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

Abstract

Twenty years (1983–2002) of hourly summer temperature data from the epilimnion of Müggelsee, 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.

A Change of Climate Provokes a Change of Paradigm: Taking Leave of Two Tacit Assumptions about Physical Lake Forcing

“…In neighbouring Lake Constance, STRAILE et al (2003a) found that the mean lake temperature had increased by 0.017 K yr -1 from 1962 to 1998. Based on hourly measurements of the surface temperature of Müggelsee, a shallow, polymictic lake in northern Germany, from 1983, WILHELM et al (2006 found a significant increasing trend in the daily minimum surface temperature, but not in the daily maximum surface temperature, resulting in a long-term increase in the daily surface temperature range. In northern Italy, on the south-facing slope of the Alps, AMBROSETTI and BARBANTI (1999) found that water temperatures in Lago Maggiore had increased at all depths between 1963 and 1998, with the increase being especially noticeable in the deep water at 300-370 m. They also found similar increases in shorter series of measurements from three neighbouring lakes -Lago di Garda, Lago d'Iseo and Lago d'Orta.…”

Effects of water warming on bank filtration: experimental enclosure studies

“…Empirical studies show that surface water temperature closely follows air temperature but it is also influenced by wind speed, humidity, cloud cover and clear-sky solar radiation as a predetermined variable [6,7]. Long-term temperature monitoring studies are mainly performed in the centre of a lake [8], water of littoral zones under calm conditions are liable to stronger heating and cooling with an up-or downwelling of water.…”

The Impact of Climate Change on Lakes in Central Europe

“…As shown by Wilhelm et al (2006), the rate of increase of the daily night time minimum exceeds that of the daily maximum. This day-night asymmetry in epilimnetic temperatures is influenced by the flux of heat across the air-water interface which, in turn, depends on wind speed, relative humidity and cloud cover.…”

“…Data from Müggelsee demonstrate that the impact of storms is critically dependent on the antecedent conditions i.e. whether they occur suddenly or build up gradually (Wilhelm et al, 2006).…”