Comparative Study on Vertical Circulation in Deep Lakes: Lake Biwa and Lake Ikeda

摘要

IPCC (Intergovenmental Panel on Climate Change) reported that the globally mean atmospheric temperature on the earth surface will increase by several degrees in the following 100 years. The increase in the atmospheric temperature may have unfavorable effects on the physical environment in deep lakes. In particular, the effects of global warming on the overturn are worried because the overturn plays a key role in supplying oxygen to the bottom layer in deep lakes of the Temperate Zone. The water temperature in the surface layer will represent the similar variation with the atmospheric temperature. The overturn will be ceased if the increasing rate of water temperature is lower in the bottom layer than in the surface layer. Hence, the vertical advection and diffusion of heat have to be examined in order to predict the variation in water temperature in the bottom layer. In the present study, water current and temperature fields were reproduced by a three-dimensional numerical model for Lake Biwa and Lake Ikeda, in which the overturn continues and often stops, respectively. Then the vertical advection and diffusivity were examined in order to understand the mechanism of the variation in water temperature in the bottom layer. As a result, time variations in water temperature were reproduced well by the three-dimensional numerical model in order to examine the mechanism of vertical advection and diffusion of water temperature. The water temperature continued increasing gradually during stratified seasons in deeper waters of Lake Biwa, whereas the increasing rate of water temperature was lower in Lake Ikeda. The absolute values of advection terms were larger in the mixing layer above thermocline because of the wind-driven current. The eddy diffusivity terms play a key role in increasing water temperature in deep waters. The value of vertical eddy diffusivity in deeper waters was larger in Lake Biwa than in Lake Ikeda even at the same depth. The larger value of vertical eddy diffusivity will contribute to the gradual increase in water temperature in deeper waters of Lake Biwa. The larger eddy diffusivity may come from the intermittent large variations in advection term, which may be attributed to wind-driven current, bottom stress, the generation of internal waves and their breaking, etc. The reason of the larger eddy diffusivity in Lake Biwa has be examined precisely as future works.