A surface velocity spiral observed with ADCP and HF radar in the Tsushima Strait

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The structure of a wind‐driven flow in the Tsushima Strait is investigated with moored acoustic Doppler current profiler (ADCP) and HF radar. Two ADCPs of high and low acoustic frequencies are simultaneously used to measure velocities in both the surface boundary layer and the interior with high resolutions. The velocity relative to an interior flow in the surface boundary layer is estimated by subtracting the reference velocity (estimated from velocities at greater depths) from a velocity in the surface layer, and complex principal component analysis (PCA) of the lagged wind stress and the relative velocity is performed. Despite a short (2 weeks) observation period of relatively calm and variable wind, a clockwise velocity spiral similar to a theoretical Ekman spiral is detected as the first mode of PCA. Ekman transport estimated from the relative velocities of the first mode agrees best with Ekman transport expected from wind stress of the first mode with 11–13 hours time lag, for which the explained variance of the first mode is also largest. This indicates that a wind‐driven flow is balanced with wind stress after 11–13 hours, half of the inertial period at this latitude. Eddy viscosity is also inferred from wind stress and the relative velocities of the first mode. It is found to increase from O(10−3) m2 s−1 at greater depth to O(10−2) m2 s−1 near the sea surface.