Abstract. Seasonal ice cover on lakes and polar seas creates seasonally developing boundary layer at the ice base with specific features: fixed temperature at
the solid boundary and stable density stratification beneath. Turbulent transport in the boundary layer determines the ice growth and melting
conditions at the ice–water interface, especially in large lakes and marginal seas, where large-scale water circulation can produce highly variable
mixing conditions. Since the boundary mixing under ice is difficult to measure, existing models of ice cover dynamics usually neglect or parameterize
it in a very simplistic form. We present the first detailed observations on mixing under ice of Lake Baikal, obtained with the help of advanced acoustic
methods. The dissipation rate of the turbulent kinetic energy (TKE) was derived from correlations (structure functions) of current velocities within
the boundary layer. The range of the dissipation rate variability covered 2 orders of magnitude, demonstrating strongly turbulent
conditions. Intensity of mixing was closely connected to the mean speeds of the large-scale under-ice currents. Mixing developed on the background of
stable density (temperature) stratification, which affected the vertical structure of the boundary layer. To account for stratification effects, we
propose a model of the turbulent energy budget based on the length scale incorporating the dissipation rate and the buoyancy frequency
(Dougherty–Ozmidov scaling). The model agrees well with the observations and yields a scaling relationship for the ice–water heat flux as a function
of the shear velocity squared. The ice–water heat fluxes in the field were the largest among all reported in lakes (up to 40 W m−2) and
scaled well against the proposed relationship. The ultimate finding is that of a strong dependence of the water–ice heat flux on the shear velocity
under ice. The result suggests large errors in the heat flux estimations when the traditional “bulk” approach is applied to stratified boundary
layers. It also implies that under-ice currents may have much stronger effect on the ice melt than estimated by traditional models.
Large-Scale Climatic Controls on Lake Baikal Ice Cover