METEOTSUNAMIS ?

It was clear and sunny in Chicago on the morning of Saturday, June 26, 1954. Before dawn, a fast-moving storm out of the northwest had passed over Lake Michigan, pummeling Michigan City, Ind., on the southeastern shore of the lake with rain and 2-meter-high waves, but sparing Chicago on the southwestern shore.

With the metropolis in the midst of a heat wave, people looking to cool off had headed down to the lake to swim, fish or just stroll along the waterfront. Mae Gabriel, 48, and her husband Edward, 49, the parents of 11 children, were in the waterfront park at Montrose Harbor. Nearby, 16-year-old Ralph Stempinski was fishing with his father, Ted, on the 55-meter-long breakwater that curves out into the lake. Shortly after 9 a.m., Ralph briefly left his father on the pier. When he returned, his father, along with about 15 other anglers, was gone.

With no warning, the lakewater had risen 3 meters, surged over the pier and rushed 50 meters inland through the park. In the chaos that followed, many people were pulled alive from the water, but Ted Stempinski and Mae and Edward Gabriel were not among them. In all, at least seven people were killed.

The incoming waves had arrived from the east and struck a 40-kilometer swath of shore from Jackson Park south of the city to Wilmette in the north. The afternoon edition of the Chicago Daily News reported that a freak “tidal wave” had struck the waterfront, but there are no such tides in the Great Lakes.

One of the first scientific analyses of the 1954 Lake Michigan wave, published in 1965, attributed it to a seiche, a seesawing oscillation in the lake’s surface induced by strong winds or a sudden increase in air pressure, which reflected off the southeastern shore and sloshed west toward Chicago.

Other large wave events associated with storms had occurred in the Great Lakes (see sidebar), but the 1954 event was the deadliest in 25 years. When another fast-moving squall line passed over Lake Michigan the following month, a warning was issued, the beaches were cleared and, when a smaller wave hit that time, no one was killed or injured.

However, not all storms produce tsunami-like waves. Only recently have researchers come to more fully understand how storms trigger such sudden and massive waves, called meteotsunamis. Now they are attempting to develop models to forecast the events and prevent future tragedies like the one that happened that day in Chicago.

Seismic Tsunami Versus Meteotsunami

“Major wave events happen two to four times per year in the Great Lakes,” says Chin Wu, a civil and environmental engineer at the University of Wisconsin at Madison. “People thought they were seiches or storm surges, which occur when the water is pushed, but they’re not,” Wu says. “This is different.”

Meteotsunami waves have many of the same characteristics as tsunamis, but are triggered by storms rather than earthquakes or landslides.

“The storm is far away, but it is pumping energy into the water system, which can propagate very far to the shore,” Wu says.

Tsunamis are more than just big waves, and they differ from waves typically seen at the beach in several important ways.

First, they have extremely long wavelengths, often in excess of 100 kilometers, which means the waves can pass unnoticed in open water and travel great distances while losing very little energy, sometimes causing catastrophic impacts half a world away. Second, because they have such long wavelengths, they behave like shallow-water waves, the speeds of which are dependent on water depth. In deep water, a tsunami travels at speeds up to hundreds of kilometers per hour. When it enters shallow coastal waters, it slows and the waves rapidly gain amplitude, often reaching dozens of meters in height. The highest recorded wave height was a 576-meter-high local tsunami generated by a landslide in Lituya Bay, Alaska, in 1958.

Major seismic, or geophysical, tsunamis are rare, but meteorological tsunamis are rarer still, with the largest occurring mainly in a few locations worldwide, including parts of the Mediterranean. Despite the risk they pose, and their worldwide occurrence, the phenomenon is not well known. Awareness of meteotsunamis has risen recently in the scientific community, says Paul Whitmore, director of the National Tsunami Warning Center in Palmer, Alaska, along with a general increase in tsunami research spurred by several recent tsunami-related disasters.

There are several differences between seismic tsunamis and meteotsunamis, with one of the main ones being how much energy is involved.

