Northern South China Sea Circulation Observational Studies

Based on the shipboard ADCP (Acoustic Doppler Current Profiler) data collected in thenorthern of South China Sea (SCS), the characteristics of tidal and residual current, the physicalmechanisms for the residual current in the Qiongzhou Strait (QS), the characteristics of SouthChina Sea warm current, western boundary current and mesoscale eddy and temperature andsalinity are analyzed.In spring2013, thirty-three repeat shipboard Acoustic Doppler Current Profile (ADCP)surveys were conducted to measure the tidal current in the Qiongzhou Strait (QS). The majortidal currents and residual current along a section across the QS were estimated for the first timeusing a modified tidal harmonic analysis method based on the inverse technique. A simplesimulation and comparisons with previous observations demonstrated that the tidal currentsestimated using the modified tidal harmonic analysis method in this study are reasonable, and themodified tidal harmonic analysis method was able to control the magnitude and deviation of theestimation error. The direction of the major axis of tidal current ellipses is generally along thestrait. The diurnal tidal constituents are dominant among the five tidal current constituents (K1,O1, M2, S2and MSF). The ratio of the amplitudes of O1, K1, M2, S2and MSF averaged along thesection across the QS is1:0.74:0.34:0.51:0.52. The residual current along the entire section is allwestward, and the averaged velocity over the section and the associated volume transportthrough the section is19.3±2.4cm s-1and-0.182±0.0220Sv (Sv=106×m3s-1), respectively, inwhich the second value denotes the uncertainty of first value. Dynamic analysis indicates thattidal current activity is more dominant than mean current and eddy activity, and tidal rectificationand sea level difference between two entrances of the QS are important in maintaining theresidual current through the strait.The observation results in the northern of South China Sea show that the western boundarycurrent with a width of140km and a vertical range of210m above the continental shelf. Thevariation ranges of maximum velocity (volume transport) were26.9-67.0cms-1(0.74-9.70Sv,1Sv=106m3s-1) and20.3-61.0cms-1(0.45-13.29Sv) under the northeast and southwest monsoon,respectively. On the other hand, the warm current with a horizontal and vertical ranges of45kmand700m near the continental slope. The variation ranges of maximum velocity (volumetransport) were6.1-29.0cms-1(0.14-2.44Sv) and12.3-31.2cms-1(1.10-5.68Sv) under thenortheast and southwest monsoon, respectively. The warm current flowed northeastward alongthe500-1000m isobath roughly, but flowed southward deflection near114°E sometimes.The shipboard ADCP survey successful captured the process of mutual disturbance betweenthe SCS western boundary currents and mesoscale eddy, and obtained the vertical structurescompletely. The anticyclonic eddy has a radius of about180km and vertical depth of about1000 m under the northeast monsoon, but the cyclonic eddy has a radius of about135km and verticaldepth of about780m under the southwest monsoon, the strength less than that of under thenortheast monsoon slightly. Furthermore, some changes in the velocity structure of the warmcurrent and western boundary of the same section were confirmed under the northeast monsoon.Moreover, the warm current was confirmed in the region east of Vietnam Peninsula near15°N,110°E, which is also the most west location comparing with all the previous studies. We canobtain valuable knowledge for South China Sea warm current, western boundary current andmesoscale eddy through this study.