Deep circulation in the northwest corner of the Pacific Ocean

We deployed for two years a line of nine current-meter moorings bearing ins truments at depths of 2, 3. and 4 km running southeast of Hokkaido, to meas ure currents above the continental slope, Kuril Trench, and Hokkaido Rise. The mean flow was directed southwestward above the continental slope, north eastward above the trench and upper rise (except at one mooring), and westw ard onto the lower rise. The mean currents were highly barotropic, except a bove the continental slope, and unexpectedly swift (8 cm s(-1) in the trenc h). The velocity pattern above the Hokkaido Rise is like that observed earl ier above the Aleutian Rise at Long. 175 degreesW, and may be due, as sugge sted for the latter, to varying topographic beta associated with the curvat ure of the bottom profile of the rise. Thermal-wind fields from three CTD s ections along the mooring line, while consistent among themselves, were unl ike the observed mean shear, and therefore useless for estimating mean tran sports. Estimates based on the direct current measurements alone, for depth s greater than 2000 m, are 4 x 10(6) m(3) s(-1) southwestward above the con tinental slope and 20 x 10(6) m(3) s(-1) northeastward in the trench; but t he former might be too small, the latter too large, by as much as 10 x 10(6 ) m(3) s(-1) because of the relatively broad mooring spacing. These measure ments, in combination with many others reported earlier, unequivocally desc ribe swift deep southward flow along the inshore sides of the Izu-Ogasawara , Japan, and Kuril Trenches, and opposed flow along their offshore sides, a s well as above their axes (except in the Izu-Ogasawara Trench). The southw ard Row may be, at least in part, the recirculation western-boundary curren t predicted for the northern North Pacific. although the oceanic geometry i s different from, and more complicated than, that of the classic analytical predictive models. Reasons for the strong opposed flow are obscure. Water properties reveal that deep water spreads into the Izu-Ogasawara Trench, an d probably into the Japan Trench as well, flows northward through the Kuril Trench, and, at least at some levels, around the northern end of the Emper or Seamount Chain to fill the long Aleutian Trench. This plus eastward flow s through the Main Gap in the Emperor Seamount Chain and the passage betwee n the Hess Rise and the Hawaiian Ridge seem to be the principal deep outflo ws from the Northwest Pacific Basin into the Northeast Pacific Basin. The M eiji Sediment Drift, lying along the eastern flank of the far northern Empe ror Seamount Chain, is composed of material from the Bering Sea. To account for its deposition, we conjecture that the deep Kamchatka Current, present ly carrying this material southward, splits at the latitude of the northern most Emperors, one branch flowing eastward as a zonal jet, and continuing s outhward along the eastern flank of the Seamount Chain as a deep western-bo undary current. Descriptive ambiguities and dynamical puzzles are considere d. (C) 2001 Elsevier Science Ltd. All rights reserved.