Abstract Characterizing tidal circulation at a high spatiotemporal resolution is essential for understanding submesoscale transport and mixing in marine protected areas (MPAs). This study presents the first application of high‐frequency Coastal Acoustic Tomography (CAT) to synoptically resolve transient eddy‐like structures at 2–3 km scales in an open‐island coastal environment. A six‐station CAT array deployed off Nanji Island, China, reconstructed depth‐averaged currents every 3 min at ∼1.5 km resolution over a ∼26‐hr observation period. CAT velocities closely matched independent Acoustic Doppler Current Profiler (ADCP) measurements ( R > 0.95; RMSE < 0.09 m s −1 ), confirming the accuracy of the tomographic inversion. The flow was dominated by the M2 constituent, with northwestward flood and southeastward ebb currents superimposed on a persistent northeastward residual flow. During flood–ebb transitions, short‐lived eddies formed and persisted for ∼40 min. Deformation and kinetic energy flux convergence diagnostics suggest that tidal phase‐dependent strain intensification favored eddy formation, producing localized retention zones that favor particle retention and reduce lateral dispersion. Horizontal eddy diffusivities estimated from strain‐based and Lagrangian methods range from 2.56 to 10.54 m 2 s −1 and peak during flood tides. Using an assumed representative horizontal nitrate gradient (∂ C /∂ x = 1.0 × 10 −6 mmol m −4 ), these estimates imply lateral nutrient flux magnitudes of 0.24–0.92 mmol m −2 d −1 , suggesting strong tidal phase modulation of biogeochemical exchange. These findings highlight the roles of tidal phase‐dependent flow variability and transient eddy activity in regulating transport and mixing in island‐shelf systems and show that high‐frequency CAT can resolve physical processes relevant to adaptive MPA management.
Liu et al. (Wed,) studied this question.