From its characteristic propagation, the Kyucho is theoretically due to internal Kelvin shock waves (Yamagata, 1981), or the density current in a rotating fluid (Kubokawa and Hanawa, 1984b). But, important physical properties of the Kyucho, such as offshore and vertical scales, and current speed have not been sufficiently clarified from the observations, because most observations have been limited to temperature measurements at the coast. Therefore, more detailed information from the measurements is required to clarify the physical process of the Kyucho in Sagami Bay.
On the night of January 9, 1994, the fishing set-net at station KO, Iocated at the western side of the bay head (Fig. 1), was shifted 600 m southward and destroyed by a strong current, i.e., the Kyucho. We carried out measurements of the surface current at two stations and of temperature at four stations near the coast. In this paper, we describe the detailed features of this Kyucho, using the current and temperature records at the moored stations along the bay coast. The generation and propagation system of strong coastal-current is studied through analyses of the Kyucho, in relation to the passing of the front.
2. Observations and data
We carried out the current and temperature measurements at 10 m depth at station EN (water depth of 70 m), 1.5 km south of station KO. In addition, the current and temperature measurements were continuously carried out at 3 m depth at the observation tower (HI in Fig. 1) belonging to the National Research Institute for Earth Science and Disaster Prevention. The temperature measurements were made at two other stations, i.e., the mouth (JG) and head (KA) of the bay shown in Fig. 1 Before the Kyucho began, NOAA IR-images were taken with slight cloud cover in and around Sagami Bay. CTD observations were made at five stations (Fig. 1) in Sagami Bay on January 6, 1994. Sea level changes in and around Sagami Bay are expected in relation to current changes, and they are available for our analysis of the Kyucho. The sea level data are useful for analysis after removing both the tidal effect, by the 48-h tide-killer filter (Thompson, 1983), and the atmospheric pressure effect.
3. Results
3.1. Temperature and current variations near the coast
Fig. 2a shows the time series of the current vector and temperature at station EN after a 1-h running average to remove the short-period fluctuations. The tidal currents had amplitudes of 0.1-0.2 ms-1 throughout the observational period. A strong southward current suddenly appeared at 20:00 on January 9, and its maximum speed was 0.72 ms-1 at 23:20 on the same day. The strong current occurred together with a steep temperature rise of about 2℃.
