Impact of secular climate change on the thermal structure of a large temperate central European lake

Strong climate-related secular trends are apparent in a 52-yr long (1947-1998) uninterrupted series of monthly temperature profiles from Lake Zurich, a large, deep (136 m), temperate lake on the Swiss Plateau. Decadal mean water temperatures have undergone a secular increase at all depths, reflecting the high degree of regional warming that occurred in the European Alpine area during the 20th century. From the 1950s to the 1990s, high warming rates (similar to0.24 K per decade) in the uppermost 20 m of the take (i.e., the epi/metalimnion) combined with lower warming rates (similar to0.13 K per decade) below 20 m (i.e., in the hypolimnion), have resulted in a 20% increase in thermal stability and a consequent extension of 2-3 weeks in the stratification period. In common with many other parts of the world, 20th-century climate change on the Swiss Plateau has involved a steep secular increase in daily minimum (nighttime) air temperatures, but not in daily maximum (daytime) air temperatures. With respect to both secular change and decadal-scale variability, the temporal structure of the temperature of the surface mixed layer of Lake Zurich faithfully reflects that of the regional daily minimum air temperature, but not that of the daily maximum. The processes responsible for longer-term changes in the temperature structure of the lake therefore act during the night, presumably by suppressing nighttime convective cooling of the surface mixed layer. Application of a one-box heat exchange model suggests that the observed secular changes in thermal structure are due to shifts in the nighttime rate of emission of infrared radiation from the atmosphere and in the nighttime rates of latent and sensible heat exchange at the air-water interface. The increase in hypolimnetic temperatures is mainly a result of the increased prevalence of warm winters in Europe.