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Ocean currents in the southern East China Sea (ECS) are complex and have long lacked systematic observational data. Observations from coastal radars in northern Taiwan, along with several drifters and satellite data, reveal a detailed ECS surface flow structure and thermal front. Drifter trajectories followed the seasonal flow field and were oscillatory and trapped due to strong tidal currents. Three drifters crossed the oceanic front off the northern coast of Taiwan following the tidal motion with a rapid change in the sea surface temperature by 2.5°C, which was recorded within a small distance of 500 m. A significant seasonal water mass exchange cycle occurred in the southern ECS. Starting in October, the Kuroshio Current intrudes onto the ECS shelf, causing the formation of a southwestward Northern Taiwan Coastal Current that mixes with the China Coastal Current to form a southward flow in the northern Taiwan Strait. Beginning in March, the ocean current off northwestern Taiwan shifts northward and then gradually flows eastward into the Kuroshio region. With satellite‐derived and model‐simulated surface currents contradicting each other over long periods of time, hourly coastal radar data have not only successfully explained the spatiotemporal variations in the sea surface temperature, salinity, and chlorophyll concentration observed by satellites but have also resolved the long‐standing disputes over the surface currents in the southern ECS. Plain Language Summary: In the southern East China Sea (ECS), the ocean surface currents observed by satellite altimeters and estimated based on numerical models have considerably differed for a long time, especially in winter. The data observed by shore‐based high‐frequency radar have yielded new discoveries. Beginning in October, the Kuroshio Current intrudes onto the ECS shelf and pushes Kuroshio waters with high temperatures, high salinities, and low chlorophyll concentrations to the west, causing the current from northern Taiwan to flow southwestward into the Taiwan Strait (TS). Then, starting in March, ocean currents off northwestern Taiwan turn northward, and the water off northern Taiwan is pushed eastward back to the Kuroshio region. These flow field variations cause a significant oceanic front in the 121°E−122°E area in the southern ECS. We deployed four surface velocity program (SVP) drifters to observe the ocean flow and used virtual drifters to simulate the distribution probability of the track in each month. The drifters basically followed the movement direction of the seasonal flow field but could have been affected by the significant tidal currents and become trapped. With this study, we aim to better understand the southern ECS environment from a geophysical perspective. Key Points: Coastal radar data and long‐term multisatellite data are used to observe the formation of oceanic fronts driven by ocean currentsThe Kuroshio Current's intrusion of the shelf causes the Northern Taiwan Coastal Current to flow southward along western Taiwan in winterVirtual drifter experiments present the possible trajectories of and seasonal changes in near‐surface ocean currents [ABSTRACT FROM AUTHOR]

Using five mooring array observations in the coastal water of the East China Sea (ECS) in winter 2006, the authors identify three kinds of low-frequency waves using the ensemble empirical mode decomposition (EEMD) method. The analysis indicates that the periods of the waves varied from 2 to 10 days, which are consistent with coastal-trapped wave (CTW) modes: the Kelvin wave (KW) mode, the first shelf wave (SW1) mode, and the second shelf wave (SW2) mode. An analytical model is established and the dispersion relation of the waves from the analytical method agrees well with the observations. The wind-forced, coastal-trapped wave theory is then applied. The calculation shows that over a wide shelf, the forcing term of wind stress curl plays an important role in shaping the CTW. Numerical solutions reproduce the sea level variation and the alongshore current. The results show that the sea level variation mainly resulted from the KW mode, but the alongshore current resulted from both the KW and SW1 modes. [ABSTRACT FROM AUTHOR]