IN SITU OBSERVATIONS
Recently, we made an effort to measure the Kuroshio and its recirculation south of Shikoku, Japan, and estimate their absolute volume and heat transports. The observation line was chosen to coincide with a subsatellite track of the altimetry satellite TOPEX/POSEIDON, which has been measuringthe sea surface height since 1992 (7). In October 1993, we deployed nine moorings equipped with 33 current meters along the observation line to start the intensive survey. In September 1994, we recovered all those current meters and deployed similar moorings, which were recovered in November 1995, to finish the intensive survey. During this two-year long intensive survey period, a group called ASUKA (Affiliated Surveys of the Kuroshio off Cape Ashizuri) carried out hydrographic surveys using conductivity-temperature-depth recorders (CTD) and/or expendable bathy-thermographs (XBT) along the line repeatedly, in order to estimate upper layer velocities, which cannot be measured adequately by those moored current meters; totally 42 repeat sections were obtained for the Kuroshio region. This effort was intended to be the western boundary current array PCM5 of the World Ocean Circulation Experiment. Sea level was measured continuously at Cape Ashizuri by a governmental agency.
RESULTS
The moored current meter data show that the Kuroshio is directed to ENE, almost perpendicular to the ASUKA line, in both the 1993-1994 and 1994-1995 annual mean fields. The vertical section of the meanvelocity component normal to the observation line shows that the Kuroshio reaches down to 2,000m depth on the offshore side but does not reach to the bottom on the continental slope, where the velocity component is found to be almost zero or slightly negative.
Geostrophic velocities are calculated from repeated hydrographic survey data, with reference to observed velocities at mid and deep layers; five-day mean velocities are used for the reference velocities. These geostrophic velocities are called here absolute geostrophic velocities, in order to distinguish them from ordinary geostrophic velocities calculated under the assumption of no-motion at some abyssal depths.
The transport of the Kuroshio is estimated from the absolute geostrophic velocities obtained above. Here the Kuroshio is simply defined as the entire eastward flow, and so the areal integral is made from the coastal station to the offshore edge of the Kuroshio, and from the sea surface to 1,000m depth. The 1,000m depth is not satisfactorily deep to estimate the whole Kuroshio transport, but it is a compromise with utilizing abundant XBT data reaching only 800m depth, which allow us to calculate geostrophic velocity shear down to 1,000m depth using a very tight experimental relationship. As is shown below, the transport is mostly confined in the upper 1,000m layer. The estimated transport varies from 27 to 85 Sv (1 Sv = 106m3/sec) for 25 selected hydrographic surveys carried out during two years. The profile of the sea surface dynamic topography (SSDT) is also estimated on the basis of the horizontal profile of absolute geostrophic velocities at the sea surface.
