Continuous Current Measurements in Lake Biwa-III-

Results of Long-Term Observation over One Year

Abstract

In this paper we report on lake currents in Lake Biwa based on the results of long-term observations of lake currents using the Eulerian method and the characteristics of periodical current variation.
Currents of fixed direction in a thermal stratification period were observed by current meters in the epilimnion, which reveal the existence of gyres but do not confirm their existence in non-stratification periods. The water temperature of the hypolimnion is about 7°C throughout the year, while the maximum water temperature in the epilimnion rises to about 30°C in the summer, a difference of more than 20°C. There is not much fluctuation in the currents during the nonstratification period. During thermocline formation, when the water temperature of the epilimnion was more than 10°C, the fluctuation in currents gained in intensity, while the average current velocity varied irregularly.
A spectrum analysis based on long-term observation data of lake currents was performed using FFT (fast Fourier transform). In periods of 80-90 hours and 24 hours, wind-forced oscillation was observed. The diurnal variations in the 24-hour-period during non-stratification were marked. In a 40-50-hour-period, an internal wave was observed, although not clearly. In a 20-hour-period, inertia oscillation was observed, and an internal Poincare wave was occasionally observed in a 11-17-hour-period.