Changes in the Climate And a Windier Lake Superior

Chalk up another effect of climate change: it’s getting windier over Lake Superior.

That is the conclusion of a study by scientists who have looked at the effects of increasing surface water temperatures in the lake and air temperatures over it. The water has warmed faster than the air, creating instability in the air mass that results in stronger winds.

Ankur R. Desai of the University of Wisconsin-Madison, an author of the study in Nature Geoscience, said the effect was due to ice, or lack of it.

“Less ice in the winter means stronger winds in the summer,” Dr. Desai said.

Ice coverage of Lake Superior has declined in recent decades, which means that the lake starts to warm sooner, becoming stratified. The earlier this stratification occurs, Dr. Desai said, the warmer the top layer gets in the summer.

Data from buoys and satellites showed that this warming outpaced that of the air above it. That means the thermal gradient between the two was reduced.

A large thermal gradient makes for stable air, which is why the wind often dies down at night, when the ground cools. In this case, with a smaller gradient, instability increased. This was confirmed by data showing that average wind speeds over the lake have increased by 5 percent per decade since the 1980s.

The researchers also showed that higher winds increase the speed of currents in the lake. That, Dr. Desai said, should increase the amount of mixing of the lake’s layers, affecting nutrient distribution of nutrients and, ultimately, the organisms in the water.

© 2009 by the New York Times Company, Tue Nov 17 2009