Does Small-Scale Meander Travel Eastwards and Trigger Large-Scale

Does Small-Scale Meander Travel Eastwards and
Trigger Large-Scale Meander of the Kuroshio?

Kouichi Fujita¹, Yutaka Nagata² and Shozo Yoshida²

(received 1998/1/20, revised 1998/4/29, accepted 1998/5/7)

ABSTRACT It has been found that the formation of the Large Meander of the Kuroshio occurs in the following way: small-scale meanders of the Kuroshio are generated near the tip of Cape Toi and propagate eastward, and some of these small-scale meanders suddenly develop into the Large Meander just after passing Cape Shionomisaki (the tip of the Kii Peninsula). This paper reexamines this scenario. It was found that the eastward propagation occurs only over the eastern margin of the meander, and its width increases as the position of the western margin does not change significantly. Just after the eastern margin passes by Cape Muroto, a small but strong small-scale eddy is cut off from the eastern portion of the elongated meander. At first, the rapid development of the eddy takes place just off the Kii Channel. It should be noted that any small-scale meander was never detected just off Shikoku.
(Key Words: Kuroshio, straight path, meandering path, small-scale meander, Cape Toi) ^{1 Fisheries Research Institute of Mie}
^{2 Marine Information Research Center, Japan Hydrographic Association}

Introduction The Kuroshio has two stable path patterns to the south of Honshu, Japan: a straight path flowing eastward after passing the tip of the Kii Peninsula and a large-meandering path flowing around a large, cold water mass, formed to the southeast of the Kii Peninsula (the Large Meander of the Kuroshio). These two paths are very stable and either pattern usually persists for periods ranging from several months to several years. Yoshida (1961) and Shoji (1972) reported that small-scale meanders of the Kuroshio are often generated near the tip of Cape Toi (see Fig. 1 for locations of capes and other places) and propagate eastwards, and that some of such small-scale meanders suddenly develop into the Large Meander just after passing by Cape Shionomisaki (the tip of the Kii Peninsula). The evolution of the Kuroshio path in generation stages of the Large Meander is shown for the cases in March-June 1959 and in January-June 1969 in Fig. 1 from Shoji (1972). This scenario of the generation of the Large Meander is widely accepted, and Kawabe (1980) and Fujimoto et al. (1988) described the Large Meander generations that occurred in 1975 and 1986 in a similar way. The variations of the Kuroshio path prior to the Large Meander are shown in Fig. 2 and Fig. 3. All of these discussions are based on the Prompt Report of the Oceanographic Condition (PROC: Kaiyo-Sokuho), published bimonthly by the Hydrographic Department, Maritime Safety Agency.

Fig. 1 Evolution of the Kuroshio path prior to the generation of the Large Meander of the Kuroshio in the period from March to August, 1962 (upper figure) and that from January to July, 1969. (Shoji, 1972). A: Cape Toi, B: Cape Ashizuri, C: Cape Muroto, D: Cape Shionomisaki, E: Cape Daio, F: Bungo Channel, G: Tosa Bay, and H: Kii Channel.
Fig. 2 Evolution of the Kuroshio path prior to the generation of the Large-Meander of the Kuroshio in the period from March to July (left figure) and August, 1975 (right figure). (Kawabe, 1980)
Fig. 3 Evolution of the Kuroshio path prior to the generation of the Large-Meander of the Kuroshio in the period from July to December, 1986. (Fujimoto et al., 1988).

Using PROC, Fujita (1997) described the variation of the Kuroshio path along the southern coast of Japan in the period from 1975 to 1992. He set 9 reference lines and analyzed the variations of the distances of the Kuroshio axis measured from coast along these lines. Cross covariance functions among these distances, however, did not indicate such eastward "travel" of any small-scale meander along the southern coast of Japan. Here, we shall re-analyze the evolution of the Kuroshio path. Evolution of the small-scale meander generated just off Cape Toi After 1950, the Large Meander of the Kuroshio (A type) was generated 6 times: in 1959, 1969, 1975, 1981, 1986, and 1989. The coverage of the observations is considerably good for the cases of Meanders 1986 and 1989 (especially in 1986, as the Hydrographic Department of the 5th Regional Maritime Safety Headquarters carried out an extensive observation program this year). On the other hand, the observations off Shikoku were scarce in 1981, and the evolution of the Kuroshio is hardly discussed. The variations of the Kuroshio path to the south of Honshu cited from PROC are shown in Fig. 4 and Fig. 5 for the periods from July to December in 1986 and in 1989, respectively. In these figures, the Kuroshio path and the current velocities measured by GEK are shown. The positions of current vectors indicate those of available oceanic observations in each half-month period.

