They may not wipe out entire cities or occur after earthquakes, but two University of Wisconsin researchers say the Great Lakes have tsunamis that can wreak havoc of their own.
The freshwater phenomena have been dubbed meteotsunamis - short for meteorological, or caused by weather - and are different from rip currents, seiches or storm surge floods.
Meteotsunamis are a rapid rise in water level formed by strong storms that move from land over water. The rise in water level often occurs under any visible waves on the surface.
Once that push in water level hits shallow water near shore, the research shows, they can peak up to 18 feet tall on the Great Lakes, although most are much smaller.
Seiches, by comparison, are slower-building changes in water level often spurred by varying barometric pressure differences over the same lake, with water levels sloshing back and forth across the lake. Storm surges push water up onto land during prolonged wind events, literally pushing water from one end of the lake to the other.
With a meteotsunami, the increase in water level usually passes quickly, researcher Adam Bechle told the News Tribune. The events seem to be spurred by both the storm intensity and rapid change in barometric pressure.
Bechle and co-University of Wisconsin-Madison researcher Chin Wu now believe that several historical disasters on the Great Lakes previously attributed to seiches were actually caused by meteotsunamis. Their results were published in a recent edition of the "Journal of Geophysical Research: Oceans" and released Monday by University of Wisconsin Sea Grant.
It's the first study to link Great Lakes wave events to meteotsunamis.
Bechle told the News Tribune Monday that the scientists applied their hunch to wave-related disasters and incidents on the Great Lakes and compared historical water level data with historical radar data showing when storms hit.
"The meteotsunami may only produce a couple of feet difference in water level. But pair that with a 2-foot wave on top and suddenly there's 4 feet of water coming at you when you only expected 2-foot waves. That can be a problem,'' Bechle said.
In one case in 1954 in Montrose Harbor in Chicago, people along the waterfront who were fishing took cover during a storm, only to return to piers on the waterfront when the storm passed. That's when a single wall of water hit, knocking people into the lake and causing seven drownings.
"We think that's a classic case of a Great Lakes meteotsunami and not a seiche,'' Bechle said. "We've seen the same sort of pattern in events on all of the Great Lakes at various times."
Wu, a professor with the Department of Civil and Environmental Engineering, estimates that on average, about 80 meteotsunamis occur in the Great Lakes every year. Most are small, only five or so are larger than 2 feet.
Wu happened to witness a meteotsunami on the St. Louis River estuary while he was working on an unrelated project on June 29, 2015. As a storm front passed overhead, Wu photographed what he estimated was a 2-foot high meteotsunami rise in water level beyond the smaller waves on the surface.
Wu said he was in a boat just off Duluth's Raleigh Street.
"The water was calm at 11 a.m. Around 11:20 a.m. a squall line storm with wind of approximately 31 mph and a rapid jump in atmospheric pressure'' hit the area, Wu told the News Tribune. The boat he was in was pushed toward the Wisconsin shore and "within two minutes the water levels'' went up about 2 feet in addition to the choppy waves spurred by the wind.
The researchers found that spring is the most likely season for the mini-tsunamis and the pair are working to forecast what type of storm events are mostly likely to cause them to give people a heads-up before they occur.
Joined by David Kristovich from the Prairie Research Institute at the University of Illinois at Urbana-Champaign, Wu and Bechle analyzed 20 years of water level records from Lake Michigan. They compared possible meteotsunami events with historical radar imagery showing when storms occured to find the link.
Other Great Lakes events attributed to seiches but which the researchers say most likely were caused by meteotsunamis include:
• Sept. 5, 2014: Sault Ste. Marie, Ontario, Lake Superior experienced shoreline flooding from a single water-level surge.
• July 13, 1995: Large waves were reported on lakes Superior, Huron and Erie, but no deaths.
• July 4, 1929: A 20-foot wave surged over the pier in Grand Haven State Park, Grand Haven, Mich., killing 10 people.
• July 13, 1938: A 10-foot wave struck Holland State Park in Holland, Mich., drowning five people.
• May 27, 2012: Three swimmers were rescued after a meteotsunami swept them a half-mile into Lake Erie near Madison, Ohio.
Bechle said most meteotsunamis happen during April through June, the beginning of the season for convective thunderstorms.
"There's a strong association between convective thunderstorms and meteotsunamis," Bechle said. "It's not a definitive cause, but they occur very close in time to each other."
The researchers found the largest meteotsunamis in southern Lake Michigan, near Chicago, where the shallow depth and concave floor of Lake Michigan appear to play a role in storm-spurred water levels.
The researchers have developed a mathematical formula that can be used to forecast meteotsunamis and they are working with Eric Anderson at NOAA's Great Lakes Environmental Research Lab in Ann Arbor, Mich., to establish a forecasting method to provide meteotsunami warnings.
Meteotsunamis also are known to occur on oceans where they have caused considerable damage with waves up to 40 feet high, the researchers note in their report. Regular ocean tsunamis, often called tidal waves, are spurred by underwater earthquakes and are not weather related.