Seismic tsunamis triggered by earthquakes or landslides receive one extremely large energy input from the initial disturbance, whereas a meteotsunami requires continued energy input from the atmosphere for it to propagate. Thus, the maximum amplitude or wave height that meteotsunami waves can attain is much lower, with the largest observed waves reaching no more than 6 meters, Whitmore says. The lower energy level of meteotsunamis is also why they are always a local phenomenon. Unlike a seismic tsunami, which can have a global reach, meteotsunamis are usually geographically coincident with the storm that triggered them.

Nevertheless, certain meteotsunamis can still be deadly and destructive, though it takes a specific combination of events to cause the destructive ones, Whitmore says. These are the events that researchers are most interested in understanding and forecasting.

How a Storm Triggers a Wave

Historical catalogs of tsunamis include many instances of “tsunami-like” waves of “unknown origin” for which no seismic or geophysical cause can be found, including many in the Mediterranean Sea, Adriatic Sea, English Channel, and off Japan and the West Coast of North America. Although it has long been understood that such events were storm-related, researchers only recently elucidated the specific mechanism of meteo­tsunami generation.

The key trigger is a sudden change in atmospheric pressure over the water’s surface, which initiates the wave — but that alone cannot sustain it. To produce the resonance needed to sustain and amplify a meteotsunami, the storm front must be moving at the same speed as the water wave. Wave speed is dependent on water depth, so changes in water depth play a critical role in the propagation of the wave both at sea and when it enters shallow water, which is called shoaling.

“The strongest meteotsunamis appear in funnel-shaped bays and harbors with a wide shelf in front of them,” says Ivica Vilibić, a physical oceanographer and meteotsunami researcher at the Institute of Oceanography and Fisheries in Split, Croatia. A flat shelf is necessary for so-called Proudman resonance to occur, which transfers energy from the atmosphere to the ocean.

Because local bathymetry plays an important role, the conditions that cause a meteotsunami in the Mediterranean are not necessarily the same conditions that cause one in the U.S.

“Croatia has both a wide [continental] shelf and lots of deep bays. The U.S. East Coast, with a wide shelf, was hit by several destructive meteo­tsunamis in the last few decades,” Vilibić says. However “the U.S. West Coast, having a narrow shelf, is not exposed to destructive meteotsunamis, only to moderate ones.”

Major meteotsunamis, like major seismic tsunamis, may be rare, Whitmore says, but we need to worry about both.

“The U.S. East Coast is actually at a higher risk of being hit by a meteotsunami than by a seismic tsunami,” Whitmore says. “While they’re not very common, they do occur, and without any warning. We have a tsunami warning system set up for the East Coast to warn of traditionally generated tsunamis, but we’re not set up for this.”

The current warning system, which relies on the early detection of seismic signals from earthquakes, landslides and submarine volcanic activity that may generate a tsunami, is incapable of detecting potential meteorological sources of tsunamis.
 In 2010, Whitmore headed efforts at NOAA to launch a project with the goals of understanding the environmental, meteorological and bathymetric forces that cause meteo­tsunamis, as well as developing ways to forecast them and establishing a warning system for the U.S. East Coast.

Funding for the two-year NOAA project was cut due to budget constraints. But the research team, led by Vilibić, had already made strides toward defining the conditions under which meteotsunamis occur and understanding what meteorological data need to be monitored to detect them.

“What they were able to do was go back through historic meteorological data and figure out that [sudden atmospheric] pressure jumps are responsible,” Whitmore says.

That information proved useful when a mysterious water wave struck the U.S. East Coast last summer.

East Coast Meteotsunami of June 2013

At about 3:30 p.m. on June 13, 2013, Brian Cohen was spearfishing from his boat in Barnegat Bay, N.J., when he saw anomalously high 2-meter waves suddenly crossing the inlet. The waves knocked several anglers off the jetty, and strong rip currents, which coincided with the outgoing tide that afternoon, pulled several scuba divers out over a breakwater. Cohen quickly headed his boat back to shore so he would not be sucked over the breakwater as well. The outrush of water continued for one to two minutes, eventually exposing the breakwater, which is usually submerged under about a meter of water.

