The structure of the Kuroshio southwest of Kyushu: velocity, transport and potential vorticity fields

Abstract

A triangular CTD/ADCP survey was made across the Kuroshio southwest of Kyu shu abroad the R. V. Thompson during January 1986. Due to relatively poor navigation data, a simple averaging technique has been used to convert the ADCP data taken between CTD station pairs into an average absolute velocity normal to the station pair, with a maximum error varying from about ±5 cm s−1 to less than ±0.1 cm s−1 depending on the type of navigation data. The average ADCP velocity at 60 m (or 10 m over the shelf) was then used as the reference velocity to calculate the absolute geostrophic velocity through the sides of the study triangle. The results show that the ADCP velocity shear was in reasonably good agreement with the geostrophic shear of the Kuroshio. The Kuroshio entered the study triangle as a coherent current and then split around a tall seamount into two branches as it left the area. The volume transport of the Kuroshio southwest of Kyushu in January 1986 was 30.3 ± 2.0 Sv, and the advective temperature transport was 27.6 ± 1.8 × 1014W. These values are similar to those reported for the Gulf Stream in the Florida Strait, and a roughly linear correlation exists between temperature and volume transports in both regions. Mass conservation within the study triangle allowed construction of a streamfunction that showed the presence of cyclonic and anti-cyclonic mesoscale eddies to the north and northwest of the core of the Kuroshio. Potential vorticity estimated from the absolute geostrophic velocity field was conserved along streamlines on potential density surfaces except perhaps near a seamount where curvature vorticity must be considered, and the path of the Kuroshio could be traced by the core of maximum potential vorticity. Finally, the Kuroshio was potentially unstable as it flowed along the continental margin in the Okinawa Trough because the gradient of potential vorticity on potential density surfaces changed sign across the Kuroshio. This helps explain the mesoscale frontal and eddy features observed in the cyclonic side of the Kuroshio in the East China Sea.