Turbulent mixing and heat fluxes under lake ice: the role of seiche oscillations

Abstract

We performed a field study on mixing and vertical heat transport under ice cover of an Arctic lake. Mixing intensities were estimated from small-scale oscillations of water temperature and turbulent kinetic energy dissipation rates derived from current velocity fluctuations. Well-developed turbulent conditions prevailed in the stably stratified interfacial layer separating the ice base from the warmer deep waters. The source of turbulent mixing was identified as whole-lake (barotropic) oscillations of the water body driven by strong wind events over the ice surface. We derive a scaling of ice-water heat flux based on 5 dissipative Kolmogorov scales and successfully tested against measured dissipation rates and under-ice temperature gradients.The results discard the conventional assumption of nearly conductive heat transport within the stratified under-ice layer and suggest contribution of the basal heat flux into the melt of ice cover is higher than commonly assumed. Decline of the seasonal ice cover in the Arctic is currently gaining recognition as a major indicator of climate change. The heat transfer at the ice-water interface remains the least studied among the mechanisms governing the growth and melting of seasonal ice. The outcomes of 10 the study find application in heat budget of seasonal ice on inland and coastal waters. Copyright statement. Author(s) 2018 1 IntroductionSeasonal formation of ice cover is an essential feature of the hydrological regime of temperate and polar climatic zones. Freshwater lakes are, in this regard, a special class of hydrological objects, both in terms of thermal and hydrodynamic processes that 15 control the formation and melting of ice, and from the point of view of the impact of the ice regime of the global freshwater budget. The majority of world lakes is concentrated in the northern temperate and boreal environments between 40 • N and 80 • N and have the potential to freeze seasonally . Seasonally ice-covered lake systems-Lake Baikal, Laurentian Great Lakes, European Great Lakes Ladoga and Onego, lake systems of Fennoscandia and Northern Canadaaccumulate the bulk of the world's surface freshwater. Their seasonal ice regime determines climatic balance of precipitation 20 and evaporation, as well as the ecosystem state and the water quality of lakes themselves. The lakes of high latitudes remain less studied than temperate lakes, while arctic regions are reported to have the strongest air temperature increase (Screen and 1 Hydrol. Earth Syst. Sci. Discuss., https://doi.

Integrating Perspectives to Understand Lake Ice Dynamics in a Changing World

“…As such, under‐ice stratification suppresses upward heat transfer into the ice layer. However, under‐ice currents and internal waves, which historically have been considered negligible, can increase heating up to an order of magnitude, accelerating ice melt (Aslamov et al, 2014; Kirillin et al, 2018, 2020). In turn, stratification conditions in the lower atmosphere vary widely by season but are typically stable during the melting period.…”

“…Such experiments thus have the potential to provide the ultimate test of ecological theory and offer parameter acquisition for physical models that are often unavailable for a large number of lakes or at the appropriate spatial scales. Continuous and precise high‐resolution temperature measurements in experimental setups enable understanding heat transfer at the ice‐water and ice‐atmosphere interfaces, and thus stratification pattern during winter as well as ice melt affected by under‐ice currents and internal waves (Aslamov et al, 2014; Kirillin et al, 2018). Whole ecosystem experiments under controlled conditions allow variables to be isolated or combined, thereby providing increased confidence in the causal links between organismal response and particular global change stressors such as warming and consequently changes in ice and snow coverage of lakes, rivers, and ocean bays (Berger et al, 2014).…”

Fine scale structure of convective mixed layer in ice-covered lake

“…Recent autonomous in situ measurements of flow velocities and temperature have enabled direct observation of radiatively driven convection in ice-covered lakes [2,12,13]. The derived data trigger the progress in studying the energetic of the process, including estimations of buoyancy flux, dissipation rates, and mixing efficiency.…”