[A-OS19] Sea level rise under global warming and its impact on coastal areas

[AOS19-02] Mechanism of unusually high sea levels around the southern coasts of Japan in early September 1971

★Invited Papers

*Nariaki Hirose¹, Norihisa Usui¹, Kei Sakamoto^1,2, Nadao Kohno¹, Goro Yamanaka¹ (1.Meteorological Research Institute, Japan Meteorological Agency, 2.Japan Meteorological Agency)

Keywords:unusually high sea level, coastal-trapped waves, Kuroshio, coastal ocean assimilation system

To reduce disaster risk to coastal communities, it is essential to reveal the mechanisms of hazardous events in the coastal area. In early September 1971, coastal flooding occurred along the southern coasts of Japan from Tokai to Kanto regions within a few days, in spite of no severe weather conditions. High sea level anomalies (SLAs) by 20 - 40 cm lasted for more than a week and could not be explained as storm surges and high waves, so that, the high SLAs event attracted wide social attention at that time. Previous studies suggested two possible factors for this unusually high sea level (UHSL) event: coastal trapped waves induced by strong wind associated with a typhoon, and Kuroshio warm water approaching coasts. However, these two factors remained qualitative considerations, and the relative importance between them has not been quantitatively clarified yet. Here, the UHSL event is examined by using a coastal ocean assimilation system with 2-km resolution, which is similar to the operational system run by Japan Meteorological Agency. The control assimilation experiment successfully represented the observed SLA. To separate the possible factors, two sensitivity experiments were performed. The first sensitivity experiment, where the wind forcing is replaced by climatological wind, revealed that coastal trapped waves which occurred along the southeastern coasts of Japan propagated westward along the southern coasts of Japan by about 2.3 m/s speed, and that the sea level increased by about 10 to 15 cm from Boso Peninsula to Sagami Bay. The second sensitivity experiment, where temperature and salinity fields are constrained by the state before the UHSL event, showed the approach of the Kuroshio warm water toward the Suruga Bay caused the sea level rise by about 30 cm from the bay to the west. The high SLAs in the control experiment were approximately explained by a sum of those in the sensitivity experiments, so that it is strongly suggested that the UHSL event in early September 1971 is caused by the superposition of the coastal trapped waves and the warm water intrusions by the Kuroshio. These findings would help us to understand the spatial dependence of sea level rise along the coasts of Japan under the future global warming.