Deep waters warming in Italian lakes: secular variations

The hypolimnion of 5 deep lakes (Maggiore, Como, Garda, Iseo e Orta) in the subalpine region contain a layer of water characterised by a sort of "climatic memory". The climatic memory was traced with continuity from the end of the 19th century to the beginning of 20th century and from 1950 to 2007. It showed a series of warming and cooling phases. Using the winter mixing depth, the heat exchange within the water body and the correlation Kz-N2 during the stratification phase the climatic memory heat content was determined. The analysis of the heat content of indicates its dependence not only on the ongoing rise of air temperature, but also on particular hydro-meteorological events such as cold water intrusion at depth. It was also observed that the pattern of all the 5 subalpine lakes' climatic memory is similar and that it is comparable to that found in the historical series of deep water temperatures in other lakes and seas around the world. In particular, drastic drops in heat content have been observed simultaneously in different environments suggesting that triggering events act on a global scale. Therefore, the climatic memory response shows the complexity of the ongoing climatic change.