Solitary Internal Waves in Lake Biwa

Abstract

This paper describes the behavior of large-amplitude nonlinear internal waves observed in Lake Biwa. A thermistor chain was moored in November of 1992, when the water column in the lake was still well stratified. After a continuous strong north-west wind, a wave train with two distinctive parts was observed traveling from the west to the east. First, a sharp front which caused a temperature decrease of 3.5℃ within two minutes appeared. Next, 10 spatially coherent pulses and a lower-amplitude wave train that was less coherent then followed it. We interpret the initial pulses as solitons according to the nonlinear wave theory. The trailing waves can be well-explained by the infinitesimal wave theory. It can be shown by a two layer model (with a thick homogeneous epilimnion, a thin hypolimnion and the thermocline simplified to an interface) that the wave lengths of the solitons are about 51m to 63m and their speeds are estimated within a range of 18.92cm/s to 22.06cm/s. Thermocline elevations which are predicted by the two-layer model are consistent with the characteristics of the solitons. Subsequent analysis shows that the solitons with periods of 6 to 10 minutes were generated in the western coastal area approximately 5km away from the sampling station.