[A-OS22] Ocean circulation and material cycle in coastal seas

[AOS22-03] Seasonal and interannual variability in Kyucho and Bottom Intrusion in the Bungo Channel

*Akihiko Morimoto¹, Ren Yamaoka¹, Shitaro Tokuda¹ (1.Ehime University)

Keywords:Bottom Intrusion, Bungo Channel, Kyucho

The Bungo Channel, one of entrances of the Seto Island Sea, is strongly influenced by perturbations from the Pacific Ocean such as the Kuroshio approaching to the channel and frontal eddy of the Kuroshio. The phenomena called Kyucho and Bottom Intrusion have been well known as sudden intrusion event of water masses in the Pacific Ocean into the Bungo Channel. As the studies in terms of the Kyucho have been conducted since 1980s, seasonality and spring-neap tide variations, and generation mechanism of the Kyucho were revealed. Long term variability of the Kyucho intensity was investigated by Takeoka et al. (2000), and it was found that the intensity drastically decreased in 1990s. On the other hand, studies in terms of the Bottom Intrusion was limited because it was difficult to capture the phenomena since the Bottom Intrusion occurred in the bottom layer. Kaneda et al. (2002) revealed the seasonality and spring-neap tidal variation in the Bottom Intrusion from mooring water temperature data at 68 m depth in the small bay of eastern coast of the Bungo Channel. In addition, Takeoka et al. (2000) showed long term variation in the intensity of the Bottom Intrusion from 1972 to 2000 with use of monthly hydrographic data.
As there is no report in terms of long term variation in Kyucho and Bottom Intrusion (BI) after 2000, we investigated interannual and seasonal variations in Kyucho and Bottom Intrusion after 2000 by using water temperature data which were observed in Uchiumi Bay from 1997 to 2016. Data interval is 30 min. and observed depths are 2, 5, 10, 15, 20, 30, 40, 50, and 60 m depth. High frequency signals were removed and make the daily data.
Number of occurrences of the BI were greater from May to November and maximum BI occurred in August (1.45 times/month). Intensity of the BI (water temperature drop range during the BI) were the almost the same (1.4 – 2.0 °C) throughout the year. It was not recognized the relationship between occurrence of the BI and moon age from our dataset although Kaneda et al. (2002) suggested the relation. We investigate the relation between the Kuroshio distance from the Cape Ashizuri and occurrence of the BI. The BI occurred in the almost the cases when the Kuroshio existed within 30 NM from the Cape Ashizuri. However, the BI occurred 4 times during the Kuroshio located 70 NM far from the Cape Ashizuri. As for long term variation in the BI, we could not recognize a clear long term trend. In the presentation, we will also show results of variations of Kyucho, and discuss the relationship between Kyucho and BI.