This complexity in the current structure and its Variable nature could be caused by the passage of a small-scale eddy passing by Cape Shionomisaki.
7. Discussions and Conclusions
Though the Prompt Reports of Oceanic Conditions published bimonthly by the Hydrographic Department indicate that the Kuroshio was in its straight path at the times of the four cruises of the R/V Seisui-maru of Mie University, the current field in the vicinity of Cape Shionomisaki exhibited considerable changes, cruise by cruise. It is suggested that such changes may be caused by the passage of small-scale eddies in the region under consideration or by temporary departure of the Kuroshio axis from the coast, and that this could be monitored by checking the daily sea level difference between Kushimoto and Uragami.
Fujita (1997) obtained the threshold value (25 cm) of the sea level difference in order to identify whether the Kuroshio takes a straight path or a large meandering path. Though he had analyzed only the phenomena having a time-scale of a half month, this threshold value is shown to be applicable for much shorter time scales in order to detect whether the strong current zone of the Kuroshio touches the coast near Cape Shionomisaki or leaves the cape significantly.
The structure of the Kii Bifurcation Current (or the Eastward Current) is very stable when the daily sea level difference is considerably larger than the threshold value and does not show significant changes. But its structure, or the current divergence of the Kii Bifurcation Current along the standard line, is changed according to the change of the daily sea level difference. When the sea level difference decreases below the threshold value, even for a short period, the Kii Bifurcation Current would disappear.
It was also shown that the hourly average sea level difference between Kushimoto and Uragami represents a very short time-scale variation in the oceanic condition near Cape Shionomisaki. However, it has not been clarified why the structure of the Kii Bifurcation Current does not exhibit significant variation having tidal periods, and why the tidal fluctuation appears sporadically in the hourly sea level difference curve. Further investigations would be necessary to understand such short time-scale fluctuations.
Acknowledgements
We would like to thank to Dr. Koichi Fujita of the Fisheries Research Center of Mie and Mr. Yasuhiro Kaku of the Wakayama Research Center of Agriculture, Forestry and Fisheries for their kind support and valuable discussions. We also thank Captain Mr. Isamu Ishikura and the crew of the R/V Seisui-maru of Mie University for their help in making observations: the observations were very difficult and dangerous as the region is located exactly in one of the busiest navigation routs near Japan.
References
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