You would be surprised at how often meteorologists get asked about tsunamis. A tsunami is defined by Geology.com as,
a large ocean wave that is caused by sudden motion on the ocean floor. This sudden motion could be an earthquake, a powerful volcanic eruption, or an underwater landslide.
So by this definition it is clear that a tsunami is a geologic or oceanographic process. It is not a meteorological process. Ahhhh, but have you heard of a meteorological tsunami or "meteotsunami?" It appears one happened this weekend in Naples, Florida. What are they?
Water levels at Naples, FL. January 17th, 2016. Courtesy of National Weather Service via... [+]
I consulted the peer-reviewed scientific literature for a robust definition. Charitha B. Pattiaratchi, and E. M. S. Wijeratne published a paper in Philosophical Transactions A entitled, "Are meteotsunamis an underrated hazard? In the paper they describe a meteorological tsunamis as,
water-level oscillations which are similar to waves generated by seismic activity (‘tsunami waves’), except they have a meteorological origin and are not generated through seismic activity, volcanic explosions or submarine landslides
Meteotsunami generation. Courtesy of UCAR Comet Program.
The term first appeared in 1961 by A. Defant, but there is scientific research dating back to the 1930s. They are primarily caused by the propagation of an abrupt atmospheric pressure change and associated wind gusts. Meteorological events like squalls, frontal passages, and gravity waves can all generate meteotsunamis. Small sea-level disturbances can amplify to produce destructive events albeit not to the level of significant geologic-driven tsunamis in the Pacific Ocean in recent years (e.g, Japan 2011). Believe it or not, there is an excellent resource website recommended by the National Weather Service Tsunami Warning Program that breaks down the key ingredients for meteotsunamis. The website notes the following,
Atmospheric gravity waves occur when air parcels are lifted by bouyancy and pulled back down by gravity in oscillatory fashion. They are caused by wind passing over terrestrial land forms, velocity shear of the polar jet stream and radiation from space.... These energy and pressure fluctuations translate to water surface and generate meteotsunamis. Meteotsunamis are created when there is a sharp atmospheric jump of 2-10 mBar in about 3 minutes due to pressure fronts and cold fronts. These pressure jumps usually result in thunderstorms and tornadoes, but when passing over seas they can cause sharp differences in water level. Squall lines are defined by wind gusts of over 25 mph for over a minute but not sustained for over 10 minutes (6). These high wind speeds create wave trains with long wavelengths but are not affected by fetch as much as typical wind-generated waves.
In addition to a meteorological disturbance, there must be resonance between the atmospheric disturbance and the wave speed in the deep water. In other words, they need to be traveling at about the same speed. The final ingredient required for a meteotsunamis is some amplification by the harbor or bay. The shape of the bay can significantly determine the likelihood of amplification.
This weekend a powerful squall line bearing tornadoes moved into Southwest Florida on the morning of January 17th, 2016. The American Meteorological Society Glossary of Meteorology describes a squall line as,
A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms. The squall line is a type of mesoscale convective system distinguished from other types by a larger length-to-width ratio.
Severe weather along SW coast of Florida, January 17, 2016. Courtesy of G. Postel/Weather Channel on... [+]
For completeness, the Glossary defines a squall as,
A strong wind characterized by a sudden onset, a duration of the order of minutes, and then a rather sudden decrease in speed. In U.S. observational practice, a squall is reported only if a wind speed of 16 knots or higher is sustained for at least two minutes (thereby distinguishing it from a gust). In nautical use, a severe local storm considered as a whole, that is, winds and cloud mass and (if any) precipitation, thunder and lightning.
As reported by NBC-2.com, this particular squall line (with 80 mph wind) arrived at high tide and raised water levels significantly. The National Weather Service believes that this event was a meteotsunami because water levels coincided with storms moving inland. The water levels were nearly 6 feet higher than a normal high tide at over 7 feet. A meteotsunami is also suspected because periodic "peaks and valleys" in the levels, indicative of an oscillatory wave, were observed.
As I close, click on this link for a cool example of a meteotsunami. Oh, and by the way, UTEP Professor Tom Gill points out that apparently the Professor on the old TV show Gilligan's Island understood this phenomenon and hoped to leverage it to escape. Who knew?
