The kyucho in the Bungo Channel plays an important role in determining the marine environment in the channel. The kyucho promotes water exchange in the bays along the eastern coast (Koizumi, 1991), suppressing eutrophication due to fish farming. Moreover, it plays an important role in maintaining biological production in the channel. Since the warm water of the kyucho originates from the Kuroshio which contains poor nutrients and as a result is transparent, the water in the Bungo Channel turns transparent due to the occurrence of the kyucho. Therefore, the kyucho is called “sumishio” (transparent sea water in Japanese) by local fishermen. However, blooms of phytoplankton (mainly diatoms) usually occur after the kyucho. Koizumi and Kohno (1994) and Koizumi et al. (1997) revealed that these blooms are caused by a combination of the kyucho and bottom intrusions (see Subsection 4.1, as for the bottom intrusion).