Continuous current measurements in Lake Biwa (I) - Method and some results.

A method of continuous measurements of lake currents and some results obtained in the north basin of Lake Biwa are presented. Two types of current meters were used, both being able to record the current speed and direction and equipped with either a water temperature sensor or a pressure sensor : one is an Aanderaa RCM-4 current meter and another one is a DCM-3B current meter manufactured at Osaka Electro-Communication University. Adopting the subsurface mooring buoy system, observations have been carried out at several locations in the northern part of the north basin of Lake Biwa, mainly in summer and winter since 1981. Analysis of the current measurement data shows the following features of lake currents : (1) In summer, when the lake water is stratified, a counter-clockwise gyre develops above the thermo cline. Surface currents are prominent and stable. (2) In winter, when the lake water is not stratified, surface currents are weak and usually controlled by wind stresses over the lake, so that current directions are deflected to the right-hand side from down wind directions. (3) Both in summer and winter, deep currents are, in general, very weak and have no stable directions. Sometimes, however, the current speed becomes greater than 10 cm·s-1 after a strong wind in summer.