About the same time, rapid fluctuations in the water level at the mouth of Falmouth Harbor in Massachusetts, 350 kilometers farther north, were detected as the water rose and fell about 0.3 meters in less than 10 minutes, causing rapid currents in the harbor.

About five hours earlier, a derecho — a high-speed windstorm associated with a band of fast-moving thunderstorms — had traveled from the Midwest to New Jersey and offshore into the mid-Atlantic.

Tsunami researchers and oceanographers immediately suspected the wave event was a meteotsunami triggered by the storm. But once the storm moved offshore, the scientists had data from just two NOAA buoys off the coast of New Jersey — a DART (Deep-ocean Assessment and Reporting of Tsunamis) buoy that detects deep-water pressure changes, and a weather buoy — with which to work.

In July 2013, Richard Signell, a physical oceanographer at the U.S. Geological Survey (USGS) in Woods Hole, Mass., presented a preliminary analysis of the event (posted at www.youtube.com/watch?v=IL4LGb2w75E), analyzing air pressure and water level fluctuations that occurred up and down the East Coast that day, along with radar records of the storm’s passage.

Clocking the derecho as it passed the DART and weather buoys, Signell noted that given the speed of the storm and the water depths on the continental shelf at those locations, the waves induced by the storm would fall within the tsunami frequency band. A model reconstruction of the storm as it moved out to sea revealed a large wave reflecting off the shelf break and heading back toward Barnegat Bay — where Brian Cohen was spearfishing that day.

“It looks plausible that this [wave event] could have been caused by the front,” Signell said.

However, scientists also considered that the waves could have been triggered by another source known to occur off the East Coast: a submarine landslide. Researchers at NOAA’s Pacific Marine Environmental Laboratory quickly input the water level fluctuations observed up and down the East Coast into models, which suggested the most likely location for a landslide source was Hudson Canyon off the coast of New York. In the summer of 2013, the NOAA research vessel Okeanos Explorer was on a cruise in the mid-Atlantic and was redirected to Hudson Canyon to look for any evidence of a disturbance.

“We are fortunate to have a number of mapping surveys from Hudson Canyon and, therefore, we could compare the morphology of the canyon from before and after the June event,” says Jason Chaytor, a research geologist at USGS in Woods Hole, Mass., who studies submarine landslides. “Analysis of the pre- and post-event bathymetry and shallow-subsurface mapping data showed no evidence of new landslides along the canyon,” Chaytor says. The NOAA ship also mapped the head of Atlantis Canyon, another potential landslide location, “but again, no significant change was detected,” he says.

In the months since, a team of researchers from NOAA’s tsunami warning centers in Alaska and Hawaii and the National Data Buoy Center in Mississippi has conducted further analyses of the June 2013 event. They reported the results at the annual meeting of the American Geophysical Union in San Francisco, Calif., in December.

“The presence of this storm, the lack of a seismic source, and the fact that tsunami arrival times at tide stations and deep ocean-bottom pressure sensors cannot be attributed to a ‘point-source’ suggest this tsunami was caused by atmospheric forces, that is, a meteotsunami,” the team wrote in the abstract.

The researchers used the 2013 event as a test case to see if prediction might be possible.

“This event caught everyone’s attention because it was so pronounced up and down the coast, and it caused some injuries,” says co-author William Knight, a physical scientist and oceanographer at NOAA’s National Tsunami Warning Center in Palmer, Alaska.

The group gathered air pressure data from barometers at land-based stations in the path of the storm, along with sea-level gauges along the coast and DART buoys offshore, and plugged the data into a tsunami-forecast model that the team has been developing.

“The only thing we changed was replacing the earthquake source with an atmospheric pressure source, which was a pretty straightforward modification,” Knight says. “We were trying to determine if the warning center could provide any kind of advance warning to people on the East Coast.”

The model showed promising results, but several things need to happen before that determination can be made, he says, including testing the model’s predictive strength on several more historical events. So far, “we only have one stake in the ground here,” Knight says. “We need to look at earlier cases [of meteotsunamis] to make sure that we can identify a candidate [weather] system.”

