[A-OS16] Coastal ocean circulation and material cycle

[AOS16-P15] Monitoring and numerical simulations of flow fields in Lake Kasumigaura

*Sohta Yoshida¹, Eiji Masunaga¹, Tatsumi Kitamura², Takao Ouchi² (1.Ibaraki University, 2.Ibaraki Kasumigaura Environmental Science Center)

Keywords:hypoxia, mixing, wind stress, shallow lakes

Water quality problems such as eutrophication and hypoxia events in lakes and reservoirs have been reported all over the world, seriously affecting water ecosystems and water quality managements. Wind stress is the primary energy source for mixing in closed lakes, thus it is important for understanding water circulations owing to wind forcing. The study site of Lake Kasumigaura is a closed shallow lake in Japan and numerous water environment issues have been reported for this region. However, few studies have reported physical structures associated with flow fields and water circulations in the lake. We analyzed long-term velocity data obtained from the Acoustic Doppler Current Profilers (ADCPs) installed on the bottom of the lake and also conducted high-resolution numerical simulations using SUNTANS to investigate water circulations in Lake Kasumigaura. Model results showed a good agreements with observed field data. Cross-shore winds generate a typical wind-induced vertical flow structure, upwelling and downwelling on the upwind and downwind sides, respectively. By contrast, when the wind rection is along shore, the flow speed (kinetic energy) is largely enhanced along the coast, but the flow speed is not remarkably enhanced in the middle of the lake. The suppressed kinetic energy in the middle of the lake is caused by counter-currents owing to the enhanced along coast currents. These results indicate that responses of water circulations to wind stress are significantly influenced by the size of lakes and the coastal topography. The results also suggest that wind-induced currents are largely intensified along the coast resulting in large basin scale water circulations.