The sea surface velocity measurements obtained during the period of 2014–2016 using two high-frequency radars (HFR), which were installed on the northern coast of Vietnam, were reprocessed by using a variational approach (EOF/2dVar) to provide an extended dataset. The high temporal resolution of measurements and large spatial coverage of the radar data enabled the assessment of the surface circulation in the data-poor region of the southern Gulf of Tonkin (GoT) and the characterization of its temporal variations in a wide range of scales: from tidal frequency to annual cycle. It was found that tidal motions account for a large percentage (approximately 60%) of surface current variability in the GoT. The temporal variability of the kinetic energy of the surface currents is tightly correlated with wind variations in the monsoon-dominated atmospheric circulation system. Stratification, induced by the discharge of the Red River, also affects the tidal currents within the coastal region (~ 40 km wide) by modifying the tidal ellipses polarizations. Close to the shore, the prominent coastal flow experiences large temporal variations in response to wind and buoyancy forcing. The seasonal variability of the zonal wind component was found to play a significant role in coastal flow speed variations through the cross-shore mass exchange. The Red River freshwater discharge controls the seaward extension of the coastal flow and causes asymmetry in the seasonal variability of flow velocity. The data obtained and analysis results could be useful for the improvement of the regional circulation models, for further studies of the physical and biological processes in the GoT, and for marine resource management.

使用变分方法（EOF / 2dVar）对使用2014年至2016年期间安装在越南北部海岸的两个高频雷达（HFR）获得的海面速度测量值进行重新处理，以提供扩展的数据集。测量的高时间分辨率和雷达数据的大空间覆盖范围使得能够评估东南部Tonkin湾（GoT）数据贫乏地区的地表环流并表征其在各种尺度上的时间变化：从潮汐频率到年周期。已经发现，潮汐运动占GoT中表面电流变化的很大一部分（大约60％）。在季风为主的大气环流系统中，表面电流动能的时间变化与风的变化紧密相关。红河泄洪引起的分层也通过改变潮汐椭圆极化来影响沿海地区（约40 km宽）内的潮流。在靠近海岸的地方，突出的沿海水流随风和浮力的作用而经历较大的时间变化。发现纬向风分量的季节变化在通过跨岸质量交换的沿海流速变化中起重要作用。红河淡水流量控制着沿海水流向海的延伸，并导致流速季节性变化的不对称性。