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A distinguishing feature of the morphology of the Pacific basin is the prevalence of subduction trenches. The role of such trenches has received little attention in terms of their impact upon ocean general circulation and climate. We consider observations near the Japan Trench together with theoretical ideas addressing ocean circulation.
In the deep area of Japan Trench, located about 150 km east from northern Honshu, one observes southward flow on the inner slope and the northward flow on the outer slope (Horiuch et al., 1993; Hallock and Teague, 1995). In 1994 and 1995, moorings were deployed by JAMSTEC near the floor of Japan Trench Inner Slope around 40°N. One mooring deployed for 275 days at depth of about 5800 m and another mooring deployed for 20 days at depth of about 4200 m (Mitsuzawa et al., 1995).
Observations showed:
1) Southward flow along the trench axis is observed above the inner slope of Japan Trench; mean speed is 3-7 cm/sec and maximum speed 15-20 cm/sec.
2) Current changes depend mainly on the M2 tide.
3) A video camera equipped in a mooring system shows sediments re-suspended by the current with mean concentrations of sediment 0.59 g/l.
We obtain southward deep current on the inner slope of trench, similar to that reported by Hallock and Teague (1995). Likewise, on the Izu-Ogasawara Trench, located southeast of Honshu as extension from Japan Trench, observed southward flow on the inner slope and northward flow on the outer slope are reported by Hashimoto et al. (1995). These results suggest continuous deep circulation from Izu-Ogasawara to Aleutian Trench. Overall a view emerges describing current shear across the axes of trenches, with oppositely directed flows tending to lie against the "right side". This circulation may persist in the water column above the trenches, interacting with wind-forced circulation of the upper ocean.
by K. Mitsuzawa and G. Holloway (Institute of Ocean Sciences, Canada)
Horiuchi, K., H. Momma, K. Mitsuzawa, and K. Fujioka. 1993. Proc. of Jamstec Deep Sea Res. (in Japanese with abstract in English), 41-48.
Hallock, Z. R. and W. J. Teague. 1995. NRL-SSC Contribution No. JA7332-95-0049.
Mitsuzawa, K., H. Momma, K. Horiuchi, M. Miyamoto, and M. Aoki. 1995. Proc. of Jamstec Deep Sea Symp. (in japanese), 155-156
Mitsuzawa, K., Y. Kaiho, H. Momma, and H. Hotta. 1995. Proc. of Autumn Conference of Japan Oceanogr. Soc. (in Japanese), 214-215.
Hashimoto, T., S. Fujio, and K. Taira. 1995. Proc. of Autumn Conference of Japan Oceanogr. Soc. (in Japanese), 42-43.
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