Hypolimnetic turbulence generation associated with superposition of large-scale internal waves in a strongly stratified lake: Lake Biwa, Japan
A 48-h microstructure experiment captured the variation of turbulence in Lake Biwa, Japan, during a strong stratification period, 1 week after a typhoon event. A free-fall microstructure profiler (TurboMAP) and a fine-scale profiler (F-probe) were deployed. An array of five ADCPs positioned close to the experiment site provided current measurements associated with the internal-wave field for a period of one week. Strong winds related to the passage of a typhoon close to the lake generated two low-frequency internal waves: a Kelvin wave and an inertial wave. Both waves were in the first baroclinic mode. This study pictures the superposition of two large-scale internal waves and a stronger current resulting from the two waves being in phase. The synchronization of the waves provoked enhanced shear in the hypolimnion, more than 10 m above the bottom, as well as high dissipation (10−7 W kg−1) and vertical diffusivity reaching 10−4 m2 s−1. The enhanced shear event seems to be related to the current amplitude since it occurred when the current amplitude was increased by the diel wind. Assuming the same turbulence intensity for the enhanced hypolimnetic shear events, this superposition may affect nutrient transfer in the hypolimnion. Also, we witnessed sediment resuspension, consisting of high turbidity and high fluorescence intensity, likely due to a combination of bottom stress and enhanced turbulence.
KeywordsLarge-scale internal waves Superposition Hypolimnion Turbulence Sediment resuspension
We thank the crew of the R/V Hakken of the Lake Biwa Environmental Research Institute for their assistance during the field experiment. We also acknowledge the technicians who fixed the TurboMAP probes. This work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan. We also acknowledge the comments provided by two anonymous reviewers on the manuscript, and Ms Rosalie Rutka for her editorial work on the manuscript.
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