Interactions Among Evaporation, Ice Cover, and Water Temperature on Lake Superior: Decadal, Interannual, and Seasonal Variability

Lake Superior, the largest freshwater lake in the world by surface area, has enormous impacts on the regional weather and climate. The lake also comprises over half of the total water volume in the Great Lakes system and is an important resource for commercial shipping, water supplies, hydropower, recreation, and aquatic ecosystems. Water temperature and evaporation on Lake Superior have been found to be increasing in recent decades, while ice cover has been decreasing at a very rapid pace. A careful analysis of the long-term trends, however, shows that these changes have not been linear through time. Rather, a step-change occurred in 1997/98 that resulted in a drop in ice duration of nearly 40 days, a 3°C increase in summer water temperature, and a near doubling of July-August evaporation rates. Linear regression analysis of data on either side of this step change shows trends which are largely insignificant and even opposite in sign from those of the step change. Using time-lagged correlation and composite analysis, interactions among ice cover, water temperature, and evaporation are explored across seasonal and interannual timescales. Fall evaporation rates are found to be significantly correlated with ice cover in the following winter, presumably as a result of strong latent heat flux leading to rapid ice onset and growth. Similarly, ice cover is found to be a strong determinant of summer water temperature. This, in turn, can lead to changes in late-summer evaporation rates. Quantifying these complex interactions is important for assessing the potential impacts of future climate change on large-lake systems. Key to this understanding is the direct measurement of lake surface processes such as evaporation and sensible heat flux. As such, this study includes an analysis of the first direct observations of nearshore evaporation rates on the Great Lakes, using eddy covariance data collected from a monitoring station on Granite Island (near Marquette, Michigan). The data are analyzed for the period October 2010 to April 2012 to explore the seasonal and interannual variations in latent and sensible heat fluxes over Lake Superior, as well as some of the primary climatic factors driving this variability. Adviser: John D. Lenter