Assessing change in the overturning behavior of the Laurentian Great Lakes using remotely sensed lake surface water temperatures

Most large temperate lakes experience overturning every spring and fall as surface water moves past 4 degrees C, the temperature of maximum density for freshwater. These semiannual, lake-wide overturning events play an important role regulating the thermal structure, deep-water ventilation, nutrient supply, water circulation, and nearshore water quality of the lakes. The general pattern of overturning has long been known from field observations and models, but its timing, duration, detailed spatio-temporal progression and seasonal and inter-annual variability remain largely undocumented, particularly in the context of recent climate-driven changes in lake thermal dynamics. Here, we used a reconstructed record of daily and spatially-explicit lake surface water temperatures (LSWT) to analyze the migration of the 4 degrees C thermal front as it progressed from the shorelines to the deep parts of the Laurentian Great Lakes during every overturning event between June 1995 to April 2012. The analysis revealed a strong asymmetry in the timing and duration of overturning between spring and fall, and no relationship with the lake-averaged LSWT or its rate of change. Key differences in the average spatio-temporal progression of overturning were also observed between spring and fall, with the spring progression being largely driven by latitude and water depth and the fall progression being less predictable and influenced by other factors such as wind. Narrow regions of very slow overturning progression were also identified, revealing areas of the lakes where persistent 4 degrees C thermal bars are likely to re-occur every year. The timing and duration of these seasonal overturning events varied between years by as much as one and two months, respectively, with a direct impact on the duration of lake-wide stratification. In 2012, Lakes Michigan and Ontario experienced an incomplete fall overturning, leading only to a partial winter stratification. Lakes Michigan and Ontario were more susceptible to experience an incomplete overturning than the other Laurentian Great Lakes, seemingly due to a combination of comparatively milder winter air temperatures and lower lake dynamic ratio (steepness of bottom slope). Overall, the duration of lake-wide winter stratification was found to be strongly correlated with mean winter air temperatures, and a simple trend analysis suggested that rising temperatures could lead to more frequent incomplete fall overturnings and partial winter stratifications in Lakes Michigan and Ontario over the next few decades. This study demonstrated that remote sensing provides an unparalleled tool for assessing the long-term variability in the overturning behavior of large lakes in the context of climate change.