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Observationally estimated transport of the Kuroshio south of Japan

 

Shiro Imawaki*1, Hiroshi Uchida*2 and ASUKA Group (Affiliated Surveys of the Kuroshio off Cape Ashizuri)

*1 Research Institute for Applied Mechanics, Kyushu University

*2 Japan Science and Technology Corporation

 

The ASUKA Group carried out oceanographic observations along a line crossing the Kuroshio south of Japan, which was chosen to coincide with a subsatellite track of the altimeter satellite TOPEX / POSEIDON. From October 1993 to November 1995, they maintained moored current meters and carried out repeated hydrographic sections. From those in-situ data, geostrophic velocities normal to the observation line are estimated, referring to velocities observed at mid and deep layers, more than twenty times. From those velocities, the volume transport of the Kuroshio (transport of the entire eastward flow) for the upper 1,000m is estimated. The transport is found to be proportional to the sea level difference across the Kuroshio.

Having this relationship and using TOPEX / POSEIDON altimetry data, a time series of the Kuroshio transport is obtained for five years (1992-1997). The result shows large fluctuations around an average of 57Sv (1Sv is million cubic meters per second). Most of the fluctuations are associated with fluctuations of both the stationary and propagating eddies superimposed on the Kuroshio system. Eliminating the transport associated with those local recirculations, the transport of the Kuroshio as a throughflow is estimated. The result shows smaller fluctuations around an average of 42Sv. The seasonal signal in the observed transport is small compared with the transport of western boundary current expected from the interiorSverdrup transport at mid-latitudes estimated on the basis of observed surface wind field. There may be some seasonality, however, that it is low in September and high in December, similar tendency as the theoretical transport. The observed interannual fluctuation is compared with fluctuation of the geostrophic (baroclinic) transport of the Kuroshio obtained along the 137。? meridian.

 

 

 

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