The structure of the Kuroshio west of Kyushu

Abstract

A triangular CTD/ADCP survey was made across the Kuroshio west of Kyushu aboard the R/V Thompson during January, 1986 in order to investigate the water properties and flow field in the Kuroshio. A similar CTD survey was made in July, 1986 aboard the R/V Washington to study the seasonal variability in the Kuroshio. The Kuroshio in this region exhibited a marked seasonal change in its near-surface stratification and water properties. In January, the Kuroshio water was separated from the vertically well-mixed coastal water over the shelf by a strong front located near the shelf break. Horizontal mixing between the Kuroshio and coastal water was observed but was limited near the shelf break. In July, surface coastal water extended far past the shelf break over the Kuroshio region near the surface, and in turn, Kuroshio water intruded onto the shelf near the bottom. Mixing between the Kuroshio and coastal water was found over much of the mid and outer shelf and upper slope, spanning a cross-stream distance of 75 km. In addition, evidence of deep vertical mixing within the Kuroshio itself was found near 32.0°N and 128.2°E, most likely due to internal tidal mixing over the slope. Since Loran C navigation coverage in the study region was poor during the R/V Thompson cruise, a simple averaging technique has been used to convert the ADCP data into an absolute velocity. An error analysis shows that the total error in the absolute ADCP velocity was about ±5 cm/s. The absolute geostrophic velocity using the absolute Doppler velocity at 60 m as the reference velocity was then calculated for the sides of the triangle. The results show that the ADCP velocity shear was in good agreement with the geostrophic shear in the Kuroshio. The Kuroshio flowed through the western section as a coherent current, then split into two streams around a tall seamount as it left through the eastern section. Some recirculation also occurred between the core of the Kuroshio and the slope as well as near the seamount. The geostrophic velocity field calculated relative to the bottom missed some of the important features of the true flow field such as splitting of the Kuroshio and the recirculation in the slope region. The volume, salt and heat transports of the Kuroshio during the January 1986 survey have been cakulated using the absolute geostrophic velocity and CTD data. The volume transport of the Kuroshio west of Kyushu in January 1986 was 31.7± 2.0 Sv, which is comparable to that of the Gulf Stream in the Florida Strait. The volume transport through the triangle was conserved within measurement uncertainty, so that a streamfuction field can be defined by the transport. The resulting streamlines clearly show the structure of the flow field in the Kuroshio and its adjacent currents during the survey. The advective heat transport of the Kuroshio west of Kyushu in January 1986 was 28.2 ± 1.8 x 1014 W. The salt transport in January 1986 was about 108.0 ± 7.3 x 1010 kg/s, and the net salt flux was zero within measurement error. Analysis of the potential vorticity based on the January 1986 absolute geostrophic velocity field shows that the total potential vorticity in the Kuroshio may be approximately given by the product of the vertical gradient of the potential density and the sum of the planetary and relative vorticities. The distribution of relative vorticity plays a significant role in determining the structure of the potential vorticity in the Kuroshio. The path of the Kuroshio can be traced in the field of potential vorticity. Facing in the direction of the current, the axis of the maximum velocity is located to the right of the core of maximum potential vorticity. Finally, the Kuroshio was potentially unstable since the gradient of potential vorticity changed its sign on potential density surfaces across the Kuroshio.