1Faculty of Bioresources, Mie University, 1515 Kamihama-cho, Tsu, Mie 514-0008, Japan
2Wakayama Research Center of Agriculture, Forestry and Fisheries
(previously the Wakayama Prefectural Fisheries Experimental Station), 1551 Kushimoto-cho, Nishimuro, Wakayama 649-3503, Japan
3Marine Information Research Center, JHA, 7-15-4 Ginza 5F, Chuo-ku, Tokyo 104-0061, Japan
(Received 11 November 1998; in revised form 23 April 1999; accepted 27 April 1999)
The Kii Bifurcation Current is often found along the southwest coast of the Kii Peninsula, and its frequency of occurrence reaches about 70% in the period from 1988 to 1996 (Takeuchi et al., 1998a). In order to clarify the structure and short-period variability of the Kii Bifurcation Current, detailed observations were made four times on board the R/V Seisui-maru of Mie University on October 29-31, 1996, on June 24-26, 1997, October 14-16, 1997, and December 3-4, 1997. The measured horizontal structure of the Kii Bifurcation Current indicates that the eastern portion of the Current (eastward flow near Cape Shionomisaki) consists of a part of the current zone of the Kuroshio. It is shown that the current structure, including the Kii Bifurcation Current in the vicinity of Cape Shionomisaki, is stable when the Kuroshio is flowing in a stationary straight path, but that the current structure is considerably changed when small-scale eddies pass by the cape. Such short-period variation can be monitored by using the daily variation of the sea level difference between Kushimoto and Uragami. In particular, in the case of October 29-31, 1996, when an eminent small-scale eddy passed by Cape Shionomisaki, and when the Kuroshio axis tentatively moved southwards about 50 km apart from the coast, the Kii Bifurcation Current seems to have disappeared.
Keywords:
Bifurcation Current,
Kii Peninsula,
Kuroshio,
current structure,
temporal variation.
1. Introduction
Along the southwest coast of the Kii Peninsula, a bifurcation current is often observed (the area under consideration is shown in Fig. 1 together with a typical temperature field at the time of the straight Kuroshio path). This current is called Furiwake-shio in Japanese, and we shall refer to it as the Kii Bifurcation Current (Nagata et al., 1999a). Takeuchi et al. (1998a) analyzed the ADCP data obtained on board the R/V Wakayama of the Wakayama Prefectural Fisheries Experimental Station (now called the Wakayama Research Center of Agriculture, Forestry and Fisheries) in the period from 1988 to 1996. They selected 258 cruises in which observation lines run parallel to the coast line and almost along the 100 m depth contour (the standard line). It was shown that the frequency of occurrence of the Kii Bifurcation Current reaches about 70% in the analyzed period. It was also shown that the frequency of occurrence depends on the flow nature of the Kuroshio: frequency is high when the Kuroshio takes a straight path, and low when it takes a large meander path off Enshunada (the so-called Kuroshio Large Meander).
Nagata et al. (1999a) analyzed the data mainly obtained on board the Seisui-maru of Mie University and the Wakayama of the Wakayama Fisheries Experimental Station in June 1996 (no ADCP observation was made for the Kii Bifurcation Current), and discussed the detailed current structure in the vicinity of the Kii Peninsula at the time when the Kuroshio flowed in the straight path. They concluded that the strong current zone of the Kuroshio touches the coast and bottom slope near Cape Shionomisaki. The coastal water to the west of the cape is completely separated from that to the east under this condition. To the west of the cape, the warm offshore Kuroshio Water intrudes well into the coastal area. Thus a large contrast of the water quality is generated between the coastal waters to the west and to the east of the cape, producing the considerable sea level change between the tide stations at Kushimoto and Uragami.
