The existence of eddy on the north boundary of Kuroshio has been suggested through the current field measurement at the Tokara Straight by using high-frequency ocean surface radar (HFOSR) system installed on Yakushima Island. The cores of eddy were about 10km in radius and about 1.5m/s in tangential velocity, and propagated eastward at about 0.7 m/s (about a half velocity of Kuroshio). The HFOSR system observed two eddies a day around the spring tide, and one eddy a day in the other period.
The Communications Research Laboratory and the National Space Development Agency of Japan have collaborated to develop an airborne high-resolution multiparameter Synthetic Aperture Radar (SAR) called Pi-SAR. The Pi-SAR is a dual frequency radar operating at L-band and X-band frequencies. The X-band radar, moreover, has an interferometric function by which topographic mapping of the ground surface is achieved. The Pi-SAR observed the southern sea off Yakushima Island with about 20km swath and 120km long. On the SAR images, two large eddies were identified clearly. The scale of them was about 10km in radius as same as the eddies observed by HFOSR system. The distance between them was about 70km. The observation was done between a spring tide and a neap tide. On the assumption that the eddies were produced with 24 hours period, the propagation velocity is estimated as 0.8 m/s. The estimation agrees with the propagation velocity measured by HFOSR system.
These eddies may be produced by the interaction between Kuroshio and tidal current at the western sea off Yakushima Island. The HFOSR observation showed that such eddies are propagated to eastward more far than Tanegashima Island. Because such a strong eddy may affect to the mass exchange between Kuroshio water and coastal water, the behavior of these eddies is important to understand the effect of Kuroshio to the marine environment. |
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