Deep water warming in lakes: An indicator of climatic change

Abstract

The deep waters of Lake Maggiore, Italy (zmax =370 m) contain a sort of "climatic memory", represented by variations in the caloric content, which has been located in time and space, and its energetic value calculated on the basis of the depth of the winter mixing, the heat exchange in deep waters and the Kz-N2 correlation's during the summer stratification. This analysis, relating to the period 1963-1998, demonstrates that the caloric content in the hypolimnion at the end of the limnological year, i.e. when deep waters are formed, depends strictly on winter meteorological parameters (e.g., wind run, air temperature and solar radiation), as well as on the quantity of heat that can reach the deep layers before and after the onset of thermal stratification. The amount of heat that can reach the deep hypolimnion is around 2-4% of the total heat budget of the lake; while its percentage is small, this quantity of heat can assume considerable importance, and lead to a progressive increase in the heat content of deep waters. The same percentage has been found in the other deep Italian lakes in the sudalpine region, i.e. lakes Como, Iseo, Orta and Garda (zmax between 143 and 410 m). All the lakes also have the same temporal evolution of the climatic memory: both the increasing overall trend and the variations (both positive and negative) occur simultaneously. The pattern we observed is quite similar to that found in historical series of deep water temperatures in lakes in Central Italy (L. Bolsena), North of the Alps, equatorial Africa (L. Victoria) as well as in the Mediterranean below a depth of 2000 m and in the Greenland Sea at depths between 200 and 2000 m. Such a world-wide distribution of the phenomenon suggests that it may be regarded as part of an ongoing process of global climate change.