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[1] Large differences between the upstream and lee side flow characteristics of an isolated island in the Kuroshio have been identified from a three-dimensional velocity survey and from vertical profiles of fine- and micro-structure. In the island wake, the relative vorticity is O(10f), the horizontal current divergence indicates upwelling of O(0.01 m s−1), and the rate of dissipation of kinetic energy is O(10−4 W kg−1). Isopycnal surfaces shoal by 60 m on the lee side and surface nitrate concentration increases seven-fold. Flow blockage by the island and the Izu-Ogasawara Ridge on its flanks, induces horizontal and vertical flow separation. The associated lateral and vertical shear drive the upwelling and the vertical mixing in the wake and produces a very pronounced “island mass effect.”