Methodological Approaches and Results of an Analysis of the Climatic Seasonal Course of Stable Stratification Parameters of a Dimictic Lake (Case Study of the Central Part of Lake Ladoga)

On the basis of general physical concepts of the thermic of a dimictic lake and the proposed methodological developments, the average climatic course of the parameters of the thermocline during the period of stable stratification for the central part of Lake Ladoga is reproduced. We consider the joint distribution of the seasonal variation of the water surface temperature; the temperature of the mixed layer, which differs by no more than 0.5°C; and the deepening of the upper boundary of the thermocline, the depth of the maximum density gradient, and the depth of the lower boundary of the thermocline, which characterize their climatic course. Statistical analysis of vertical temperature profiles measured over a long period of time from 1897 to 2017 made it possible for the first time to develop empirical quantitative relationships between stratification parameters with high determination coefficients. Based on the difference in the physical characteristics of the processes of heating and cooling of the lake surface, useful regression relationships were found for each of the periods which describe the seasonal variation of the thermal parameters of Lake Ladoga. The rates of change in water temperature and change in the parameters of the thermocline are estimated. The forms of the proposed approximation dependences can be used to study the thermic of other dimictic lakes.