中文摘要：

利用旋转谱分析、调和分析和低通滤波的方法,对1987年8月至1988年1月南海北部东沙附近陆架坡折带处定点海流观测资料进行分析,研究了该区域定常余流、潮流和低频流的特征。各观测层次定常余流基本为西向流,垂向呈现较强的正压性。潮流以日分潮和半日分潮为主,呈顺时针方向旋转,全日潮流椭圆长轴普遍大于半日潮流,冬季K1分潮振幅在近底层明显增大,海流在中间层存在明显的惯性振荡。从能量角度分析,剩余流占海流总能量比例较大,定常余流能量主要存在于沿岸线方向,而垂直于岸线方向的能量主要由潮流和剩余流构成。低频流存在显著的季节变化,1988年1月呈现明显的顺时针旋转形态。冬季海表面风应力与次表层低频流有较强的相关性。结合OFES（Ocean General Circulation Model for the Earth Simulator）模拟结果,利用动量平衡分析的方法探究了动量方程中各项对低频流的贡献以及1988年1月次表层出现北向流的动力机制。冬季低频流具有较强的地转流特征,垂向分布受海水层化影响;东北风松弛和反气旋涡的联合作用是次表层出现偏北向流动的主要原因。

英文摘要：

Rotary spectral, harmonic and low-pass filter analyses were performed to examine the characteristics of the steady, tidal and subtidal currents over the shelf break in the northern South China Sea （NSCS） based on the observed current data derived from a mooring current meter from August 1987 to January 1988. Steady currents were mainly westward at each layer, showing a barotropic feature in the vertical. Observed tidal currents were dominated by diurnal and semi-diurnal tides with a clockwise rotation. Major axes of the diurnal tidal ellipses were longer than those of the semi-diurnal ones. Amplitude of K1 constituent obviously increased near the bottom in winter. There was a prominent inertial oscillation in the intermediate layer. Energy of the residual current was larger than other components in both along-and cross-shore directions. Energy of the steady current mainly existed in the along-shore direction, while energy in the cross-shore direction was controlled by the tidal current and residual current. Subtidal current featured a remarkable seasonal variation with a clockwise rotation at 50 and 100m layers in January 1988. A high correlation was achieved between the sea surface wind stress and subtidal current velocity at the subsurface layer in winter. Combining with the model-computed results from OFES （Ocean General Circulation Model for the Earth Simulator）, the momentum budget was analyzed to investigate the contributions of the momentum balance terms to the subtidal current over the shelf break and the dynamical mechanism of the northward flow at the subsurface layer in January 1988. The results suggest that the subtidal current was prominently geostrophic in winter, and the northward subtidal flow was driven by both the northeasterly wind relaxation and the anticyclonic eddy passing the station in January 1988.