How a volcanic eruption helped prove a 200-year-old theory
The Hunga Tonga-Hunga The Ha’apai eruption reached an explosive crescendo on January 15, 2022. Its rapid release of energy fueled an ocean tsunami that caused damage as far as the west coast of the United States, but it also generated waves pressure in the atmosphere that quickly spread around the world. .
The atmospheric wave pattern near the eruption was quite complicated, but thousands of miles away it appeared as an isolated wave front moving horizontally more than 650 miles per hour as it expanded outward.
NASA’s James Garvin, the chief scientist at the Goddard Space Flight Center, said NPR the space agency estimated the explosion to be about 10 megatons of TNT equivalent, about 500 times more powerful than the bomb dropped on Hiroshima, Japan, in 1945. Satellites observing with infrared sensors overhead, the wave resembled a ripple produced by dropping a stone into a pond.
The pulse recorded as atmospheric pressure disturbances lasting several minutes as it moved over North America, India, Europe, and many other places around the world. Online, people followed the progress of the pulse in real time as observers posted their barometric observations on social media. The wave traveled around the world and back in about 35 hours.
I am a meteorologist who has studied global atmospheric oscillations for nearly four decades. The wavefront expansion of the Tonga eruption was a particularly dramatic example of the phenomenon of global atmospheric wave propagation, which manifested itself after other historic explosive events, including nuclear testing.
This eruption was so powerful that it rang the atmosphere like a bell, but at a frequency too low to be heard. It is a phenomenon first theorized over 200 years ago.
The first such pressure wave that caught the attention of scientists was produced by the great eruption of Mount Krakatoa in Indonesia in 1883.
The Krakatoa Wave Pulse has been detected in barometric observations at locations around the world. Communication was slower then, of course, but within a few years scientists had combined the various individual observations and were able to plot on a map of the world the spread of the pressure front in the hours and days following the wave. ‘eruption.
The wavefront moved outward from Krakatoa and was observed making at least three full trips around the globe. The Royal Society of London published a series of maps illustrating wavefront propagation in a famous 1888 report of the eruption.
The waves observed after Krakatoa or the recent Tonga eruption are very low frequency sound waves. Spreading occurs when local pressure changes produce a force on adjacent air, which then accelerates, causing expansion or compression accompanied by pressure changes, which in turn forces air farther along of the wave path.
In our normal experience with high frequency sound waves, we expect sound to travel in straight lines, for example, from a firework rocket exploding directly at the ear of a spectator on the ground . But these global pressure pulses have the particularity of propagating only horizontally, and therefore of bending following the curvature of the Earth.
A theory of waves hugging the Earth
More than 200 years ago, the great French mathematician, physicist and astronomer Pierre-Simon de Laplace predicted such behavior.
Laplace based his theory on the physical equations governing atmospheric motion on a global scale. He predicted that there should be a class of motions in the atmosphere that spread quickly but hug the Earth’s surface. Laplace showed that the forces of gravity and atmospheric buoyancy favor horizontal air motions over vertical air motions, and one effect is to allow certain atmospheric waves to follow the curvature of the Earth.
For most of the 19th century, this seemed a somewhat abstract idea. But pressure data after Krakatoa erupted in 1883 dramatically showed that Laplace was right and that these Earth-hugging motions can be excited and propagate huge distances.
Understanding this behavior is used today to detect distant nuclear explosions. But the full implications of Laplace’s theory for the background vibration of the global atmosphere have only recently been confirmed.
ring like a bell
An eruption that makes the atmosphere ring like a bell is a manifestation of the phenomenon theorized by Laplace. The same phenomenon is also present in the form of global vibrations of the atmosphere.
These global oscillations, analogous to the sloshing of water in a bathtub, have only recently been conclusively detected.
The waves can connect the atmosphere rapidly across the globe, much like waves traveling through a musical instrument, such as a violin string, drumhead, or metal bell. The atmosphere can and does “resonate” at a set of distinct frequencies.
In 2020, my colleague from Kyoto University, Takatoshi Sakazaki, and I were able to use modern observations to confirm the implications of Laplace’s theory on the globally coherent vibrations of the atmosphere. By analyzing a newly published dataset of atmospheric pressure every hour for 38 years at sites around the world, we were able to spot the global patterns and frequencies that Laplace and others who followed him had theorized.
These global atmospheric oscillations are much too low to hear, but they are continuously excited by all other movements of the atmosphere, providing very soft but persistent “background music” to the more dramatic weather fluctuations in our atmosphere.
Laplace’s work was a first step on the way to our modern computer forecasting of the weather.
This article was originally published on The conversation through Kevin Hamilton at the University of Hawaii. Read the original article here.