Small‐scale turbulence and vertical mixing in Lake Baikal

The water column of Lake Baikal is extremely weakly#x2014;but permanently—stratified below 250 m. Despite the thickness of this relatively stagnant water mass of more than 1000 m, the water age (time since last contact with the atmosphere) is only slightly more than a decade, indicating large‐scale advective exchange. In the stratified deep water, the fate of water constituents is determined by the combined action of advective processes (deep‐water intrusions) and small‐scale turbulent vertical diffusion. Here, vertical diffusivity is addressed through the analysis of 25 temperature microstructure profiles collected in the three major basins of Lake Baikal to a depth of 600 m. In addition, in the 1,432‐m deep south basin, monthly CTD profiles and a two year record of near‐bottom currents were analyzed. Balancing turbulent kinetic energy and small‐scale temperature variance leads to the conclusions that (1) vertical diffusivity in the stratified deep water ranges from 10–90 × 10 ‐4 m ‐2 s ‐1 (between 600 and 250 m), which is three orders of magnitude more than estimated by Killworth et al. (1996), (2) the mixing efficiency is ∼0.16, comparable to that found in stronger stratification (e.g., the ocean interior), (3) turbulence under ice decays with a time scale of 40 ± 2 d and (4) the interior of the permanently stratified deep water below 250 m and the bottom boundary layer contribute roughly equally to the TKE production. The latter implies, that mixing in the deep water of Lake Baikal's three sub‐basins is dominated by bottom boundary mixing as found in smaller lakes and ocean basins.