CLIMATE CHANGE IMPACTS ON SEAWATER EXCHANGE IN THE ARIAKE SEA AND THE YATSUSHIRO SEA UNDER THE RCP SCENARIOS

Abstract

 In the Ariake Sea, red tides have caused a serious Discolorationof Nori and the number of useful bivalves such as clams. And, in the Yatsushiro Sea, red tides have caused the death of cultured fish. Seawater exchange suppresses the development of redtide of these hytoplankton species causing color bleaching of cultured Nori. To clarify climate change impacts on seawater exchange between the Ariake Sea, the Yatsushiro Sea, and the open ocean, we performed numerical simulations under the present (the end of the 20th century) and the RCP8.5 future climate conditions using an integrated river basin – coastal ocean hydroenvironment assessment model. The net seawater flow shows a clockwise flow from the open sea into the Ariake Sea via Tachibana Bay and outflow into the open sea via the Yatsushiro Sea. The seasonal characteristics of the flow through the mouth of the Ariake Sea were observed to be stronger in winter and weaker in summer, corresponding to river discharge and precipitation and evaporation in the sea area. As an effect of climate change, river discharge tended to increase in summer and decrease from autumn to winter. Under RCP8.5 which has the strongest warming tendency compared to the present climate, the net inflow from the open sea to the Ariake Sea is predicted to become more extreme with a 40% decrease during July and August and a 20% increase during January and February. In addition, the analysis by Self-organizing map revealed that the remarkable increase in inflow frequency occurs when there are strong northwesterly winds and SST of the Ariake Sea is equivalent to that of the open sea.