Boundary mixing in the thermocline of a large lake

Abstract

High-resolution measurements of near-bottom temperature stratification and current velocity were performed on the sloping boundary of a large lake at the depth of the seasonal thermocline. The measurements cover nearly the entire stratified period and reveal the periodic occurrence of strong temperature and current velocity fluctuations, which can be attributed to shoaling high-frequency internal waves with periods between 5 and 20 min. Two different techniques are applied to obtain a long-term record of dissipation rates of turbulent kinetic energy from the current velocity measurements. Shoaling of high-frequency internal waves is associated with a strong and rapid increase of turbulent dissipation rates of up to four orders of magnitude. Since the occurrence of high-frequency internal waves on the slope is correlated with the passage of the dominant basin-scale internal Kelvin wave with a period of four days, energy dissipation rates on the slope vary with the same period. Diapycnal diffusivities, estimated by combining the dissipation estimates with simultaneously measured density stratification, follow a similar dynamics and a comparison with a basin-scale diffusivity estimate based on tracer measurements reveal the importance of boundary mixing, which, at this particular site, is mainly driven by the interaction of high-frequency internal waves with the sloping boundary.