How islands stir and fertilize the upper ocean

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 (10 f ), 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.”