As if worrying about life-threatening rip currents, seiches, storm surges and rogue waves on the Great Lakes wasn't enough, now Wisconsin Sea Grant researchers are learning more about yet another cause for concern: meteotsunamis.
Often mistaken for seiches, meteotsunamis are single waves similar to, but smaller than, tsunamis caused by earthquakes or landslides. Mini-tsunamis, if you will. However, instead of being caused by seismic activity, meteotsunamis are formed by strong storms that move over the water. The term "meteotsunami" is a contraction of "meteorological tsunami," which translates from Latin and Japanese as "a harbor wave caused by weather." Storms at the right speed and intensity, moving over water that is the right depth can cause a wave front. Once the wave shoals and breaks on shore, it can reach nine to 18 feet tall.
Unlike seiches, which occur when water in the entire lake basin sloshes back and forth repeatedly, meteotsunamis are a one-time moving wave front; but like seiches, their water level changes can cause loss of life and property. In fact, researchers Chin Wu and Adam Bechle with the University of Wisconsin-Madison, discovered that several historical disasters attributed to seiches were actually caused by meteotsunamis. They also found that spring is a prime time for this relatively unknown phenomenon to happen, especially in Lake Michigan, and they are working on ways to forecast meteotsunamis to avoid loss of life and property.
Wu, a professor with the Dept. of Civil and Environmental Engineering estimates that on average, about 80 meteotsunamis larger than one foot occur in the Great Lakes every year, with about five meteotsunamis per year larger than two feet.
Wu and Bechle, a postdoctoral research associate with the same department, and David Kristovich from the Prairie Research Institute at the University of Illinois at Urbana-Champaign, analyzed 20 years of water level records from Lake Michigan to make this determination. The levels are recorded every six minutes at 10 locations by the National Oceanic and Atmospheric Administration (NOAA). The researchers compared possible meteotsunami events with radar imagery to assess whether the water level changes were caused by storm fronts. Their results were published in the Journal of Geophysical Research: Oceans.
"We found that the largest meteotsunamis in Lake Michigan occur down by Chicago at Calumet Harbor," said Bechle. "The peak of activity was in the late-spring, early summer time period. That surprised us because most of the damaging events recorded in newspapers happened in June and July, which had us thinking more meteotsunamis would happen mid-summer."
Bechle said most meteotsunamis happen during April through June -- the beginning of the season for convective thunderstorms, which may cause meteotsunamis. "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 depth of the water the storm blows over is also a factor in creation of a meteotsunami. The deeper the water, the faster the storm needs to move for a meteotsunami to form. Shallower water, like that near Calumet Harbor, takes slower storm speeds to form, which are more common.
The shape of the lake basin in that area is another factor conducive to meteotsunamis.
"If you look at Lake Michigan, the east coast of the lake is a bit concave, like a satellite dish," Bechle said. "So when a storm moves from west to east across the lake, the waves will hit the east side of the lake and get reflected back to Chicago. It's almost like it focuses the wave energy down into that southern area. We think that's why we see a lot bigger events than other places along the middle of the lakeshore."
Two well-known historic events the team found were caused by a meteotsunamis and not seiches, were shoreline wave events on the Chicago coastline in 1954. The first event on June 26 swept fishermen off of piers at Montrose Harbor, causing seven drownings. The second event on July 6 did not cause any loss of life because warnings were issued.
Other Great Lakes events attributed to seiches but most likely caused by meteotsunamis include:
July 4, 1929 – a six meter wave surged over the pier in Grand Haven State Park, Grand Haven, Mich., killing ten people.
July 13, 1938 – a three-meter wave struck Holland State Park in Holland, Mich., drowning five people.
July 13, 1995 – large waves were reported on lakes Superior, Huron and Erie, but no deaths.
May 31, 1998 – a straight-line windstorm hit the eastern shore of Lake Michigan, sinking a tugboat. Parts of western Michigan were declared a federal disaster area.
May 27, 2012 – three swimmers were rescued after a meteotsunami swept them a half-mile into Lake Erie near Madison, Ohio.
Sept. 5, 2014 – Sault Ste. Marie, Ontario, on Lake Superior experienced shoreline flooding from a water-level surge.
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 center to provide meteotsunami warnings.
"Forecasting a meteotsunami is very difficult," Wu said. "You have a short time period to work with, and the information is variable and needs to be gathered in real time. We are developing a holistic approach that takes all the data from the atmosphere and water to develop a model that can provide a warning."
Wu said meteotsunamis occur throughout the world's oceans, on the U.S. East Coast, the Mediterranean, Northern Europe, the East China Sea in Japan, Korea, Taiwan, and Western Australia.
Until the forecasting center comes online, Bechle summed up what to look for. "If you see a fast-moving squall coming at you from across the lake, especially Lake Michigan, be on the alert for a possible meteotsunami."
Marie Zhuikov, WI Sea Grant, (715) 392-3141 X1003, firstname.lastname@example.org
Moira Harrington, WI Sea Grant, (608) 263-5371, email@example.com