Eighty years of spatially coherent Austrian lake surface temperatures and their relationship to regional air temperature and the North Atlantic Oscillation

Eighty years of monthly mean lake surface temperature (LST) data from eight lakes in the northern perialpine area of Austria show a high degree of coherence among lakes in all seasons and reflect much of the temporal structure of the regional air temperature. Coherence is least in winter because of the distorting effect of varying periods of ice cover. In spring, regional coherence in meteorological driving forces that are essentially uncorrelated with air temperature (e.g., geostrophic wind speed) contribute to the coherence in LST. presumably by partially determining the timing of the onset of stratification. In summer, spatial coherence in LST appears to be related directly (via the radiation balance) and/or indirectly (via air temperature) to large-scale variations in high-altitude cloud cover. Correlations of the Austrian LSTs with (1) seasonal indices of the North Atlantic Oscillation (NAO), (2) the timing of spring ice break-up in Finland, and (3) air temperatures in northern and western Europe, suggest that from autumn to spring, spatial coherence of LST in central Europe is related to the dominance of the weather by large-scale climatic processes occurring over the North Atlantic, whereas in summer the processes responsible are more regional in nature. The influence of the NAO on LST is greatest in low-lying lakes in which periods of ice cover are infrequent and short.