Deep current structure above the Izu-Ogasawara Trench

Above the Izu-Ogasawara Trench south of Japan, direct current measurements were made at 34°N from 1987 to 1996, and hydrographic observations were carried out at 34° and 30°N in 1995. The geostrophic shears calculated from the conductivity-temperature-depth data were consistent with the shears calculated from the current data. It is found that there are opposing currents that flow southward on the western flank and northward on the eastern flank along the isobaths. In the cross-trench direction the magnitude of the mean velocity tended to increase with the distance from the deepest point of the trench and exceeded 10 cm s⁻¹ on the eastern flank. In the vertical direction the mean velocity increased with depth, but it decreased just above the bottom, probably because of friction. The southward transport above the western flank was estimated to be 5–8 Sv both at 34° and 30°N. However, the northward transport above the eastern flank increased from 5 Sv at 30°N to 22 Sv at 34°N, suggesting a large inflow from the west.