Trends and abrupt changes in 104 years of ice cover and water temperature in a dimictic lake in response to air temperature, wind speed, and water clarity drivers
Abstract. The one dimensional hydrodynamic ice model, DYRESM WQ I, was modified to simulate ice cover and thermal structure of dimictic Lake Mendota, Wisconsin, USA, over a continuous 104 year period (1911 2014) The model results were then used to examine the drivers of changes in ice cover and water temperature, focusing on the responses to shifts in air temperature, wind speed, and water clarity at multiyear timescales. Observations of the drivers include a change in the trend of warming air temperatures from 0.081 degC per decade before 1981 to 0.334 degC per decade thereafter, as well as a shift in mean wind speed from 4.44 m s 1 before 1994 to 3.74 m s 1 thereafter. Observations show that Lake Mendota has experienced significant changes in ice cover: later ice on date(9.0 days later per century) earlier ice off date (12.3 days per century) decreasing ice cover duration (21.3 days per century) while model simulations indicate a change in maximum ice thickness (12.7 cm decrease per century) Model simulations also show changes in the lake thermal regime of earlier stratification onset (12.3 days per century) later fall turnover (14.6 days per century) longer stratification duration (26.8 days per century) and decreasing summer hypolimnetic temperatures 1.4 degC per century) Correlation analysis of lake variables and driving variables revealed ice cover variables, stratification onset, epilimnetic temperature, and hypolimnetic temperature were most closely correlated with air temperature, whereas freeze over water temperature, hypolimnetic heating, and fall turnover date were more closely correlated with wind speed. Each lake variable (i.e. ice on and ice off dates, ice cover duration, maximum ice thickness, freeze over water temperature, stratification onset, fall turnover date, stratification duration, epilimnion temperature, hypolimnion temperature, and hypolimnetic heating) was averaged for the three periods (1911 1980, 1981 1993, and 1994 2014) delineated by abrupt changes in air temperature and wind speed. Average summer hypolimnetic temperature and fall turnover date exhibit significant differences between the third period and the first two periods. Changes in ice cover (ice on and ice off dates, ice cover duration, and maximum ice thickness) exhibit an abrupt change after 1994, which was related in part to the warm El Nino winter of 1997 1998. Under ice water temperature, freeze over water temperature, hypolimnetic temperature, fall turnover date, and stratification duration demonstrate a significant difference in the third period (1994 2014) when air temperature was warmest and wind speeds decreased rather abruptly. The trends in ice cover and water temperature demonstrate responses to both long term and abrupt changes in meteorological conditions that can be complemented with numerical modeling to better understand how these variables will respond in a future climate.