Abstract: By using in situ hydrographic measurements from a cruise during September 1~9, 2014 and real-time sea level anomaly (SLA) data provided by archiving validation and interpretaiton of satellite oceanographic data (AVISO), we survey three-dimensional structure of a cold eddy and eastward jet in the western South China Sea. The results indicate that the cold eddy began to appear on August 3 and disappeared on September 29, lasted for about two months. During the in situ observation, the horizontal scale of the cold eddy was about 150 km and its shape was an ellipse with the long axis located along the southwest-to-northeast direction, which was affected by an eastward jet on the southeast side of the cold eddy. And the cold eddy extended downward for more than 200 m with a vertically tilted central axis. Strong surface geostrophic flow (maximum value reaching 1.0 m•s^-1) emerged on the southeastern area of the cold eddy, which was closest to the area of eastward jet. However, during the survey, the zonal range of the eastward jet was between 109°E and 113°E, its maximum geostrophic flow velocity reached 1.2 m•s^-1 and it flowed in a northeast direction. The width of the eastward jet was approximately 100 km and the vertical structure of the eastward jet extended 150 m. The evolution of the SLA and the surface geostrophic currents during August and September showed that the development of the eastward jet and the northern cold eddy may be influenced by basin-scale wind stress in the South China Sea.

Key words: western South China Sea, cold eddy, eastward jet, three-dimensional structure