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A Surface Velocity Spiral Observed with ADCP and HF radar in the Tsushima Strait


Y. Yoshikawa, T. Matsuno, A. Masuda, K. Marubayashi, M. Ishibashi, and K. Fukudome
Research Institute for Applied Mechanics, Kyushu University


The structure of a wind-driven flow in the Tsushima Strait is investigated with moored acoustic Doppler current profiler (ADCP) and HF radar. 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 the total velocity in the surface layer, and complex Principal Component Analysis of lagged wind stress and the relative velocity is performed. Despite short (two weeks) observation period of 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 integration of the relative velocities agrees most with Ekman transport expected from wind stress for 11 - 13 hours time lag, for which 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 to find that it increases to the shallower depths.

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