Missing Volume Transport of the Kuroshio
The Kuroshio, one of the strongest current in the world, flows in the East China Sea and south of the Japanese main island. Because large amount of warm water is carried from the tropics, the Kuroshio has great impacts on climate changes in a wide area including Japan. In addition, the Kuroshio strongly affects efficiency of maritime transports and fisheries.
Although its existence has long been recognized, detailed quantitative measurements of the Kuroshio volume transport (the amount of sea water carried by the Kuroshio) have started only 10-20 years before. The results revealed that the Kuroshio volume transport in the downstream (south of Japan) is twice larger than that in the upstream (East China Sea). Analogizing rivers, this suggests existence of a "branch current" which merges into the Kuroshio south of Japan. Recently, a northeastward subsurface current was found southeast of Amami-Ohshima Island by mooring observations, so the "branch current" is expected to flow northward east of the Nansei islands. High-resolution numerical models also support this idea.

Northeastward Current Southeast of Okinawa Island
However, existence of a northeastward current was not certain in the southeast of Okinawa Island, the upstream of Amami-Ohshima Island. Plenty of meso-scale eddies that are slowly propagated from the east in the Pacific cause high temporal variability in the east of the Nansei islands, which prevents us from obtaining mean velocity field without long-term observations.
The Japanese Coastal Ocean Predictability Experiment Group (JCOPE; Climate Variations Observational Research Program, Frontier Observational Research System for Global Change), therefore, started long-term observations deploying many Inverted Echo Sounder with Pressure gauge (PIES)*¹ and Moored Acoustic Doppler Current Profiler (MADCP)*² mooring systems southeast of Okinawa Island (Figs. 1 and 2). Based on nine-month records of the mooring observations from November 2000 to September 2001, a time series of the volume transport across the observation line southeast of Okinawa Island (indicated by a broken line in Fig. 1) is obtained (Fig. 3). As expected from active meso-scale eddies, its temporal variability is found very high; the volume transport was sometimes southwestward, but turned to northwestward only a few month later, reaching to 20.8 million m³/s.
Removing these variations by averaging over a long period, the temporal mean current is first obtained southeast of Okinawa Island, manifesting northeastward 6.1 million m³/s volume transport. This corresponds to carry 5 cups of Tokyo Dome Stadium of sea water northeastward in every second. In other words, there would exist 10,000 of the Shinano Rivers, the largest river in Japan.

Revealing the Current System East of Nansei Islands
Large amount of sea water is confirmed to be carried northeastward in the southeast of Okinawa Island, as well as Amami-Ohshima Island. The obtained mean volume transport (6.1 million m³/s) accounts for a quarter of the Kuroshio volume transport in the East China Sea, so that the observation line is considered to have captured a part of the current that merges into the Kuroshio south of Japan.
In order to investigate connection of the current to the Kuroshio east of Taiwan, the JCOPE Group deployed mooring observation systems southeast of Miyako Island, the further upstream of Okinawa Island, in December, 2002 (Fig. 4). In addition, to study seasonal and interannual changes of the current and to obtain offshore velocity field and precise vertical velocity profiles, many observations are being conducted cooperating with other institutes such as Nagasaki Marine Observatory, Nagasaki University and Kagoshima University.
In near future, the entire current system east of the Nansei islands would be described more precisely. This should greatly help us to trace paths of the warm water and materials carried by the Kuroshio, which would eventually aid to estimate climate changes and to increase efficiency in maritime transport and fisheries.