The Ryukyu Current System (RCS) is a northeastward current southeast of the Ryukyu Islands. The variation of current structure in the RCS region was investigated on the basis of the absolute geostrophic velocity estimated by inverse calculation using hydrographic data. Three realizations of the RCS are examined from cruises in May, September, and October 2002. It was found that the current structure in the RCS region is divided into upper and lower layers by an isopycnal surface of 26 σθ. The currents in both layers proceed northeastward along the eastern slope, east of the Ryukyu Islands. The current in the upper layer merges with the inflow from the interior region, resulting in larger northeastward volume transport southeast of Amami-Oshima Island than southeast of Okinawa Island. The volume transport from the east reaches more than 40% of that southeast of Amami-Oshima Island. On the other hand, the current in the lower layer is not affected by inflow from the east. It is also found that the upper RCS gains heat mainly through the advection of warm water from the east, resulting in an increase in the downstream temperature by about 0.5°C on average.
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