Michida, Y.
　　　Structure of the Kuroshio and the surface Ekman layer by current data analysis of shipborne ADCP
　　　Doctoral dissertation (Univ. of Tokyo), 119pp, 1999


　Shipborne ADCP data collected by the Hydrographic Department, Japanese Maritime Safety
Agency, from 1985 to 1995 are analyzed to study the velocity structure of the Kuroshio south
of Japan and the Ekman current in the adjacent region of Japan, by making use of an advantage
of ADCPs which can measure multi-layer current velocities with high density in space and time
along a ship track. A general methodology of current measurement with a shipborne ADCP
including its principle, error sources, and correction for them is firstly described, and it is
followed by discussions on the horizontal structure of current velocity of the Kuroshio and by
the structure of the Ekman current in the surface layer.
　 The horizontal structure of the Kuroshio velocity is investigated by analyzing ADCP velocities
taken repeatedly along two meridional lines across the Kuroshio south of Japan. The profile
of the zonal velocity is different depending on whether the Kuroshio flows close to the coast
(coastal mode) or in the offshore region (offshore mode). The velocity profile for the offshore
mode is symmetric at the Kuroshio main path, while the profile for the coastal mode is
asymmetric with a sharp decrease on the coastal side from the main path. Therefore, the
amplitude of relative vorticity for the offshore mode is almost the same between the coastal
and offshore sides of the Kuroshio main path (〜0.2 x 10-4 sec-1), while that for the coastal
mode is larger by three times on the coastal side than the offshore side. The relative vorticity
on the offshore side of the Kuroshio always takes a similar value (〜0.2 x 10-4 sec-1).
　The Ekman current in the surface layer is investigated by analyzing multi-layer ADCP
velocities on the basis of the Ekman theory. The Ekman depth is estimated by two methods:
one is from the amplitude ratio of multi-layer velocities, and the other is from the angle between
them. Then the vertical eddy viscosity is calculated by using the Ekman depth. The Ekman
depth and the eddy viscosity are estimated at 41 m and 0.09 m2 sec-1 on average. They show
a similar seasonal change, being large in winter and small in summer. The Ekman depth and
the eddy viscosity in the Sea of Japan are larger than those in the Pacific Ocean around Japan.




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