Daigo Yanagimoto

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.

Current Measurements of the Japan Sea Proper Water and the Intermediate Water by ALACE Floats

The subsurface current of the Japan Sea was observed by two Autonomous Lagrangian Circulation Explorer (ALACE) floats. One float, having a 20-day cycle, was deployed on 29 July 1995 in the eastern Japan Basin and drifted in the northeastern part of the basin until 15 September 2000. The other float, with a 10-day cycle, was deployed on 4 August 1995 in the western Japan Basin and drifted in the western Japan Basin, in the Yamato Basin and around the Yamato Rise until it reached its life limit in mid-May 2000. An anticlockwise circulation in the eastern Japan Basin was observed and it was assumed to be in the upper portion of the Japan Sea Proper Water (UJSPW) or in the intermediate water. The spatial scale of the circulation increased as the depth decreased. A clockwise circulation was observed around the Yamato Rise in the UJSPW. Smaller clockwise and anticlockwise rotations were observed in the western Japan Sea, where a seasonal variation was seen in drift speed with different phase by depth. The correlation coefficient between drift speeds of two floats indicated little coherence among the subsurface circulation between the east and the west of the Japan Basin, or between the north and the south of the subpolar front.

Deep Circulation in the Shikoku Basin Measured with the SOFAR Floats

Abstract Tracking of the SOFAR floats were made from April 1988 to January 1991 by releasing twelve floats at 1000 m, 1500 m, 2000 m, 3000 m and 4000 m in the Shikoku Basin. The receivers were moored in five periods and trajectories were obtained for nine floats. Deep circulations and eddy diffusivity inferred from the trajectories of the floats are described.

Pathway and variability of deep circulation around 40°N in the northwest Pacific Ocean

To clarify the global deep-water circulation in the northwest Pacific, we conducted current observations with seven moorings at 40°N east of Japan from May 2007 to October 2008, together with hydrographic observations. By analyzing the data, while taking into consideration that the deep circulation has a northward component in this region and carries low-silica, high-dissolved-oxygen water, we clarified that the deep circulation flows within the region between 144°30′ and 146°10′E at 40°N on and east of the eastern slope of the Japan Trench with marked variability; the deep circulation flows partly on the eastern slope of the trench and mainly to the east during P1 (10 May–24 November 2007), is confined to the eastern slope of the trench during P2 (25 November 2007–20 May 2008), and flows on and to the immediate east of the eastern slope of the trench during P3 (21 May–15 October 2008). Previous studies have identified two branches of the deep circulation at lower latitudes in the western North Pacific; one flows off the western trenches and the other detours near the Shatsky Rise. It was thus concluded that the eastern branch flows westward at 38°N and then northward to the east of the trench, finally joining the western branch around 40°N during P1 and P3, whereas the eastern branch passes westward south of 38°N, joins the western branch around 38°N, and flows northward on the eastern slope of the trench during P2.