Researchers also need to develop a constellation of weather stations that provide the necessary air pressure data, he says. In this case, air pressure data were gleaned from barometers mounted on seismic stations that are part of the transportable USArray, many of which are not permanent stations.

“We really need pressure data from the land-based stations, which will allow us to make a forecast,” Knight says. “By the time the storm is offshore, being seen by ocean-based pressure sensors, it’s already too late.”

And lastly, the team will need to address what kind of warnings should be released to the public. “This is not the same kind of seismic source that we are used to, so the kind of warning we would put out to pull people back from the shore is still an open question,” Knight says.

Can Meteotsunamis Be Forecast?

Whitmore says the early results are promising and indicate that under certain conditions, and with adequate funding, East Coast meteo­tsunamis should be able to be forecast. An unusual aspect of Great Lakes and East Coast meteotsunamis actually gives tsunami researchers time to produce a forecast: As with the June 2013 event, the waves are often generated by storms moving from west to east, over land and offshore. In many Mediterranean events, the storms blow onshore.

“Part of the reason that we can [potentially forecast an event],” Knight says, “is that the actual source was the reflected wave off the Atlantic shelf break, which is eastward of the coast, which could give us at least a couple hours of lead time.”

Whitmore says good forecasts will require air pressure data recorded at much finer resolution, with correspondingly higher-resolution models, to detect the sudden atmospheric pressure jumps that trigger meteotsunamis. Current weather buoys off the coast only transmit one air pressure reading every hour, he says, but scientists need to track these air pressure fluctuations about once every minute.

At the 2012 European Geosciences Union meeting in Vienna, Austria, Vilibić and colleagues presented their preliminary results from the TMEWS project (Towards a MEteotsunami Warning System along the U.S. coastline), which showed that some models could retrospectively reproduce the conditions under which meteo­tsunamis had occurred. Thus validated, it could be the basis of an early warning system, but there is still a long way to go.

“It is quite hard, or almost impossible, with present models and systems, to forecast meteotsunamis because the source in the atmosphere is a very mesoscale process,” Vilibić says.

Currently, the world’s only meteo­tsunami warning system is operating in the Balearic Sea in the western Mediterranean; however, it is based on identifying larger-scale weather systems.

The method being used “in the Balearic Islands, Spain, is to assess synoptic conditions, which can be easily forecasted, and to assign a warning level if the conditions are close to those observed during meteotsunamis. However, the latter provides just qualitative and not quantitative forecasts, and does not tell you anything about the intensity of the potential event,” Vilibić says.

In addition to efforts to develop warning systems for the U.S. coast, there have also been attempts in the Adriatic Sea, Vilibić says, but a great deal of work remains to be done.

In Wisconsin, Wu and his team are also working on a predictive model for the Great Lakes, which experience 40 to 60 convective-type storms each year, the type of storm most likely to produce a meteotsunami.

“The [1954] Chicago tsunami hit on a perfectly calm day, with no warning,” Wu says. “One of our major goals is to be able to warn people.”

The National Weather Service currently issues broad warnings for the Great Lakes, although they are not specifically meteotsunami warnings. For example, in July 2013, a beach hazards alert was issued warning that an incoming cold front sweeping over Lake Michigan could generate “rogue waves” as high as 5 meters on the Chicago waterfront. The alert warned people to stay out of the water and away from the waterfront as waves could be “high enough to sweep an unsuspecting biker into the water.”

Meteorologists and beachgoers are not the only ones interested in forecasting meteotsunamis, however. After the tsunami triggered by the 2011 Tohoku earthquake swamped the Fukushima Daiichi nuclear plant, causing reactor meltdowns and radioactive releases that contaminated the surrounding land, atmosphere and ocean, administrators from the Nuclear Regulatory Commission contacted Wu about his research. The Palisades nuclear power station is located on the shore of Lake Michigan near South Haven, Mich. — just south of  the site of a deadly meteotsunami that struck Grand Haven in 1929.