A small-scale meander or eddy was generated just off Cape Toi around July in both 1986 and 1989. In both cases, the eastern margin of the eddy (the position of the northward current) tends to move eastwards. On the other hand, the movement of the western margin is not clear, and its position appears to be unchanged. The generation of the small-scale meander just off Cape Toi was observed prior to the Large Meander formation in both cases of 1986 and 1989: Aug. (A) and (B), Sept. (B) and Oct. (A) in 1986, and Aug. (B) and Sept. (A) and (B) in 1989 (the symbols “A” and “B,” when attached to months, indicate the first half and second half of that month, respectively). However, such a small-scale meander cannot be detected to the south off Shikoku. The eastern margin of the meander (the position of the northward current) moves eastwards, while the position of the western margin stays almost at the same position in both cases (from August (A) to October (A) in 1986 and from August (A) to September (B) in 1989). Therefore, no eastward traveling occurred, and deformation of the meander took place with only the eastern margin of the meander moving eastward in both cases. Thus, the meander was elongated, and its east-west width was increased. Just after the eastern margin of the elongated meander passed off Cape Muroto, a kink of the offshore margin of the meander occurred off Tosa Bay, and its eastern portion was separated from the western portion to form a relatively strong smaller-scale eddy (Oct. (B) in 1986 and Oct. (A) and (B) in 1989). This newly generated eddy appears to grow rapidly just off the Kii Channel before it passes Cape Shionomisaki to form the Large Meander of the Kuroshio. As the scale of this newly-generated meander is almost the same as that of the meander off Cape Toi, it would have been considered that the small-scale eddy travels eastward and reaches off the Kii Channel.

Fig. 4 Changes of the Kuroshio path and current field to the south of Japan from July to December, 1986 cited from the Prompt Report of Oceanographic Condition published bimonthly by the Hydrographic Department, Maritime Safety Agency. (A) and (B) attached to months indicate that the data are taken in the first half and the second half of the month, respectively. Vectors indicate current velocities measured by GEK. The positions of these vectors roughly correspond to those of available oceanic observations.

Fig. 5 Same as in Fig. 4 except from July to December, 1989.

Yoshida (1961) , Shoji (1972) and Kawabe (1980) described the eastward propagation of the small-scale meander, but they focused their attention only on the position of the eastern margin of the meander as seen in Fig. 1 and Fig. 2. Their figures also suggest that the Kuroshio usually flows due east from off the southern tip of Kyushu or off Cape Toi until it turns north and approaches Cape Muroto just before the formation of the Large Meander of the Kuroshio. In this stage, the Kuroshio path is located far to the south from the east coast of Kyushu to off Shikoku, and an abnormal oceanic condition is seen from Cape Toi to Cape Muroto. Tsumura (1963) and Kawabe (1980) pointed out that the varying nature of the coastal sea level is very similar in the region between the east coast of Kyushu to the west of Tokyo Bay. These two phenomena might be related, but it is hard to discuss it in the present stage as the available data is too limited. Formation and development of the strong small eddy off Kii Channel The fifth Regional Maritime Safety Headquarters at Kobe also publishes the bimonthly Prompt Report of the Regional Oceanographic Condition. In order to see detailed generation and development stages of the small, strong eddy off the Kii Channel, the oceanographic conditions are cited from the report in the period from the last half of August to the last half of November, 1986, and they are shown in Figs. 6 through 9. Oceanographic conditions shown in Fig. 6 (August 22-31: left figure and August 31-September 15: right figure) shows the stage that the eastern margin of the elongated meander reached just off Cape Muroto. It should be noted that no small-scale eddy or meander is seen to the west of the northward flowing portion of the Kuroshio just off Cape Muroto. The density of the observations was excellent in the latter period, and we can see that a counter-clockwise eddy existed in the Kii Channel. The strong current zone of the Kuroshio appears to touch the coast at Cape Shionomisaki. These conditions appear to be most favorable for the occurrence of the Bifurcation Current reported by Takeuchi et al. (1998a and b). The situation is hardly understood as a so-called small-scale meander was approaching the area off Cape Muroto.

