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The typical tsunami, in people's minds, is a wave pushed from below, either by an earthquake or by some sort of landslide. But weather events can cause them too in certain regions. Although local people in these places have their own names for these freak waves, only recently have scientists recognized them as a universal phenomenon with the name meteotsunamis.
What Makes Them Tsunamis?
The basic physical feature of a tsunami wave is its oversize scale. Unlike ordinary wind-driven waves, with wavelengths of a few meters and periods of a few seconds, tsunami waves have wavelengths of up to hundreds of kilometers and periods as long as an hour. Physicists classify them as shallow-water waves because they always feel the bottom. As these waves approach shore, the rising bottom forces them to grow in height and move closer in succession. The Japanese name tsunami, or harbor wave, refers to the way they wash ashore without warning, moving in and out in slow, damaging surges.
Meteotsunamis are the same kind of waves with the same kinds of effects, caused by rapid changes in air pressure. They have the same long periods and the same damaging behavior in harbors. The main difference is that they have less energy. Damage from them is highly selective, limited to harbors and inlets that are well aligned with the waves. In Spain's Mediterranean islands, they are called rissaga; they are rissagues in mainland Spain, marubbio in Sicily, seebär in the Baltic Sea, and abiki in Japan. They have also been documented in many more places, including the Great Lakes.
How Meteosunamis Work
A meteotsunami starts with a strong atmospheric event marked by a change in air pressure, such as a fast-moving front, a squall line, or a train of gravity waves in the wake of a mountain range. Even extreme weather changes the pressure by small amounts, equivalent to a few centimeters of sea-level height. Everything depends on the speed and timing of the force, along with the shape of the water body. When those are right, waves that start out small can grow through the resonance of the water body and a pressure source whose speed matches the wave's speed.
Next, those waves are focused as they approach shorelines of the right shape. Otherwise, they simply spread away from their source and fade out. Long, narrow harbors that point toward the incoming waves are affected worst because they offer more of the reinforcing resonance. (In this respect meteotsunamis are similar to seiche events.) So it takes an unlucky set of circumstances to create a notable meteotsunami and they are pinpoint events rather than regional hazards. But they can kill people-and more important, they can be forecasted in principle.
A large abiki ("net-dragging wave") surged into Nagasaki Bay on March 31, 1979 that reached wave heights of nearly 5 meters and left three people dead. This is Japan's most notorious site for meteotsunamis, but several other vulnerable harbors exist. For instance, a 3-meter surge was documented in nearby Urauchi Bay in 2009 that capsized 18 boats and threatened the lucrative fish-farming industry.
Spain's Balearic Islands are noted meteotsunami sites, particularly Ciutadella Harbor on the island of Menorca. The region has daily tides of about 20 centimeters, so harbors are typically not made for more energetic conditions. The rissaga ("drying event") on June 21, 1984 was more than 4 meters high and damaged 300 boats. There is video of a June 2006 rissaga in Ciutadella Harbor showing the slow waves tearing dozens of boats off their moorings and into each other. That event began with a negative wave, drawing the harbor dry before the water rushed back. Losses were tens of million euros.
The coast of Croatia, on the Adriatic Sea, recorded damaging meteotsunamis in 1978 and 2003. In some places 6-meter waves were witnessed.
The great eastern U.S. derecho of 29 June 2012 raised a meteotsunami in the Chesapeake Bay that reached 40 centimeters in height.
A 3-meter "freak wave" in Lake Michigan killed seven people as it washed over the Chicago shoreline on June 26, 1954. Later reconstructions show that it was triggered by a storm system over the north end of Lake Michigan that pushed waves down the length of the lake where they bounced off the shore and headed straight for Chicago. Just 10 days later another storm raised a meteotsunami more than a meter high. Models of these events, programmed by researcher Chin Wu and colleagues at the University of Wisconsin and the Great Lakes Environmental Research Lab, raise the promise of forecasting them when strong weather comes.