From: Robin Blackstone, ENN
Published January 21, 2014 03:24 PM

Great Lakes evaporation hypothesis up in the air

The recent Arctic blast gripping the nation will likely contribute to a rise in Great Lakes water levels in 2014, new research from the University of Michigan and Michigan State University shows. Research conducted by the two schools through the Great Lakes Integrated Sciences and Assessments Center (GLISA) shows the correlation between periods of high and low evaporation and its effect on ice cover. 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 indicating a two-way connection between ice cover and evaporation. While ice cover reduces evaporation from what would otherwise be exposed lake surface water, it also reduces lake temperature generating ice cover.

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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 results could help decision-makers define the risk associated with climate conditions affecting 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. "This will be extremely important for improving Great Lakes water-level forecasting and for understanding the long-term impacts of climate change."

Evaporation is a dominant physical process affecting the Great Lakes reaching 0.4-0.6 inches of release 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. For example, one might assume that the Great Lakes’ highest rates of evaporation occur in the 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. Because the lake was 30 to 40 degrees warmer than the overlying air in early January of 2014, the temperature contrast led to high evaporation rates and significant lake effect snowfall, defying expectations.

Read more at the Great Lakes Integrated Sciences and Assessments Center.

Broken Ice at Sunrise image on Lake Superior by Jacob Clausnitzer via Shutterstock.

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