Two cold seas off the coast of Alaska, nearly 3,000 miles from Michigan, appear to increasingly hold the key to how much the Great Lakes will ice over in winter, according to new research.
Ice and temperature conditions on the Bering Sea, in the northern Pacific Ocean west of Alaska; and the Chukchi Sea in the Arctic Ocean to Alaska's north; have a strong, negative correlation to Great Lakes maximum winter ice cover — especially since the late 1990s, researchers at the University of Michigan and the National Oceanic and Atmospheric Administration found.
In recent years, when surface water temperatures are colder and more ice exists in the Alaskan seas in November and December, the Great Lakes consistently have had relatively warmer temperatures and less ice, the scientists found. When the Alaskan seas are warmer, the Great Lakes will be more iced up that winter.
'A shift in Great Lakes ice cover'
This correlation appears stronger than more famous ones, such as El Niño, a band of warmer central Pacific Ocean water that occurs in some years and can drive weather phenomena in North America, said Jia Wang, a research scientist and ice climatologist at NOAA's Great Lakes Environmental Research Laboratory in Ann Arbor.
"Since the winter of 1997-1998, we can call this a shift in Great Lakes ice cover," he said. "The average of the ice cover is lower than before, but variability up and down is larger than before 1998."
Of those 25 winters, from 1997-98 to last winter, Great Lakes ice cover has been below its long-term average 15 times. But despite the trend, in some years the lakes still get very iced over: all of the Great Lakes together averaged 93% ice cover in the winter of 2013-14. The ice persisted into June that year on Lake Superior. In contrast, just two winters earlier, in 2011-12, Great Lakes ice cover was at an all-time low average of 12%. Maximum Great Lakes ice levels in a winter are usually reached in late February.
How Arctic air makes its way south
Scientists believe human-driven climate change, caused by greenhouse gas emissions from fossil fuel burning, is causing a general warming in sea temperatures and a trend toward reduced sea and Great Lakes ice — but also can spur more volatility in Arctic air masses and contribute to occasions where frigid air usually more entrapped around the North Pole makes its way well south into the Great Lakes region.
The higher pressure areas of the atmosphere more typically set around the Aleutian Islands in the northern Pacific, Wang said. But if sea surface temperatures are warmer in this region, the high pressure weakens, and moves inland to the northeast, well into Alaska, he said.
"It squeezes the ridge-trough system (in the upper atmosphere) and brings the cold air from the Arctic into the Great Lakes more intensely," he said.
In addition to being important to professions such as hydroelectric power generation and the shipping industry, Great Lakes ice cover also matters for Michigan's weather.
"Declining ice cover, or longer periods of the year with open lake water, combined with warmer surface temperatures, will lead to increased lake-effect precipitation in the future. In the near term, this may mean increased lake-effect snow, but as air temperatures rise, lake-effect snow will transition to lake-effect rain," state researchers with the Great Lakes Integrated Sciences and Assessments, or GLISA, an Ann Arbor-based collaboration between the University of Michigan, Michigan State University and NOAA focused on climate change's impacts and how communities can adapt to them.
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How much ice will the Great Lakes have this winter?
Wang said he plans to incorporate Bering and Chukchi sea temperatures into his laboratory's Great Lakes ice cover forecast modeling.
"It looks like this pattern will stay for some time," he said.
And what does it show for the upcoming winter? The West Coast is showing colder-than-average Pacific sea surface temperatures right now, Wang said.
"That means the high (atmospheric) pressure center would be like the situation prior to 1998, staying in the Aleutian Islands, not moving northeastward," Wang said. "So this year, Great Lakes ice cover will be mild, because the North American ridge-trough system will not be intensified."
Contact Keith Matheny: kmatheny@freepress.com.
