Yuan et al. [1994] recently reported the existence of the countercurrent east of the Nansei Islands. The model grid of Sekine and Kutsuwada [1994] is rather coarse and the vertical structure is determined by only two modes. Our experience even suggests that the interaction between baroclinic currents and the bottom topography may play a significant role as described below, particularly over the continental slope [see, for example, Salmon, 1992]; we must be careful to appreciate results from ocean models of the intermediate resolution in both vertical and horizontal direction for such a case.
Welander [1959], Gates [1966], Sarkisvan [1969] and Holland [1973] introduced the idea of the bottom torque into large-scale physical oceanography. In particular, Holland [1973] demonstrated that the bottom torque, together with the adjustment effect called JEBAR (Joint Effect of BAroclinicity and bottom Relief) due to density variations, may have a crucial effect on the steady ocean circulation. Sarkisyan and Ivanov [1971] and Mellor et al. [1982] pointed out clearly the importance of JEBAR to the annual mean transport of the Gulf Stream; the JEBAR increases the annual mean transport of the Gulf Stream and thus explains the difference between the Sverdrup transport and the observed transport of the Gulf Stream. Anderson and Corry [1985] demonstrated, using 2-layer linear ocean model, that the seasonal transport variation of Florida current cannot be reproduced without dealing with both bottom topography and oceanic baroclinicity, although they did not state the importance of the JEBAR explicitly. Recently, Sakamoto and Yamagata [1996] introduced the JEBAR to explain the seasonal variations of the Kuroshio transport within a conceptual model framework.
In the present study, we run a high-resolution eddy-resolving Pacific Ocean model to mimic the Kuroshio as much as possible and try to clarify seasonal transport variations of the Kuroshio after comparing the model performance with direct velocity measurements using the Acoustic Doppler Current Profiler in various sections south of Honshu. As suggested by Sakamoto and Yamagata [1996], the JEBAR and/or the bottom torque needs to be carefully taken into account in order to evaluate the seasonal transport variations of the Kuroshio. Therefore we have adopted the Princeton Ocean Model (POM) using a bottom-following vertical coordinate system. In section 2 the model and the procedure in conducting the present experiment are described. In section 3, the model results are compared with observations. In section 4, the seasonal variations of Kuroshio transport are discussed in terms of the torque balance. Section 5 discusses the importance of anticyclonic eddy activities near Nansei Islands to the increase of the Kuroshio transport in summer. Section 6 is reserved for results from a simple theory based on a two-layer planetary geostrophic model with a continental slope. As for details of this section, however, readers are referred to our companion paper of Sakanioto and Yaniagata [1996]. The simple theory explains qualitatively, in terms of the joint effect of baroclinicity and bottom relief, why observed seasonal transport variations of the Kuroshio are weak compared to the prediction based on the Sverdrup balance. Section 7 is devoted to a summary.
2.THE OCEAN MODEL
The model adopted in this study is the Princeton primitive equation ocean model (POM) described by Blamberg and Mellor [1987] and Mellor [1992]. The model domain extends from 115E° to 70°W, and from 20°S to 55°N. The zonal grid spacing is l/3° in the both western (west of 160°E) and eastern Pacific (east of 110°W), and 1°in the central Pacific between 150°W and 160°W. It varies linearly between 160°Band 160°W and between 110°W and l50°W. The meridional grid spacing is 1/3°everywhere. A unique feature of the model is that it is written using a bottom following, sigma coordinate in the vertical direction. The number of vertical levels is 21, and the thickness varies to accommodate to a higher resolution in the upper ocean.
The bottom topography is smoothed to reduce the error in the pressure gradient force [Haney,1991]. The maximum depth is assumed artificially to be 4000m since we are not interested in complex, deep sea circulation in the ocean interior. The prognostic variables of the model are the surface elevation η, the temperature T, the salinity S, the velocity (u, v, w), the eddy kinetic energy and the mixing length. The potential density
