A history of research into the geostrophic gyres of Lake Biwa

Abstract

The geostrophic gyre system in Lake Biwa, the largest freshwater lake in Japan, was discovered in 1926. At first, this steady flow system over the lake was assumed to be a direct response to the summer monsoon. It was recognized as a geostrophic current in 1960, and was the focus of intensive observations and theoretical studies until the 1980s. The main part of the system is the anticlockwise 'First Gyre', which occupies the largest circle that can be drawn in the lake's North Basin. The gyre is continuously present from May to December each year. Its disappearance in winter implies that its existence depends on stratification. Because a stratified structure stores energy as a set of kinetic and available potential energies, we can explain the origin of the gyre system both by a "wind stress curl" and a "topographic heat accumulation effect". Therefore, a quantitative evaluation of the energy budget is necessary. Although a rough description of the overall gyre system has been established previously, unresolved problems still remain concerning, e.g., the detailed formation and maintenance mechanisms of the gyre system, and its interactions with other phenomena in the lake.