4. Dynamical Properties of Coastal Currents in the Northwestern Part of Iriomote Island Part. 1—Sakiyama and Amitori Bays

The physical properties of coastal currents in semi-closed bays (e.g., Sakiyama and Amitori bays) in the northwestern part of Iriomote Island are discussed in this chapter, based on the results of theoretical, observational, and numerical investigations. According to the tidal flow dynamics in a single channel, these bays in this region are categorized as “deep bays,” in which little difference in phase and amplitude of the tidal levels exists from the open ocean. Ocean waters therefore flow in a bay in the incoming tide. By contrast, bay waters flow out to the open ocean in the outgoing tide. In Sakiyama Bay with its shallower waters, surface seawater exchange between the bay and ocean is active under near-calm conditions, leading to an environment in which the river water mass near the surface layer is more likely to flow out to the open ocean. By contrast, in Amitori Bay with the deeper waters, near-surface river water is stagnant under near-calm conditions. It selectively distributes to the eastern side of the bay because of the Earth’s rotation. The theory of tidal flow dynamics in a single channel suggests that differences of coastal currents between Sakiyama and Funauki bays are mainly attributable to the difference of the aspect ratio, the length to depth ratio, between the two bays. Such a difference in physical properties in the coastal currents is expected to affect the coastal ocean ecosystem distribution in this region.