Internal waves and the generation of turbulence in the thermocline of a large lake

Abstract

High-resolution thermistor chain data collected between 5 and 20 m in a large stratified freshwater lake (Lake Constance) at a water depth of 60 m reveal the frequent occurrence of large-amplitude (> 1-m) vertical density inversions that indicate overturns in the pelagic thermocline. Velocity data collected simultaneously with the temperature measurements indicate that the density inversions are mainly produced by shear instabilities. A comparison between the timing of the passage of the basin-scale internal Kelvin wave and the density inversions demonstrates a pronounced phase relationship, implying that the processes leading to the occurrence of turbulence and mixing are connected to the passage of the Kelvin wave. Two different processes are identified during which density inversions were particularly common. An increased number of density inversions and especially high dissipation rates of turbulent kinetic energy were observed when Kelvin wave induced critical shear supported the generation of large Kelvin Helmholtz billows. A particularly large number of density inversions was also associated with the passage of nonlinear highfrequency waves of large amplitudes. The density inversions typically occurred at the wave troughs, which indicates breaking of these waves. These observations indicate that self-induced shear generated by the basinscale seiche and by high-frequency internal waves leads to a significant amount of turbulence and mixing in the pelagic thermocline.