The densest water at the surface in the North Pacific and its marginal seas is produced thorough sea-ice formation over the northwestern shelf in the Sea of Okhotsk. This dense water (called Dense Shelf Water; DSW) is transported to the south as a part of the southward flowing East Sakhalin Current (ESC). Eventually, the water affected by the DSW exits to the North Pacific through straits between the Kuril Islands. Thus, the DSW is the ventilation source for the North Pacific Intermediate Water, which spreads to the entire subtropical Pacific. However, all of these processes are not well understood qualitatively mainly due to the lack of direct current measurement in the Okhotsk Sea and Kuril Straits.
From 1998 to 2001, an intensive observational study was conducted by Japanese, Russian and U.S. institutions. In order to reveal the processes described above, surface and profiling floats, and moorings were deployed in the region off Sakhalin, and the Lowered ADCP observation was carried out in Bussol′ Strait.
The surface drifters and profiling floats at the depth of about 500 m clearly revealed the existence of the ESC off the east coast of Sakhalin. It is strongly controlled by bottom topography and confined in the region shallower than 1000 m. A part of this current reached the southern tip of Sakhalin and another part turned to the east around 48N flowing eastward as far as Bussol′ Strait. Mesoscale (100-200 km in diameter) anticyclonic eddies were dominant in the Kuril Basin. Most of the floats exited to the Pacific through Bussol′ Strait.
The moorings were deployed in the region off Sakhalin from July 1998 to June 2000. These moorings revealed that the ESC extended from the surface to a depth around 1000 m. The yearly average transport is 6.7 Sv at 53N. In the slope regions, instruments were moored in the depth range of about 200-450 m, which roughly coincides the range of the DSW. The DSW transport measured at the moorings was converted to that at the freezing point. The time series of this converted transport shows that seasonal variability was quite different between 1998-1999 and 1999-2000. The estimated transport evaluated from two moorings at 53N was about 0.2 Sv for potential density larger than 26.7.
Intensive current measurements with a Lowered ADCP at the Bussol′ Strait in summer of 2001 showed that the outflow is in excess of the inflow and the net transport through the strait was 9 Sv. In the intermediate ventilated layer with potential density values of 26.8-27.1, 1.5 Sv of this net transport was observed. An extraordinarily strong diurnal tidal current was also found, with the amplitude of 1.1 m/s at the depth below 1000 m. |
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