin is mainly dominated by a cyclonic circulation in winter. The Kuroshio intrusion current is stronger than other reasons. And a meso-scale cyclonic gyre exists in the southern part in winter. In summer, SCS deep basin is dominated by an anti-cyclonic gyre, and the northeast current is quit remarkable in the central deep area. In all the southern part of SCS deep basin, the circulation is mainly anti-cyclonic. Current in the western part of central SCS is quite strong because of the western intensification. In spring and autumn, the circulations are relatively weaker due to reversal of the SCS monsoon.Middle layer circulation has a little seasonal variation. But to the west of Bashi Channel, an anti-cyclonic eddy persists regardless of season, which may be caused by the Pacific water intrusion to the east of Bashi Channel. Around 12 N ,113E, a cyclonic eddy exists for the whole year and hardly changes with time in the bottom layer of SCS deep basin. Also the bottom circulation has little seasonal variation. In the Luzon strait section, zonal velocity is mainly westward above 500 m in the middle and southern part of the strait, which means that the water flows into SCS through the strait. And in the northern part, zonal velocity is mainly eastward which means that the water flows out SCS to the Pacific Ocean. Beneath 500 m, zonal velocity is small and mainly westward. This indicates that water flows into SCS in all the middle and bottom layers. In autumn and winter, flow through the Luzon strait above 100 m is much larger than it in spring and summer in surface layers. However, in subsurface and middle layer, flow is hardly changed all the year.Simulated sea surface height anomaly (SSHA) is completely opposite between winter and summer, as well as between spring and autumn. In winter, SSHA in the deep basin reach the lowest value of the year. There are two negative SSHA centers in the western of Luzon around 19 N ,119Eand east to Vietnam near 14 N ,112E, respectively. Oppositely, in summer near the similar area there are two positive SSHA centers in 21 N ,116Eand 14 N ,110E.The simulated SSH and SSHA correspond with the current field exactly. In winter, SSH in the western of deep basin reaches it highest value of a year because of Ekman Transports; Luzon cold eddy and the cyclonic gyre on the southeast of Vietnam coast is corresponded with two negative SSHA centers and a low SSH center. In summer, SSH in the eastern part is higher than that in the western part, and the anti-cyclonic gyre in the southern part of deep basin is corresponded with a positive SSHA center.