The Effect of the Bottom Boundary on Diapycnal Mixing in Enclosed Basins

Tracers released into the pelagic, stratified center of lakes, show that interior mixing—based on both mode-related internal shear and higherfrequency internal waves – is extremely weak. After horizontal spreading, the tracers “feel” the local bottom boundaries (BBL) and the basin-scale diapycnal (vertical) mixing increases by more than one order of magnitude. Balancing kinetic energy and dissipation reveals that the observed internal energetics and mixing can be explained by classical bottom friction of the basin-scale (seiches) or inertial currents alone, without relying on further processes. The assumption that the buoyancy flux is generated within the BBL and diffusively intrudes into the interior is compatible with the observed tracer residence time scale in the interior. Within enclosed water bodies of limited extent, the BBL structure is modified due to the strong periodicity (typically several hours to days) of the near-bottom currents. This Stokes’ solution-like behavior (of the oscillating currents) can also be expected to be important in estuaries and ocean subbasins, where the 12 and 24-hours periodicity is relevant.