Partner und Internationale Organisationen
(Englisch)
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Institute of Freshwater Ecology (UK), Trinity College, Dublin (IRE), University of Helsinki (FIN), University of Uppsala (S), Austrian Academy of Sciences (A), University of Innsbruck (A), University of Constance (D), Climate Research Unit, University of East Anglia (UK)
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Abstract
(Englisch)
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A detailed investigation of a unique 52-yr (1947-1998) uninterrupted series of monthly temperature profiles from Lake Zurich revealed that decadal mean water temperatures have undergone a secular increase at all depths, reflecting the high degree of regional warming occurring in central Europe. From the 1950s to the 1990s, high warming rates in the epi/metalimnion (~0.024 K·yr-1) combined with lower warming rates in the hypolimnion (~0.013 K·yr-1) have resulted in a 20% increase in thermal stability and a consequent extension of 2-3 weeks in the stratification period at the expense of the period of homothermy. Regional warming in central Europe is known to occur mainly during nighttime. The temporal structure of the epilimnetic temperature of Lake Zurich was found faithfully to reflect that of the regional daily minimum (night-time) air temperature, but not that of the daily maximum (daytime) air temperature, implying that long-term warming processes in Lake Zurich are also acting during the night. Additionally, a strong signal from the North Atlantic Oscillation (NAO) was found in the total heat content, epilimnetic temperature and hypolimnetic temperature of the lake in winter and spring, stressing the importance of large-scale climatic influences on lake thermal structure.
Within the framework of the REFLECT project, the physical lake model 'SIMSTRAT' was developed, tested, improved and employed to make simulations of the thermal structure of various lakes, including Lake Zurich, Greifensee, Baldeggersee, Alpnachersee and Esthwaite Water. SIMSTRAT combines a buoyancy-extended k-e model with a simple seiche excitation and damping model to account for the vertical diffusivity below the thermocline. The unique data set of 50 yr of monthly temperature profiles from Lake Zurich allowed 'SIMSTRAT' to be calibrated (1948 - 1957) and validated (1958 - 1997). Hindcasts of temperature profiles agree excellently with the measured data. Both interannual and intra-annual variations in thermal structure are reproduced well during the entire 50-yr simulation, thus demonstrating the stability and good prognostic qualities of the model. Simulations conducted with raised and lowered air temperatures (Tair) suggest that an increase in Tair will lead to an increase in lake water temperature at all depths. In comparison to the continuous modelling approach taken in this study, the commonly employed discontinuous modelling approach (with no heat carryover during winter) was found to underestimate substantially the degree of long-term hypolimnetic warming that can be expected to result from an increase in Tair. Thus, whereas the discontinuous approach yields valid predictions for strictly dimictic lakes that are ice-covered each winter, heat carryover during winter makes a continuous approach necessary in lakes like Lake Zurich that are only facultatively dimictic. The significant degree of hypolimnetic warming found in this study suggests that the response of facultatively dimictic lakes to increases in Tair is likely to differ from that of the strictly dimictic lakes modelled in other investigations. In Lake Zurich, an increase in Tair is predicted to result in more frequent suppression of deeply-penetrative winter mixing events, with a potentially negative impact on the lake ecosystem. Addition of an ice module to SIMSTRAT allowed the rare ice-cover events that occur on Lake Zurich in extremely cold winters to be simulated successfully.
The analysis of historical lake surface temperature (LST) data from the central European REFLECT region in conjunction with meteorological data and with indices of the NAO revealed a high degree of spatial coherence in LST in all seasons. Synoptic-scale coherence was found to be high in spring, low but still significant in summer and winter, but insignificant in autumn. LSTs are strongly related to regional air temperature in all seasons and to a lesser extent to wind speed in spring and cloud cover in summer. LSTs are often related much more strongly to the winter NAO than air temperatures are. Short-term fluctuations in LST in summer are related to fluctuations in synoptic-scale meteorological forcing, but local factors such as ice cover or local topographic shading can result in substantial modification to this. In addition to the central European LST data, temporally overlapping historical LST data from all three REFLECT regions were investigated to detect supraregional spatial coherence, large-scale climatic forcing and the signature of the NAO. Results indicate that LSTs are strongly related to regional air temperature in all seasons, and that supra-regional (synoptic-scale) coherence in LST is high in spring, low but still significant in summer and winter, and insignificant in autumn. The NAO was found to be a relevant determinant of LST in all three REFLECT regions in winter and spring.
The effect of the NAO was also detected in long (~150 yr) time-series of historical observations of break-up on lakes in Finland and Switzerland, but also in Siberia and North America. In some lakes, weaker signals were also found from global volcanism. Because spring break-up triggers many ecologically important physical, chemical and biological processes, the detection of large-scale climate forcing signals in the timing of break-up implies that the ecology of individual lakes is linked to climate not only on local or synoptic scales, but also on a planetary wave scale. An analysis of long time-series of the timing of freezing and break-up of many lakes distributed over the Northern Hemisphere revealed a secular warming trend over the last ~150 yr in almost all lake ice datasets investigated, providing further evidence for the existence of a global warming trend and for its potential effect on lacustrine systems.
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