The effect of the exceptionally mild European winter of 2006–2007 on temperature and oxygen profiles in lakes in Switzerland: A foretaste of the future?

The European winter of 2006–2007 was unusually mild, with record high mean winter air temperatures comparable with those predicted to become the norm by the end of the current century as a result of climate warming. In Lake Zurich and Greifensee, two neighboring Swiss perialpine lakes with several decades of data, mean lake temperatures for this winter were the highest ever recorded, as was thermal stability. Associated with the high thermal stability, mean winter oxygen concentrations in Lake Zurich were unusually high in the epilimnion and metalimnion, but normal in the hypolimnion. In Greifensee, however, which is much shallower, mean winter oxygen concentrations did not deviate substantially from the norm anywhere in the water column. From 17–19 January 2007, an unusually severe cyclonic storm, "Kyrill," traversed Europe. Monthly oxygen profiles suggest that the stabilizing effect of the mild winter on the two lakes was greatest before the occurrence of the storm, and that wind mixing resulted in a deepening of the mixed layer in both lakes. The mixing was able to encompass the entire water column of Greifensee, but not of Lake Zurich. These results, supported by more limited data from two other neighboring lakes, suggest that climate warming will likely inhibit complete mixing of some deep, temperate, normally monomictic lakes in winter even when extremely intense cyclonic storms occur. In shallower lakes, however, complete mixing is unlikely to be inhibited.