Measurements of deep currents by ADCP attached on the real-time deep seafloor observatory

Three systems of real-time deep seafloor observatory were deployed in the northwest Pacific around Japan. The first system was deployed in the Sagami Bay, 7 km off Hatsushima Island, central Honshu in 1993. It was developed as a seafloor monitoring network. The observatory was renewed in March 2000. The ADCP was installed in the observatory at a renewal. The second system was deployed at the landward slope of the Nankai Trough, 120 km off Cape Muroto, Shikoku, in March 1997. The third system was deployed at the landward slope of the southern Kurile Trench, 220 km off Kushiro, Hokkaido, in August 1999. The observation system consists of bottom seismometers, tsunami pressure gauges and the cable end station which is composed of several environmental monitoring instruments. ADCP (RDI BBADCP 150 kHz) is attached on the cable end station as the bottom current monitoring instrument. Measurement data of each observatory are transmitted to the land station in real-time using a submarine electro-optical cable. Bottom current profiles are measured as ranges of about 440 m in the Sagami Bay, about 150 m in the Nankai Trough, and about 360 m in the southern Kurile Trench using the ADCPs. The following phenomena were obtained as results and topics of the observation. (1) Deep exchange flow, which was divided in the internal boundary, was observed in the Sagami Bay. (2) As the features of the deep currents, the southwestward current can be observed on the landward slope in the Nankai Trough. (3) The strong bottom eddies were observed on the slope in the southern Kurile Trench.