Long-term response of daily epilimnetic temperature extrema to climate forcing

Twenty years (1983-2002) of hourly summer temperature data from the epilimnion of Muggelsee, a shallow lake in northern Germany, showed a long-term increase, with the rate of increase of the daily minima (nighttime temperatures) exceeding that of the daily maxima (daytime temperatures). This does not simply reflect the long-term behaviour of air temperature, which did not exhibit a significant degree of day-night asymmetry. A sensitivity analysis based on a heat-balance model revealed that the daily extrema of the lake surface equilibrium temperature responded differently not only to shifts in air temperature, but also to shifts in wind speed, relative humidity, and cloud cover, suggesting that long-term changes in all four variables contribute to day-night asymmetry in the epilimnetic temperature. A comparison of nighttime and daytime estimates of the heat flux components into the lake indicates that the emission of long-wave radiation from the atmosphere is likely to be the main process responsible for day-night asymmetry in the epilimnetic temperature. Although this process is partially dependent on air temperature, it is also dependent on relative humidity and cloud cover. The influence of long-term changes in these additional driving variables on epilimnetic temperatures cannot therefore be neglected.