The mild winter spurs an earlier summer "turnover" for a Great Lake.
Swimmers may be able to remain in Lake Superior a few minutes longer and perhaps earlier than usual this year.
The lake, often numbingly cold even in midsummer, is running warm this spring after a warm winter and lack of ice cover, said Jay Austin, a University of Minnesota Duluth lake researcher. Surface water that would usually have frozen never got colder than about 35.6 degrees during the winter, he said, and relatively warm water remained unusually close to the surface without losing much heat.
Now, the lake's summer "turnover," in which the warm water from below replaces cooler surface water, may occur in early June instead of the customary early July, Austin said.
"It's going to be a great year for swimming," he said.
A similar ice-free winter two years ago led to record warm water temperatures and a busy beach at Duluth's Park Point in July 2010. But water temperatures subsequently dropped 10 degrees in three days in mid-August.
Austin and others were aboard a boat near Munising, Mich., this week inspecting temperature buoys and collecting other data.
Speaking from the boat late in the week, Austin said a buoy in the western part of the lake showed water closing in on 39 degrees, a critical freshwater temperature threshold. After that, the upper levels of the lake should get warmer, and more quickly, than usual. It's part of a twice-yearly process lake experts call "stratification." Austin expected it to reach that mark in the next several days, which would be about 10 days ahead of the earliest dates on record, set in 1998 and 2010.
"Our experience is that if the lakes start forming summer stratification early, they get a lot warmer by the middle of summer," he said.
The surface temperature of Lake Superior at Port Wing, Wis., a relatively shallow area close to the south shore of the lake east of Duluth, was 60.4 degrees Monday afternoon, according to measurements maintained by Michigan State University. That was less than 5 degrees cooler than Carson Bay on Lake Minnetonka, about 200 miles to the south. After several days of rain and clouds, however, the Port Wing temperature Friday afternoon was 47.5. Telly Mamayek, spokeswoman for the Minnehaha Creek Watershed District, said the Carson Bay temperature of 65 was "significantly" warmer than temperatures from the same date in recent years -- 54 degrees last year, 57 in 2010, 58 in 2009, 56 in 2008 and 60 in 2007.
Minnesota Sea Grant, a joint program of the National Oceanic and Atmospheric Administration and the University of Minnesota, is developing a website that will provide continual updates on weather and water conditions, including possible rip currents and contaminations, at Duluth's Park Point this summer. The miles-long sandy stretch at the west end of the lake is popular for swimming, though conditions there change rapidly.
Researchers have noted that warming in Lake Superior, normally too cold to support much aquatic life, could boost the growth of algae, microscopic plants and animals, small fish and even larger predators. At the same time it could push such species as lake trout, which prefer cold water, to lower, cooler depths.
Bill McAuliffe • 612-673-7646