Ichiro YASUDA

Hokkaido University, Japan

WOCE-P2 one-time observation was carried out with R/V Kaiyo Maru along 30°N from Japan to the U.S coasts in January-February 1994. Water-masses appeared in the section and ventilations north of 30°N are discussed with the CTDO data combined with the climatological World Ocean Atlas dataset. In the upper part of the permanent thermocline (26-26.6 ) along the section, large zonal changes of dissolved oxygen (oxygen fronts) were observed at 154°E, 180°, 160°W and 135°W, corresponding to an intense mesoscale eddy, the eastern limit of the subtropical mode water of 17-19°C, the western boundary of surface high-salinity water (subtropical front) and the appearance of the shallow salinity minimum, respectively. From 154°E to 160°W, oxygen showed vertical maximum, and horizontal maximum was seen from 180° to 160°W, indicating newly ventilated water. This water-mass corresponds to the North Pacific Central Mode Water which is formed along the northern edge of the subtropical gyre and indicated by small potential vorticity. For increasing density from 26.0 to 26.7, isopycnally minimum potential vorticity, implying formation area, shifts westward. In the lower part of the thermocline (26.7-27 ) where main salinity minimum (North Pacific Intermediate Water) is distributed, two low-salinity cores with S<34 were observed: one was in 150-154°E and the other in 160-130°W. The western core had higher oxygen, indicating newer NPIW. Isopycnally minimum potential vorticity and salinity and maximum oxygen are found in the Okhotsk Sea, suggesting that ventilated water-mass from the Okhotsk Sea is transported southward along the east coast of Japan and forms NPIW along the Kuroshio Extension and in the Mixed Water Region. The western core probably newly formed NPIW across the Kuroshio Extension; while the eastern core is relatively old NPIW which is transported along the clockwise subtropical gyre circulation. For 27.2-27.4 where vertical oxygen minimum existed, the oxygen is high west of Izu-Ogasawara Ridge, possibly due to the influence of the Antarctic Intermediate Water. East of the Ridge, isopycnal oxygen decreases eastward. Isopycnal salinity and temperature are nearly constant west of a longitude and east of the longitude they changed. The longitude shifts westward with increasing density for 27.0-27.5, suggesting eastward extent and the bottom of wind-driven subtropical gyre.

by Ichiro Yasuda¹, Kuniaki Okuda², Katsumi Yokouchi³, and Makoto Okazaki⁴

¹ Hokkaido University
² National Research Institute of Fisheries Science
³ Tohoku National Fisheries Research Institute
⁴ Far Seas National Fisheries Research Institute

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