The Upper-layer Currents Near The Continental Slope In The Northeastern South China Sea Basin

Detailed research is carried out on the vertical distribution and temporal variation of near-inertial oscillation and the seasonal evolution of the residual current especially the influence of vortex based on observation data in the northeastern South ChinaSea(SCS)for about 8 months,combined with some auxiliary data.The NIE at site A has the following several features:The NIE is mainly concentrated in the subsurface layer;the wind speed is not necessarily proportional to the near inertial energy excited in the subsurface ocean;the NIE of 475-600m in autumn and winter is stronger than that in summer;the NIE are affected by mesoscale warm eddy and background circulation.Moreover,the first energy peak of near inertial oscillation is probably triggered by the suddern wind increase on 28th Aug,which is trapped into the vortex and then brought back to the observation site.The frequency is "blue-shifted" by 5.9%when the background circulation of the fifth observation period is southwestward.The power spectrum analysis of residual current indicates the existence of many high-frequency components energy peaks associated with diurnal and semi-diurnal tides and 4d and 7d components.During November and December when the northeast monsoon is strongest,the steady residual current is weakest caused by a dipole"vortex pair" and the influence of anticyclonic vortex on residual current is earlier in deep layer.In the middle and late August and September,a current which is different from the SCS warm current flows eastward in the depth of about 500m to 675m or even deeper.The mesoscale eddy can significantly affect the residual current parallel to the isobath,but have little effect on the perpendicular component.