Winter thermal and ice regimes of small Karelian lakes against the background of regional climatic variability

This article investigates regularities of thermal and ice regimes of three small lakes of Karelia under current climatic conditions. Data of measurements of water temperature at autonomous stations and results of numerical calculations of the dates of ice-on and ice-off and the thickness of ice in these lakes using the one-dimensional parameterized model FLake in the anomalously warm winter season of 2019-2020 are analyzed. Data obtained are compared with long-term values over 1994-2019. The dates of the ice-on and ice-off on the lakes were quite close; however, on two larger lakes, intermediate ice destruction was observed at the beginning of winter, due to which the duration of the ice season differed between lakes by two weeks. The winter months of 2019-2020 were 6.4-9.4 ° C warmer than the baseline, which was reflected in a noticeably smaller ice thickness on the lakes compared to previous years of measurements (40-48 cm at the end of March 2020 compared to the values ​​in mid-April in 1994-2004 - 65-85 cm and in 2005-2018 - 50-65 cm). The decrease in ice thickness contributed to an early onset (mid-March) and long duration (more than five weeks) of spring subglacial convection. The model calculation, taking into account the atmospheric impact based on the ERA-5 re-analysis, reproduced the main features of the ice regime of the lakes, including the intermediate destruction of ice at the beginning of winter on two larger lakes. Significant regression relationships have been obtained between the dates of ice-on and ice-off on Lake Vendyurskoe, the dates of the onset and duration of spring under-ice convection, and the characteristics of the regional climate of southern Karelia (air temperature and the number of days with thaw in winter and spring months) for 1994-2020. The relationship between the dates of ice-on and the water temperature in the lake in winter is shown.