Linkage between mesoscale anti-cyclonic eddies, and the path and surface velocity of the Kuroshio is described by using surface current field derived from multi-year TOPEX/POSEIDON altimetry and ADCP (Acoustic Doppler Current Profiler) measurement of the TOLEX (Tokyo-Ogasawara Line Experiment) monitoring.
During the six years from 1993 to 1998, we identified five events in which an anti-cyclonic eddy propagated westward along the latitude of nearly 30 degrees N, collided the Kuroshio off Kyushu Island, and was advected to an area off Shikoku Island along the southern side of the Kuroshio. The surface current velocity of the Kuroshio showed a local maximum when the eddy passed the area, and the path of the Kuroshio simultaneously began to meander off Kii Peninsula, the downstream side of the velocity maximum. This small meander propagated eastward, being followed by the local acceleration of the Kuroshio. When the meander reached the Izu Ridge one or one-and-half more months later, a cyclonic eddy was formed on the coastal side of the Kuroshio. In two of the five cases, the triggering eddy came from the east and passed westward across the TOLEX line (located east of Izu Ridge) at about 29.5 degrees N, approximately six months before the development of the coastal cyclonic eddy on the same line.
We found another thirteen events where the local acceleration of the Kuroshio off Shikoku moved eastward and amplified. Although we could not identify the upstream eddy collision prior to these thirteen events, it was speculated that an anti-cyclonic eddy, which revolves clockwise in a region south of Shikoku, might influence the Kuroshio when the eddy takes northernmost position, as pointed by Ebuchi and Hanawa (2003). Dominant variability on a time scale of approximately four months is found in the surface current speed of the Kuroshio from 132 to 140 degrees E. This time scale coincides with a period of the clockwise revolution of an anti-cyclonic eddy off Shikoku detected by Ebuchi and Hanawa (2003). The variation with the same time scale is also found in the position of the Kuroshio axis off Honshu Island. The surface speed of the Kuroshio has a negative correlation with the latitudinal position of the path over the TOLEX line: that is, the Kuroshio accelerates when it meanders over the TOLEX line. The average speed of the Kuroshio was 1.14 ± 0.20 m/s for the period when the Kuroshio takes the non-large-meander offshore path, and 1.00 ± 0.20 m/s when the Kuroshio takes the non-large-meander coastal path. |
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