We applied inverse techniques to the CTD and ADCP data set obtained along a hydrographic section south off Kyushu and Shikoku, for the purpose of determining the net Kuroshio transport.
The transport of the Kuroshio main stream flowing in the East China Sea west of the Nansei Shoto Ridge has been consistently estimated at 20-30 sverdrups (Sv), which value is approximately 10-20 Sv smaller than the southward Sverdrup transport in the interior ocean at the same latitude. Thus, many studies have predicted a supplemental western boundary current (Kuroshio branch) along the eastern slope of the Nansei Shoto Ridge. However, the Kuroshio branch has not been clearly confirmed yet owing to the dominance of southward recirculation flows and meso-scale eddies there.
Our group has repeated hydrographic observations over this region and recently found frequent occurrence of a northward subsurface jet east off Amamai-ohsima, where the climatology of sea-level distribution shows an anti-cyclonic meso-scale gyre. The northward jet might be, therefore, only part of the anti-cyclonic gyre and cannot be immediately regarded as the Kuroshio branch, without verifying its contribution to the increase of the downstream Kuroshio transport. Then we planned a hydrographic section so that the transport of the northward through flow is determined in distinction from those of adhering recirculation gyres, and carried out CTD and ADCP observations along this section in May 2002, by the R/V Chofu-maru of the Nagasaki Marine Observatory, JMA. This section consists of two transects across the Kuroshio including its recirculation part and closes an area south off Kyushu-Shikoku from coast to coast. We formulated mass and property conservation requirements in this area and obtained estimates of the absolute flow field with use of inverse techniques. The result shows that the total transport of the northward flow east off Amami-ohshima was 25 Sv, but 14 Sv of which recirculated to the south, and that the remainder (11 Sv) joined the Kuroshio outflowing into the Pacific Ocean through the Tokara Strait. The total transport of the Kuroshio through flow out of the area is estimated at 33 Sv in the section south off Shikoku.
Further efforts with combination of synoptic hydrography and long-term moorings at selected locations will be necessary to determine the time-averaged flow filed because eddy activity is high both in meso and seasonal time scales there. On our poster presentation, we will show the result of inversions of ADCP data combined with the property conservation requirements. |
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