Internal energy, the work of the wind, and the thermal stability in Lake Tyrifjord, southeastern Norway

Abstract

Lake Tyrifjord consists of Holsfjord and Steinsfjord, two lakes of highly contrasting morphology, was used as the basis for a comparison of internal energy, the work of the wind, and thermal stability in two lakes under identical large-scale climatic conditions. Because of simultaneous temperature measurements available for the summer of 1973 from the two lakes, the present paper is based on these observations. The deep Holsfjord (z_max = 295 m) accumulates internal energy of an order representative for its geographical situation. Its annual heat budget in 1973 was 129 735 j cm⁻2. Steinsfjord (z_max = 22 m) accumulated 60 per cent of the latter. At the stratifications representing maximum stability in 1973, the stability S in Holsfjord exceeded the corresponding value in Steinsfjord by a factor of 37, whereas the work of the wind B in Holsfjord was only 1.45 times the Steinsfjord B value. The much greater center of volume moment under homoiothermal conditions typical of Holsfjord was responsible for the great difference between energy conditions in the two lakes, as clearly demonstrated by the B + S values. The ratio (B + S)/B shows that the water masses of Steinsfjord would maintain homoiothermal conditions throughout the heating season with an increase in B (i.e. an increase in external energy supply) of ca 40 per cent. The additional energy supply needed to maintain homoiothermal conditions in Holsfjord was of the order of 1200 per cent of B for Holsfjord. Accordingly, the metalimnetic thermal discontinuity represents a far more effective mixing barrier in Holsfjord than in Steinsfjord. Calculations of the (B + S)/B ratios for other lakes show that this ratio is useful for a judgment of the degree of stagnancy of a lake's hypolimnetic water masses.