Periods of high and low evaporation can significantly affect the timing and duration of ice cover on the Great Lakes, according to recent research funded in part by GLISA. Years with high ice cover were usually followed by cooler summer water temperatures and lower evaporation rates, but these same high-ice winters were preceded by high evaporation rates during the autumn and early winter. This indicates that connection between ice cover and evaporation is a two-way street. While ice cover reduces evaporation from what would otherwise be exposed lake surface water, evaporation is also a very effective means of reducing lake temperature to generate ice cover.
The findings carry numerous implications for the short-term variation and long-term trend of Great Lakes lake levels, which have been declining since the early 1980s and have been at a sustained low for several years. What’s more, Lake Superior underwent a regime shift during the late-1990s El Niño event, resulting in warmer summer water temperatures and winters with less ice cover.Given the long-term trend of warming lake temperatures, it’s unclear if the lakes will ever return to previous conditions.
These recent results could help decision-makers define the level of risk associated with climate conditions that may affect evaporation and water levels. "It's our hope that we will soon have the funding and infrastructure in place to maintain — and even expand — the network well into the future," said John Lenters, the study's lead investigator and a senior scientist at Ann Arbor-based LimnoTech, an environmental consulting firm. "This will be extremely important for improving Great Lakes water-level forecasting and for understanding the long-term impacts of climate change."
Evaporation is one of the dominant physical processes affecting the Great Lakes. Evaporation rates in the Great Lakes can reach 0.4-0.6 inches per day. To put this in perspective, a single day’s loss of 0.5 inches of water from surface area of the Great Lakes is roughly 20 times the amount of water that flows over Niagara Falls.
Despite its critical role, evaporation has been challenging to understand. It is invisible and often counterintuitive. For example, one might assume that the Great Lakes’ highest rates of evaporation occur in the heat of summer, but this is not the case. The highest evaporation rates typically occur in late fall and early winter, when the difference in air temperature and water temperature is greatest. In early January 2014, the lake was 30 to 40 degrees warmer than the overlying air. That temperature contrast led to high evaporation rates and significant lake effect snowfall, another striking example of how Great Lakes evaporation can defy expectations.
This project was funded by the Great Lakes Integrated Sciences + Assessments Center through a 2011 Great Lakes Climate Assessment Grant.