A 30% change in gravity wave activity in Earth’s upper atmosphere was brought on by the Southwestern North American (SWNA) megadrought in the ten years following its commencement in 2000, according to new research based on data spanning two decades.
Chester Gardner from the UIUC Department of Electrical and Computer Engineering and Chiao-Yao She from the Department of Physics at Colorado State University collaborated on a project to investigate Earth’s middle atmosphere more than 30 years ago.
Gardner and She created and proved a significant new method for detecting temperature profiles in the Earth’s upper atmosphere using sodium resonance laser radar (lidar).
They were then able to track changes in upper atmospheric gravity waves over a 20-year period at two places (Albuquerque, New Mexico, and Ft. Collins, Colorado). Geophysical Research Letters has just published their findings, which describe how the waves shifted when the drought started.
In 1994, a team led by Gardner installed a lidar system on a large telescope at the Starfire Optical Range outside of Albuquerque, NM on the Kirtland Air Force Base. Up to the end of the year 2000, the lidar system used atomic sodium as its target species to measure temperature and winds in the high atmosphere. This atomic sodium comes from cosmic dust particles that are evaporated in the atmosphere of Earth.
Using a laser beam, atomic sodium can be excited, causing it to glow. The sodium fluorescence’s backscattered light is collected by a ground-based telescope, and computers use that data to calculate sodium’s density, temperature, and radial wind velocity profiles. She’s team at CSU made similar observations at Ft. Collins, CO, eventually compiling an extensive dataset that spanned 20 years from 1990 to 2010.
A fortunate result of an earlier study was the identification of variations in wave activity throughout the megadrought. When analyzing how waves created in the lower atmosphere affected temperature and wind fluctuations in the higher atmosphere, researchers were shocked to discover significant variations in gravity wave activity following the start of the drought.
Current atmospheric models can’t see the waves because the resolution, even on the fastest computer models, is not sufficient to see the scale of these waves. Now, scientists are developing regional models at very high resolution so that the models can see the larger-scale waves. Our observations can be used to test the accuracy of those models and to validate them.
Chester Gardner
“We never expected to make observations that would yield some insight into how a drought might affect Earth’s upper atmosphere,” Gardner says.
After taking a look at Gardner’s data from Albuquerque and She’s data from Ft. Collins, what they found was a significant reduction (~30%) in wave activity after the drought began. The decrease in wave creation by tropospheric storms during the megadrought and the changing geographic distribution of precipitation events in the Western and Midwestern United States may be linked to variations in gravity wave activity. Simply explained, less precipitation results in fewer storms, and since storms produce waves, less precipitation is present.
What is a Megadrought?
A megadrought is a prolonged period of drought lasting two decades or longer. Megadrought in the Southwestern United States and Canada began in 2000 and is still going strong 22 years later with no end in sight.
The SWNA megadrought, according to Gardner and She, is noteworthy not only for the length of time it lasted but also for the size of the affected region, which stretches from northern Mexico to the northern borders of Wyoming and Oregon as well as from the Pacific Coast to the eastern borders of Wyoming, Colorado, and New Mexico.
This particular megadrought is believed to be the driest 22-year period in the region since 800 CE. It’s been argued that human-induced warming of the Earth’s atmosphere may have contributed more than 40% of the severity of the megadrought.
Gravity Waves (Not Gravitational Waves)
At the boundary between two mediums, gravity waves not to be confused with gravitational waves related to cosmic events like black holes occur when gravity or buoyancy attempt to reestablish equilibrium.
According to Gardner, this effect is comparable to throwing a stone into a body of water. The pond’s surface is moved by the stone, forcing water lower. When the water is buoyant again, it begins to oscillate and create an outward-moving ring of waves. These are gravity waves.
Storm convection, which initiates gravity waves by creating vertical movement that results in oscillation, is one way waves are produced in the lower atmosphere. Air moving across topographical features like mountains can also produce waves by pushing the air upward.
In contrast to the air-water interface, where waves in a pond only go in one direction, gravity waves in the atmosphere travel in all directions. These waves regulate the upper atmosphere’s worldwide circulation and can affect satellite orbits and space weather.
Impactful Results
This research, according to Gardner and She, is important because it shows how regional changes in the lower atmosphere can affect the higher atmosphere. They think this is the first time an upper atmosphere regional climatic effect has been seen.
The outcomes can be used to evaluate and test the next generation of regional high-resolution atmospheric computer models that can resolve the lidar-observed small-scale waves.
Gardner explains, “Current atmospheric models can’t see the waves because the resolution, even on the fastest computer models, is not sufficient to see the scale of these waves. Now, scientists are developing regional models at very high resolution so that the models can see the larger-scale waves. Our observations can be used to test the accuracy of those models and to validate them.”