Fig. 7 (left: September 16 to 30, and right: October 1 to 15) indicates the stage that the eastern margin of the elongated meander was passing by Cape Muroto, but , due to limited observation density, details such as the small eddy activity are not clear. Fig. 8 (left: October 16-31, and right: November 1 to 5) shows the generation stage of the small strong eddy just off the Kii Channel. On October16-31, the cold counter-clockwise eddy inside the Kii Channel appears to be enhanced, and another counter-clockwise eddy is generated to its south as a kink of the southern margin of the elongated meander occurring off Tosa Bay. The rapid development of the small, strong eddy may be attributed to an interaction with the counter-clockwise eddy in the Kii Channel. The strength of the cold eddy in the Kii Channel is believed to be correlated with the distance of the Kuroshio axis, measured southward from Cape Shionomisaki (Baba, 1989). It should also be noted that the existence of the Bifurcation Current is also suggested in this stage.

Fig. 6 The Kuroshio path and current field off Shikoku and the Kii Peninsula in the stage that the eastern margin of the elongated meander reaches off Cape Muroto, cited from the Prompt Report of the Regional Oceanographic Condition published bimonthly by the fifth Regional Maritime Safety Headquarters, Maritime Safety Agency, at Kobe. Left: August 22-31, 1986, and right: August 31-September 15.

Fig. 7. Same as in Fig. 6 except in the stage that the eastern margin of the elongated meander passed by Cape Muroto. Left: September 16-30, 1986, and right: October 1-15.

Fig. 8 Same as in Fig. 6 except in the stage that the small and strong eddy is formed off the Kii Channel. Left: October 16-31, 1986, and right: November 1-5 The small, strong eddy passed by Cape Shionomisaki in the period from November 5 to November 20, as seen in Fig. 9 (left: November 5-15, and right: November 15-20). It should be noted that the clockwise eddy exists in the Kii Channel in this stage, and that the eddy passing by Cape Shionomisaki tends to suck waters from the Kii Channel. The Bifurcation Current often disappears when the Kuroshio is flowing in a meandering path or when the eddies are just passing by Cape Shionomisaki (Takeuchi et al., 1998a and b). It should also be noted that the Kuroshio tends to flow in a nearshore path along the coast, from east of Kyushu to Cape Muroto, when the eddy had passed by Cape Shionomisaki and the Kuroshio Large Meander had been formed. As discussed by Tsumura (1963) and Kawabe (1980), such a variation of the Kuroshio path may produce the similar nature of sea level variation in the region between the east coast of Kyushu to the west of Tokyo Bay.

The separation of a small, strong eddy from the elongated meander and its rapid growth just off the Kii Channel was also observed in the case of the Large Meander in 1989; the small-scale meander generated in July 1989 had developed into the wide meander by September (B), as seen in Fig. 5. It can be seen that the relatively strong, small eddy was generated in the eastern portion of the elongated meander and off the Kii Channel (October (B) in Fig. 5) just before the formation of the Large Meander (November (B) in Fig. 5).

Concluding remarks It has been found that the formation of the Large Meander of the Kuroshio occurs in the following way: small-scale meanders of the Kuroshio are generated near the tip of Cape Toi and propagate eastward, and some of these small-scale meanders suddenly develop into the Large Meander just after passing Cape Shionomisaki (the tip of the Kii Peninsula). This study reexamined this scenario. We found that this scenario is not accurate, but not necessarily incorrect. The eastward propagation occurs only for the eastern margin of the meander, and its width increases as the position of the western margin does not change significantly. Just after the eastern margin of the elongated meander passes by Cape Muroto, a small but strong small-scale eddy is cut off from the eastern portion of the elongated meander. The rapid development of the eddy at first takes place just off the Kii Channel.

This new scenario will be examined by more elaborate studies in the future, as the available data are not good enough in the cases discussed above. We hope that our suggestions are useful enough in such future investigations.

Fig. 9. Same as in Fig. 6 except in the stage that the small meander was passing by Cape Shionomisaki. Left: November 5-15, 1986, and right: November 15-20.
Acknowledgment The authors wish to express their thanks to Prof. Yoshihiko Sekine of Mie University, Mr. Junichi Takeuchi of the Wakayama Prefetural Fisheries Experimental Station, and Mr. Yoshitake Ito and Mr. Akifumi Nishimura of the Mie Prefecture Government, and Ms Keiko Tajima of the Marine Information Center, JHA, for their encouragement and help. References Baba, N. (1989) The current variation in the south adjacent area of Kii Channel and the eastward propagation of the Kuroshio small meander in 1986. Tech. Bull. Hydrogr., 7, 17-24 (in Japanese).

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Shoji, D. (1972) Time variation of the Kuroshio south of Japan. In, Kuroshio - Its Physical Aspects-, ed. H. Stommel and K. Yoshida, Univ. of Tokyo Press, 217-234.

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