Winter 2005

UW Sea Grant Research

Where the Wind Blows
Shifting Winds could be Stirring up Green Bay

By Kathleen Schmitt

Summer winds blowing across northern Lake Michigan have shifted dramatically over the past 15 years. Researchers supported by UW Sea Grant say the change in the weather might trap tons of sediments and nutrients in Green Bay instead of letting them drift out to the open water of Lake Michigan.

James Waples, a biogeochemist at UW-Milwaukee’s Great Lakes WATER Institute, first noticed the shift in wind direction while studying air/water gas exchanges in Green Bay. Trying to solve a discrepancy in annual gas exchange estimates for two particular years, he dove into a database of local hourly wind vectors.

"I noticed that while wind speeds were, on average, the same each year, the average wind direction was quite different," Waples said.

The finding prompted him to pore over more than 20 years of data collected from meteorological buoys throughout the Great Lakes. He found a large-scale shift in summer wind direction over the entire Great Lakes basin, beginning around 1990. Over Green Bay, average summer wind directions changed from southwesterly to easterly.

"This shifting wind field may change the circulation pattern of Green Bay during the summer," said J. Val Klump, director of the Great Lakes WATER Institute and a partner on the current study. He said that in the summer, most of Green Bay separates into layers of warm and cold water. Typical winds blowing along the length of Green Bay usually push warm surface water out to Lake Michigan. At the same time, colder water from the lake moves into the bottom of the bay.

An increase in easterly winds blowing across Green Bay could essentially shut down this "conveyer belt" to Lake Michigan, Klump said. That means instead of traveling out to Lake Michigan, more sediments would remain in Green Bay, which receives roughly 27 dump trucks of it from the Fox River every day.

Waples and Klump are taking cores of the bay’s bottom to determine how fast sediments are accumulating, and they’re performing other tests to find out how many times sediments resuspend in the water column before they settle down for good. After collecting their final year’s worth of data this summer, they’ll compare their results to data Klump and colleagues collected in the 1980s as part of another Sea Grant study of sediment resuspension and transport in Green Bay.

The researchers hypothesize that the shift in wind direction is mixing up Green Bay, breaking down the normal summer layers of warm and cold water, and creating a much warmer and more oxygenated bottom habitat. Waples said these changes could dramatically alter the biogeochemistry of the system. Klump added that the shifting winds might also shed light on why total phosphorus levels in lower Green Bay have increased over 20 percent since 1999, while the amounts flowing out of the Fox River have remained relatively stable.

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University of Wisconsin Sea Grant Institute & University of Wisconsin Water Resources Institute.

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