This study investigates the physical processes governing subsurface tidal current dynamics in the coastal waters off northeastern Taiwan using a year-long in situ current profile dataset. A 600-kHz Acoustic Doppler Current Profiler was deployed at a water depth of 31 m from 6 June 2023 to 11 May 2024, providing high-resolution measurements with 10-minute temporal resolution and 1-m vertical resolution. These observations enable detailed examination of barotropic and baroclinic tidal current structures under varying stratification conditions. Harmonic analysis indicates that while sea level variability is dominated by the M₂ tidal constituent, tidal current energetics are primarily controlled by the higher-harmonic M4 constituent, reflecting the influence of shallow-water nonlinear processes. This decoupling between sea level and current responses highlights the complexity of nearshore tidal dynamics beyond surface elevation. Seasonal modulation reveals stronger barotropic tidal currents during winter, suggesting enhanced vertical coherence of the flow throughout the water column. Baroclinic tidal currents are predominantly characterized by the first baroclinic mode, with zero-crossing interfaces located at depths of 14–18 m. Under strongly stratified summer conditions, M₂ tidal currents exhibit a transition to a second-baroclinic-mode structure, featuring dual zero-crossing interfaces near 7 m and 22 m. This mode transition implies intensified vertical shear and redistribution of tidal energy across the water column. Empirical orthogonal function analysis further confirms that the dominant modes of current variability are dynamically linked to the vertical structure of stratification-controlled tidal motions. These results demonstrate that subsurface current profiles reveal critical aspects of coastal tidal dynamics that cannot be inferred from sea level alone, providing insight into the processes controlling vertical current structure and energy distributions in stratified coastal environments.
 

coastal ocean,nearshore circulation,shelf dynamics