A Numerical Study of the Kuroshio System

Historical hydrographic surveys show that the Kuroshio flowing along the East China Sea (hereafter, ECS) continental shelf flows out through the Tokara Strait and feeds the Kuroshio south of Japan. However, the volume transport through the Tokara Strait is only half (20 ~ 25 Sv) of that of the Kuroshio south of Japan (45 ~ 50 Sv) suggesting the existence of "a missing component of the Kuroshio transport" somewhere, Many researchers have been thinking that there should be a northeastward current along the Pacific side of the Ryukyu Islands (hereafter, RI), which supplies the missing transport to the Kuroshio. However, until recent years, any currents with enough volume transport to supply the missing transport had not been detected along the Pacific side of the RI due to a unique current structure with a subsurface core and strong variability. In recent years, several observations based on direct measurements using Inverted Echo Sounders with pressure gauges (PIES), moored current meters or ADCPs combined with hydrographic observations have been successful in detecting a-current with a unique structure, the Ryukyu Current (hereafter, the RC), along the I~I. This paper describes the structure and variability of the RC generated with the Pacific Ocean circulation model, compares them with observations and tries to clarify the structure of the RC all the way along the RI and the formation mechanism of the RC, "the northeastward undercurrent".