Warmer means windier on world's biggest lake

Washington, November 18 (ANI): Scientists have found that rising water temperatures are kicking up more powerful winds on Lake Superior in the US, with consequences for currents, biological cycles, pollution and more on the world's largest lake and its smaller brethren.

Since 1985, surface water temperatures measured by lake buoys have climbed 1.2 degrees per decade, about 15 percent faster than the air above the lake and twice as fast as warming over nearby land.

"The lake's thermal budget is very sensitive to the amount of ice cover over the winter," said Ankur Desai, atmospheric and oceanic sciences professor at the University of Wisconsin-Madison.

"There is less ice on Lake Superior during the winter, and consequently the water absorbs more heat," he added.

A wide temperature differential between water and air makes for a more stable atmosphere with calmer winds over the relatively cold water.

However, as warming water closes the gap, as in Lake Superior's case, the atmosphere gets more turbulent.

"You get more powerful winds," Desai said. "We've seen a 5 percent increase per decade in average wind speed since 1985," he added.

Desai, fellow atmospheric and oceanic sciences professor Galen McKinley and graduate research assistant Val Bennington of UW-Madison and physics professor Jay Austin of the University of Minnesota-Duluth used more than 20 years of temperature and wind data collected by three lake buoys and Earth-observing satellites to model Superior's water and wind system in three dimensions.

"We can look at how the currents are changing based on changes in the wind," McKinley said. "What we saw was a significant increase in the speed of the currents, nearly 10 percent per decade," he added.

In theory, that increase in wind and current strength would make for more mixing within the lake and, in turn, a boost in the growth of organisms that make up the earliest links in the food chain.

But Lake Superior's chlorophyll levels - a measure of the presence of phytoplankton -have been falling.

The effect went largely without explanation until the researchers' modeling showed that the period of temperature stratification was growing alongside surface temperatures and wind and current speed.

"The warming of the lake is counteracting the mixing we would expect," said McKinley, as the annual period warm water "shoals," or remains shallow, grows longer by a few days every decade.

A warmer Lake Superior may also have consequences for the movement of airborne pollutants to and from lakeshore communities. (ANI)