A meteotsunami is a powerful natural phenomenon involving large waves that can suddenly impact coastal areas. The word combines “meteorological,” referencing its atmospheric origin, with “tsunami,” indicating its wave characteristics. This event demonstrates a powerful connection between weather systems and the ocean, capable of creating destructive waves far from any seismic activity.
Defining the Meteotsunami
A meteotsunami is a tsunami-like sea wave that originates from a meteorological disturbance, rather than a geological event. It is a long-period wave in the ocean or a large lake, capable of traveling at high speeds. The wave periods typically range from two minutes to two hours, falling within the tsunami frequency band.
The physical appearance of a meteotsunami often differs from the towering waves associated with seismic tsunamis. It frequently manifests as a rapid change in the water level, sometimes appearing as a sudden drawdown followed by a powerful surge. Most meteotsunamis are smaller, generally reaching heights of two meters or less, though rare events have been recorded up to six meters.
The fundamental difference lies in the source of energy: a seismic tsunami is an impulse wave caused by a sudden, massive displacement of water from an event like an earthquake or underwater landslide. Conversely, a meteotsunami is a forced wave, continuously driven by an atmospheric pressure disturbance moving across the water surface. This sustained energy input from the weather system allows the wave to propagate over long distances and maintain its considerable speed.
The initial displacement of the water surface is caused by rapid changes in barometric pressure, such as those found within fast-moving squall lines or severe thunderstorms. A significant drop in atmospheric pressure effectively “pulls” the water level up in a localized area. This localized bulge of water then travels along with the atmospheric disturbance, forming the initial long-period wave.
The Role of Atmospheric Resonance
The power of a meteotsunami results from a process of energy transfer and amplification known as Proudman Resonance. This resonance occurs when the speed of the atmospheric pressure disturbance, such as a squall line, closely matches the speed of the long ocean wave it generates. When these two speeds synchronize, the atmospheric forcing continuously pumps energy into the water wave over an extended period.
The speed of a long ocean wave in shallow water is determined by the water depth. This relationship means that a specific water depth corresponds to a specific wave speed for a given location. If a weather system moves at the exact speed corresponding to the wave speed for that water depth, the resonance condition is met, leading to maximum wave amplification.
This mechanism explains why bathymetry, or underwater topography, is significant. A shallow continental shelf is particularly conducive to this phenomenon because shallower water slows the ocean wave speed, making it more likely to match the speed of a typical meteorological event. As the forced wave moves onto the shelf, the decreasing depth can lead to full resonance, causing the wave amplitude to increase rapidly.
Maximum wave growth is proportional to the distance the atmospheric disturbance travels at the resonant speed. Although the initial wave amplitude caused by the pressure change might be small, this continuous coupling can amplify the water wave by a factor of five to ten. The disturbance may travel for hundreds of kilometers, allowing the wave to gain substantial energy before reaching the shore.
Coastal Effects and Global Hotspots
When a meteotsunami approaches the coast, it leads to sudden, dramatic changes in water level that pose a threat to coastal communities. The wave causes an extremely rapid rise in sea level, followed by an equally quick retreat, often creating dangerous rip currents that can sweep people off piers and beaches. The suddenness of the event makes it hazardous for swimmers and boaters, as there is little visible warning until the surge arrives.
In confined areas like harbors and inlets, the incoming wave often excites a secondary phenomenon called a seiche. A seiche is a standing wave characterized by rhythmic oscillations of water within the enclosed basin. This secondary effect, known as harbor resonance, can further amplify the initial meteotsunami wave, sustaining dangerous water level fluctuations for several hours.
Meteotsunamis are a global phenomenon, but they occur most frequently in specific locations known as hotspots, where bathymetry and weather patterns are ideal for Proudman Resonance. Primary hotspots include:
- The Mediterranean Sea, particularly the Adriatic Sea.
- Certain bays in the Western Mediterranean.
- The Great Lakes region in North America.
- Lake Michigan, which sees an average of over 100 events per year across the system.
Historical events illustrate the potential danger. In 1954, a meteotsunami struck the Chicago waterfront on Lake Michigan, resulting in seven fatalities. Another notable event occurred in 1979 in Nagasaki Bay, Japan, where a five-meter wave killed three people. These incidents underscore that meteotsunamis are powerful enough to cause significant damage and loss of life.