Cooperative study of measurements of the volume and heat transports of the Kuroshio and Kuroshio Countercurrent

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To measure the Kuroshio and its recirculation (Kuroshio Countercurrent) south of Japan, a group called ASUKA (affiliated surveys of the Kuroshio off Cape Ashizuri) carried out observations along a line crossing the Kuroshio, which was chosen to coincide with a subsatellite track of the altimetry satellite TOPEX/POSEIDON.In October 1993, they deployed nine moorings equipped with 33 current meters and two ADCPs (acoustic Doppler current profilers), and ten IESs (inverted echo sounders), which were maintained for two years until November 1995. During this intensive observation period, they carried out measurements by CTD (conductivity-temperature-depth recorder), XBT (expendable bathy-thermograph) and towed-ADCP as frequently as possible to estimate upper layr velocities, which cannot be measured adequately by the moored instruments. This Grant-in Aid supported activities of the ASUKA group.From first-year moored current meter date and repeated hydrography data, we estimated geostrophic velocities referred to velocities observed at a nominal depth of 700 m. The absolute volume transport (estimated as the sum of those geostrophic velocities) of the Kuroshio for the upper 1000 m was found to have a very tight relationship with the difference of sea level across the Kuroshio. Combination of this relationship and TOPEX/POSEIDON altimetry data obtained every 10 days provided us, for the first time, with the time series of the Kuroshio transport for two years. The estimated transport fluctuates much around an average of 53 Sv (1 Sv =10^6 m^3/sec). The annual cycle is not apparent in the time series but some semi-annual cycle is dominated.