Deep current measurements at 38°N east of Japan

Abstract

Deep currents east of Japan were investigated using direct current measurements and hydrographic observations. Current data, recorded for 9 months along 38°N, showed a pair of currents, one southward and the other northward, flowing along local isobaths above the western and eastern flanks. respectively, of the Japan Trench. Geostrophic velocities calculated from conductivity‐temperature‐depth (CTD) data showed a similar velocity pattern to the current measurements. The transports referring to the mean current velocities were estimated. Below 2000 m, the deep transports above the trench were southward 5.5 Sv and northward 13.5 Sv, and the boundary was at the deepest point of the trench. The mean current measured above the abyssal plain to the east of the trench was westward at 38°N. Since the bottom water properties above the eastern flank of the trench were colder and more saline than those on the abyssal plain, the westward current does not enter the trench but rather is expected to turn northward just east of the trench. Its northward transport was estimated to be 6.2 Sv, and the total northward transport was estimated to be 19.7 Sv at 38°N. A pair of oppositely flowing deep currents is confirmed in the Japan Trench. While the transports were similar to those of the Izu‐Ogasawara and Kuril‐Kamchatka trenches, whose ends connect with the Japan Trench, the current structure resembles that at the Izu‐Ogasawara Trench rather than that at the Kuril‐Kamchatka Trench.