Observational Analysis of the Formation Reasons and Evolution Law of Winter Counter-Wind Current in Jiazi Sea Area of Northeastern South China Sea

Based on the observational data of wind, current, and sea surface temperature in the Jiazi sea area of the northeastern South China Sea in 2018 and the satellite remote sensing data of sea surface temperature in the northern South China Sea, this paper explores the formation reasons and evolution law of winter counter-wind currents in the Jiazi sea area of the northeastern South China Sea. The results show that: (1) The counter-wind current in the Jiazi sea occurs only in certain time periods instead of the entire winter; (2) When the eastern component of wind stress weakens, the eastern component of seafloor friction also weakens to some extent. A high-frequency northeast current often occurs in the bottom layer of the sea area, indicating that the formation of winter counter-wind current in the Jiazi sea area is a result of the concerted action of wind stress, the baroclinic effect, and geostrophic effect; (3) When the counter-wind current is formed, there is a low temperature water mass in the northwest of Jiazi and a high-temperature water mass in the southeast. The baroclinic effect causes the sea water to flow to the shore and produce a westward flow on the shore, and northeastward counter-wind current occurs on convergent sea water on the shore due to the baroclinic effect and geostrophic effect (Ekman effect). Therefore, two different current systems are formed in the northeastern South China Sea in winter with 116° E as the boundary. The appearance of cold water masses in the northwest of 116° E sea area and warm water masses in the southeast of the South China Sea is the key to the formation of both the two different current systems with 116° E as the boundary and the winter counter-wind flow; (4) The formation and disappearance of the counter-wind current can be divided into four stages: in the first stage, the northeast monsoon gradually relaxes to become the southeast wind, forming the northwest current; in the second stage, the warm water masses on the west side of the Luzon Strait flow to the coastal waters due to the northwest current, forming a significant onshore pressure gradient force; in the third stage, a high-temperature seawater convergence zone is formed in the Jiazi sea area, forming southwest and northeast pressure gradient forces, and the northwest coastal current forms a counter-wind current under the combined action of pressure gradient force and geostrophic effect; in the fourth stage, the northeast monsoon intensifies and the counter-wind current weakens gradually until it disappears, and the sea water flows to the southwest again.