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Lake Superior Forecast: Warm Water

Researchers at the Large Lakes Observatory (LLO) of the University of Minnesota Duluth predict that the surface of the open waters of Lake Superior will be exceptionally warm this summer. They suspect that water temperatures may exceed the high recorded during the strong El Niño summer of 1998, 68 degrees Fahrenheit (20 degrees Celsius). The average water temperature of Lake Superior's surface waters in August is about 59 degrees Fahrenheit (15 degrees Celsius). (It is already about 59 degrees at NOAA's Western Lake Superior buoy.)

Jay Austin, associate professor of physics, and his collaborator Steve Colman, professor of geology, report that this past winter's low ice coverage led to an early spring turnover and an early onset of summer stratification in the world's largest freshwater lake by surface area. Early stratification will allow the lake's surface waters to warm for a longer time. This year, the lake began forming a warm surface layer in early- to mid-June rather than the normal mid-July. From now until autumn, the surface waters will continue to warm and blanket the colder waters. When Austin noticed that the lake was headed for early stratification, he said that this "typically means that it's going to be a very warm year in Lake Superior."

Lake turnover happens as the winds "turn over" the lake, mixing the water column until the water reaches about 39 degrees Fahrenheit (4 degrees Celsius). At that temperature, the water column begins to "stratify," where warmer water forms a layer over the colder waters below.

Austin and Colman grabbed headlines in 2007 for showing that summer water temperatures were warming twice as fast as air temperatures over the last 30 years, based on data from NOAA buoys in Lake Superior. The reason for the warming was partly due to increasing air temperatures, but also strikingly related to winter ice cover. The less winter ice, the earlier the summer stratification begins, and the longer the summer heating season for the surface waters. Austin said, "After watching last winterís ice cover, I suspected that Lake Superior was in for warm summer surface water temperatures as much as five months ago. The early stratification date this year adds weight to my suspicions."

Lake Superior's warming waters could alter the productivity of microbes and algae, affect commercial and sport fishing, and create a longer recreational summer season. Swimmers might rejoice but lake trout may have to move deeper or further offshore.

To learn more about how a changing climate is affecting Lake Superior, visit Minnesota Sea Grant's climate portal at: www.seagrant.umn.edu/climate. For access to the University of Minnesota Duluthís meteorological buoy data, including real-time water column temperature, see: www.d.umn.edu/buoys/.

Posted on July 9, 2010

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