In the past century, the historical air temperature record suggests a warming trend in southern Wisconsin, particularly during winter and spring seasons (Kucharik et al. 2010). These changes should in turn have multiple effects on area lakes, such as later freezing and earlier ice breakup (Magnuson et al. 2000), warmer epilimnetic temperature (Arhonditsis et al. 2004, Dobiesz and Lester 2009), earlier onset of thermal stratification (Gerten and Adrian 2001), stronger temperature gradient across thermocline (Livingstone 2003), change in thermocline depth (Schindler et al. 1990), and prolonged stratification period (Wilhelm and Adrian 2008).
To determine the consequences of these climate changes on ice cover and thermal structure of Lake Mendota, long-term, 100-year (1911-2010) simulations were performed using the DYRESM-WQI model. Results show later freezing (9.8 days/100yr), earlier ice breakup (10.8 days/100yr), and shorter ice duration (20.8 days/100yr; graphs on left). In agreement with previous studies (e.g., Magnuson et al. 2000), the mean absolute difference between the model results and observations is 2.6 days and 4.1 days for ice-on date and ice-off date, respectively. The model also captures the inter-annual variation of ice dates observed in other studies (e.g., Anderson et al. 1996; significant earlier ice breakup during El Niño events in 1965, 1972, 1976, and 1982).
Model results also indicate the earlier onset of thermal stratification (6.8 days/100yr), later fall overturn (12.1 days/100yr), and prolonged stratification period (18.9 days/100yr; graphs below). Earlier onset of stratification is most likely due to earlier ice breakup and the warmer April (strong correlation between April air temperature and onset dates are found). The simulated hypolimnetic temperature shows a slightly decreasing trend (about -1 oC/100yr). Nevertheless we did not find a significant trend for mid-summer epilimnetic temperature.
To the best of our knowledge, this study presents the first attempt to continuously model both ice cover and thermal structure of a dimictic lake over a period of as long as a century. The model successfully reproduced the inter-annual variations and long term trend in ice cover that agree well with the observations. Most importantly, the results indicate the impacts of warmer winter/spring (i.e. earlier ice breakup) can be transmitted to the fallowing summer (i.e. cooler hypolimnetic temperature, earlier onset of stratification, and longer stratified period).
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