It has been pointed out by numerous authors that estimates of the vertical mixing obtained by microstructure profiling techniques in lakes and the ocean can, in general, not explain the total mixing suggested by the global budgets of matter and heat. Searching for additional mixing mechanisms, boundary mixing has been identified as a key process. In this study, a boundary mixing process driven by a gravitationally unstable bottom boundary layer (BBL) is investigated. Such unstable BBLs in stably stratified basins are the result of strong up-slope currents, typically caused by internal seiching in lakes or by long internal-inertial waves in the coastal ocean. Due to the fact that in a turbulent BBL, the up-slope speed increases with increasing distance from the sediment, heavy (i.e. cold or salty) water in the BBL may be advected above lighter (i.e. warmer or fresher) water, leading to convective mixing. Inversely, down-slope currents lead to a suppression of mixing. We plan to investigate this process with an interdisciplinary approach: our primary study site will be Lake Constance, where the BBL dynamics and turbulence will be studied in detail for a limnological set-up. Our oceanographic study site will be the Baltic Sea, where a similar mixing process takes place. A numerical turbulence model will be used to analyze this process at the measuring sites, study the basin-wide effect, and construct a general theory.
