Naumenko M.A.

Сlimatic quantitative relationships are established between the surface water and air temperatures, the areas of the stratified (Twsur > 4 °C) and the isothermal area (Twsur < 4 °C) in Lake Ladoga during vernal frontal zone (thermobar) period. Initial data were based on the temporal spatial distributions of the temperature of the water surface of Lake Ladoga and the upper air temperature. These empirical regression models indicate a stable relationship between the selected parameters and allow their use in the analysis of climatic variations. The climatic values (norms) of the surface water and air temperature and the dates of their occurrence during the existence of the spring thermal zone on Lake Ladoga are determined, relative to which deviations should be estimated.The spring minimum temperature of the water surface in the deepwater part of Lake Ladoga during the appearance of the thermal frontal zone is a kind of indicator (“memory”) of the winter state of the lake and the value from which heating begins after ice melting and which largely determines the date of disappearance of the front. The approximation of the temporal course of the surface water temperature’s minimum in the deep-water zone of the lake allows us to estimate the time of onset of full thermal stratification in Lake Ladoga. Interannual variations in the date of disappearance of the isotherm on the water surface of 4 °C changes from one to four weeks in relation to norm, this is due to differences from year to year of the minimum surface temperature of the lake in the deepest part of the lake, determined by winter conditions, wind conditions, cloud cover and, therefore, the intensity of lake-atmosphere heat exchange in Lake Ladoga. The rate of increase in the water surface temperature of the stratified zone is much greater than the rate of heating of the isothermal region.

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