Summer temperatures, autumn winds, and thermal structure under the ice in arctic lakes of varying morphometry

Ice‐covered lakes differ strongly from their ice‐free counterparts in their temperatures, heat budgets, and hydrodynamics. Nevertheless, due to a lack of detailed measurements, processes under the ice and their drivers are still understudied. Here we present a multiyear study of five arctic lakes ranging from 1 to 150 ha focusing on temperature and thermal stratification during ~ 200 d of ice cover. Mean water temperatures during the ice‐covered period, calculated from ice‐on until the end of February, ranged from 1.5°C to 3.1°C. They depended on lake size, water temperatures in summer, and conditions during the weakly stratified period before ice‐on. Temperatures of the smaller lakes were primarily controlled by water temperatures in summer, while heat loss during the weakly stratified period contributed more for the two comparably larger lakes whose surface area exceeded 10 ha and depth 10 m. Heat budgets indicated consistent losses of ~−1 W m−2 through the ice. Solar radiation during fall contributed significantly to the heat budget in years with reduced snow cover and drove penetrative convection. Sediment heat fluxes during early winter were up to 6 W m−2 in the smaller lakes and more than 10 W m−2 in 150 ha Toolik Lake. The extent of stratification at ice‐on depended on the magnitude of winds in the preceding day or two. The combined factors led to strong interannual and between‐lake variability in lakes of the same geographical region and will improve the understanding of arctic lakes in a changing environment.